Eddy Current Manual I Volume 1 Test Method

Home , Eddy current

I AECL-7523 Rev. 1 i

• ATOMIC ENERGY aT—33 L'ENERGIE ATOMIQUE 1 OF CANADA LIMITED TmJ DU CANADA LIMITEE I

I EDDY CURRENT MANUAL I VOLUME 1 TEST METHOD

Manuel d'essai par courant de Foucault Methode d'essai

V.S. CECCO, G. VAN DRUNEN and F.L. SHARP

Chalk River Nuclear Laboratories Laboratoires nucleaires de Chalk River

Chalk River, Ontario

November 1981 novembre

Revised 1983 revise I

I ATOMIC ENERGY OF CANADA LIMITED I I EDDY CURRENT MANUAL VOLUME 1

TEST METHOD

V.S. Cecco, G. Van Orunen and F.L. Sharp

Chalk River Nuclear Laboratories Chalk River, Ontario KOJ 1J0 1981 OCTOBER REVISED 1983 SEPTEMBER

AECL-7523 REV. 1 L'ENERGIE ATOMIQUE DU CANADA, LIMITEE

Manuel par courant de Foucault

Volume 1

Méthode d'essais

V.S. Cecco, G. Van Drunen et F.L. Sharp

Résumé

Ce manuel de référence et d'instruction a pour but de fournir â ceux qui font des essais par courant de Foucault les principes fondamentaux de la technique et les connaissances voulues pour interpréter conme il faut les résultats souvent compliqués de ces essais. Une approche non rigoureuse est employée pour simplifier les complexes phénomènes physiques. L'accent est mis sur un choix approprié de fréquences d'essai et sur l'interprétation des signaux. La détection et le diagnostic des défauts font l'objet d'une attention particulière. La conception et la réalisation des sondes sont traitées de façon approfondie car les sondes jouent un rôle clé dans les essais par courant de Foucault. Les avantages et les limitations des divers types de sondes sont indiqués.

La théorie électromagnétique, l'instrumentation, les méthodes d'essai et les analyses de signaux sont décrites. Les réponses des sondes permettent d'avoir une compréhension fondamentale du comportement des courants de Foucault, à condition d'avoir recours aux déductions simplifiées indiquées dans le manuel pour tester les paramètres. Les signaux des courants de Foucault sont présentés sur des diagrammes de plans d'impédance tout au long du manuel, car il s'agit là de l'information la plus commune affichée sur les instruments universels modernes. L'emploi du "retard de phase" dans l'analyse des signaux est décrit en détail. Pour compléter la théorie, des exemples pratiques sont donnés. Ces exemples ont pour but de rendre les inspections plus performantes et ils montrent comment les principes de base s'appliquent au diagnostic des signaux réels.

Laboratoires nucléaires de Chalk River Chalk River, Ontario KOJ 1J0

Novembre 1981 Revisé September 1983 AECL-7523 REV. 1 1 I I ATOMIC ENERGY OF CANADA LIMITED EDDY CURRENT MANUAL

I VOLUME 1

I TEST METHOD

V.S. Cecco, G. Van Drunen and F.L. Sharp

ABSTRACT

This training and reference manual was assembled to provide those involved in eddy current testing with both the fundamental principles of the technique as well as the knowledge to deal with often conplicated test results. A non-rigorous approach is used to simplify complex physical phenomena. Emphasis is placed on proper choice of test frequency and signal interpretation. Defect detection and diagnosis receive particular attention. Design and construction of probes are covered extensively since, probes play a key role in eddy current testing. The advantages and limitations of various probe types are discussed.

Electromagnetic theory, instrumentation, test methods and signal analysis are covered. Simplified derivations of probe response to test parameters are presented to develop a basic understanding of eddy current behaviour. Eddy current signals are presented on impedance plane diagrams throughout the manual since this is the most common display on modern, general purpose instruments. The use of ''phase lag" in signal analysis is covered in detail. To supplement theory, practical examples are presented to develop proficiency in performing inspections, and to illustrate how basic principles are applied to diagnose real signals.

Chalk River Nuclear Laboratories Chalk River, Ontario KOJ 1J0

1981 NOVEMBER REVISED 1983 SEPTEMBER AECL-7 523 REV. 1 I I -iii-

I ACKNOWLEDGEMENTS

This manual is an accumulation of knowledge and experience I obtained by the NDT Development Branch (formerly Quality Control Branch) of CRNL through its 12 years of existence. I The authors are indebted to the other members of the Nondestructive Testing Development Branch especially C.R. Bax, H.W. Ghent, J.R. Carter, G.A. Leakey and I W. Pantermoller who assisted in collecting some of the data in the manual and made many constructive criticisms. I I

All rights reserved. No part of this report may be reproduced by any means, nor transmitted, nor translated into a machine language without the written permission of Atomic Energy of Canada Limited Research Company. -iv-

TABLE OF CONTENTS

CHAPTER 1 - INTRODUCTION PAGE

1.1 EDDY CURRENT TESTING 1 1.2 PURPOSE OF THIS MANUAL 1 1.3 HISTORICAL PERSPECTIVE 2

CHAPTER 2 - ED71Y CURRENT FUNDAMENTALS 2.1 BASIC EQUIPMENT 5 2.2 GENERATION OF EDDY CURRENTS 6 2.2.1 Introduction 6 2.2.2 Magnetic Field Around a Coil 6 2.2.3 Equations Governing Generation of Eddy Currents 8

2.3 FUNDAMENTAL PROPERTIES OF EDDY CURRENT FLOW 10 2.4 SKIN EFFECT 11 2.4.1 Standard Depth of Penetration 12 2.4.2 Depth of Penetration in Finite Thickness Samples 13 2.4.3 Standard Phase Lag 14 2.4.4 Phase Lag in Finite Thickness Samples 16 2.5 SUMMARY 17 2.6 WORKED EXAMPLES 18 2.6.1 Standard Depth of Penetration and Phase Lag 18

CHAPTER 3 - ELECTRICAL CIRCUITS AND PROBE IMPEDANCE 3.1 INTRODUCTION 19 3.2 IMPEDANCE EQUATIONS AND DEFINITIONS 19 3.3 SINUSOIDS, PHASORS AND ELECTRICAL CIRCUITS 21 3.4 MODEL OF PROBE IN PRESENCE OF TEST MATERIAL 23 3.5 SIMPLIFIED IMPEDANCE DIAGRAMS 25 3.5.1 Derivation of Probe Impedance for Probe/ Sample Combination 25 3.5.2 Correlation Between Coil Impedance and Sample Properties 28

3.6 SUMMARY 30 3.7 WORKED EXAMPLES 31 3.7.1 Probe Impedance in Air 31 3.7.2 Probe Impedance Adjacent to Sample 32 3.7.3 Voltage-Current Relationship 32 I I I CHAPTER 4 - INSTRUMENTATION PAGE I 4.1 INTRODUCTION 33 4.2 BRIDGE CIRCUITS 34 4.2.1 Simple Bridge Circuit 34 4.2.2 Typical Bridge Circuit in Eddy Current Instruments 36 4.2.3 Bridge Circuit in Crack Detectors 37

4.3 RESONANCE CIRCUIT AND EQUATIONS 38 4.4 EDDY CURRENT INSTRUMENTS 40 4.4.1 General Purpose Instrument (Impedance Method) 40 t.4.2 Crack Detectors 42 4.4.3 Material Sorting and Conductivity Instruments 44 4.5 SEND-RECEIVE EDDY CURRENT SYSTEMS 45

4.5.1 Hall-Effect Detector 46 4.5.2 Send-Receive Coils and Lift-Off Compensation 47 4.6 MULTIFREQUENCY EQUIPMENT 48 4.7 PULSED EDDY CURRENT EQUIPMENT 49 4.8 SPECIAL TECHNIQUES 50 4.9 RECORDING EQUIPMENT 51 4.9.1 Frequency Response 53

4.10 SUMMARY 53 4.11 WORKED EXAMPLES 54 4.11.1 Impedance at Resonance 54 a -vi- 1 CHAPTER 5 - TESTING WITH SURFACE PROBES

'AGE 1

5.1 INTRODUCTION 55 5.2 SURFACE PROBES 55 1 5.2.1 Probe Types 56 59 I 5.2.2 Directional Properties 60 5.2.2.1 Sensitivity at Centre of a Coil 61 I 5.2.3 Probe Inductance 65 5.3 PARAMETERS AFFECTING SENSITIVITY TO DEFECTS 1 65 5.3.1 Sensitivity with Lift-off and Defect Depth 66 5.3.2 Effect of Defect Length 1 5.4 COMPARISON BETWEEN SURFACE AND THROUGH-WALL INSPECTION 67 5.5 IMPEDANCE GRAPH DISPLAY 69 5.5.1 Effect of Resistivity 72 I 5.5.2 Effect of Permeability 72 5.5.3 Effect of Thickness 72 5.5.4 Effect of Frequency 72 1 5.5.5 Effect of Probe Diameter 73 5.5.6 Comparison of Experimental and Computer Impedance Diagrams 73 i 5.6 CHARACTERISTIC PARAMETER 74 5.7 DEFINITION OF "PHASE" TERMINOLOGY 77 5.8 SELECTION OF TEST FREQUENCY 78 if 5.8.1 Inspecting for Defects 78 5.8.2 Measuring Resistivity 80 a 5.8.3 Measuring Thickness 83 5.8.4 Measuring Thickness of a Non-conducting Layer on a Conductor 84 5.8.5 Measuring Thickness of a Conducting Layer on I a Conductor 84 5.9 PROBE-CABLE RESONANCE 85 1 5.10 SUMMARY 86 5.11 WORKED EXAMPLES 88 i 5.11.1 Effective Probe Diameter 88 5.11.2 Characteristic Parameter 88 i i i -vii- I

CHAPTER 6 - SURFACE PROBE SIGNAL ANALYSIS I PAGE 6.1 INTRODUCTION 89 6.2 EDDY CURRENT SIGNAL CHARACTERISTICS 89 I 6.2.1 Defect Signal Amplitude 89 6.2.2 Defect Signal Phase 91 I 6.3 EFFECT OF MATERIAL VARIATIONS AND DEFECTS IN A FINITE THICKNESS 93 6.A COIL IMPEDANCE CHANGES WITH DEFECTS 97 I 6.4.1 Surface Defect Measurement 97 6.4.2 Subsurface Defect Measurement 97 6.5 COIL IMPEDANCE CHANGES WITH OTHER VARIABLES 98 6.5.1 Ferromagnetic Indications 98 6.5.2 Electrical Resistivity 100 6.5.3 Signals from Changes in Surface Geometry 100 6.6 CALIBRATION DEFECTS 101 6.7 SUMMARY 104

CHAPTER 7 - TESTING OF TUBES AND CYLINDRICAL COMPONENTS 7.1 INTRODUCTION 105 7.2 PROBES FOR TUBES AND CYLINDRICAL COMPONENTS 105 7.2.1 Probe Types 105 7.2.2 Comparing Differential and Absolute Probes 107 7.2.3 Directional Properties 109 7.2.4 Probe Inductance 110 7.2.5 Probe-Cable Resonance 112

7.3 IMPEDANCE PLANE DIAGRAMS 113 7.3.1 Solid Cylinders . 115 7.3.1.1 Sensitivity in Centre of a Cylinder 116 7.3.2 Tubes 118 7.3.3 Characteristic Frequency for Tubes 120 7.3.4 Computer Generated Impedance Diagrams 122 7.4 CHOICE OF TEST FREQUENCY 123

7.4.1 Test Frequency for Solid Cylinders 123 7.4.2 Test Frequency for Tubes 124 7.5 PROBES FOR DETECTING CIRCUMFERENTIAL CRACKS 125 7.6 SUMMARY 12 8 7.7 WORKED EXAMPLES 129 7.7.1 Calculate f/fg to operate at knee location, for a cylinder 129 7.7.2 (a) Calculate optimum test frequency for Cube inspection 129 (b) Determine operating point for above frequency 130 (c) Calculate frequency to discriminate ferromagnetic indications 130 -viii- I CHAPTER 8 - TUBE TESTING - SIGNAL ANALYSIS PAGE I 8.1 INTRODUCTION 131 8.2 EDDY CURRENT SIGNALS 131 8.2.1 Defect Signal Characteristics 131 I 8.2.2 Effect of Test Frequency 135 8.2.3 Calibration Tubes and Simple Defects 138 am 8.2.4 Vectorial Addition and Defects at Baffle Plates 142 I 8.2.5 Tube Inspection at Tubesheets 146 ™ 8.2.6 Testing Tubes with Internal Surface Probes 147 8.3 ANOMALOUS EDDY CURRENT SIGNALS 149 jg 8.3.1 Ferromagnetic Inclusions and Deposits 149 8.3.2 Conducting Deposits 153 H 8.4 MULTIFREQUENCY EDDY CURRENT TESTING 155 8.4.1 Background 155 • 8.4.2 Multifrequincy Testing of Dented Tubes 158 • 8.5 SUMMARY 162 I CHAPTER 9 - METALLURGICAL PROPERTIES AND TESTING FERRO- MAGNETIC MATERIALS 9.1 INTRODUCTION ' 163 I 9.2 ELECTRICAL CONDUCTIVITY 163 9.2.1 Factors Affecting Resistivity 163 I 9.2.2 Material Sorting by Resistivity 166 9.3 MAGNETIC PROPERTIES 168 I 9.3.1 Magnetic Hysteresis 169 9.3.2 Magnetic Permeability 17U m 9.3.3 Factors Affecting Magnetic Permeability 172 • 9.4 TESTING MAGNETIC MATERIALS 174 9.4.1 Simplified Impedance Digrams 174 I 9.4.2 Impedance Diagrams 176 9.4.3 Material Sorting by Magnetic Permeability 178 9.4.4 Testing for Defects in Magnetic Materials 178 ft 9.5 SUMMARY 184 9.6 WORKED EXAMPLES 185 I 9.6.1 Calculate Conductivity 185 9.6.2 Calculate Magnetic Permeability 185 9.6.3 Calculate Standard Depth of Penetration 186 I CHAPTER 10 - DEFINITIONS, REFERENCES AND INDEX 10.1 DEFINITIONS 187 10.2 REFERENCES 194 f 10.3 INDEX 195 I -ix- I I NOMENCLATURE I SYMBOL QUANTITY SI UNIT A Cross-Sectional area met re r Radius metre 1 Length metre t Thickness metre w Width met re D Diameter metre ^ B Magnetic flux density weber/meter or ttes la C Capacitance farads f Test frequency hertz

l- h Optimum tube testing frequency hertz Characteristic or Limit g frequency hertz

fr Resonant frequency hertz H Magnetic field intensity amperes/meter or (Magnetizing force) lenze I Current amperes £ J Current density amperes/meter L Self Inductance henry N Number of turns (Windings) dimens ionless Pc Characteristic Parameter dimensionless R° Res'is tance ohm RL Resistive load ohm V Electric potential volt X Depth below the surface metre Inductive Reactance ohm xLi L Capacitive Reactance ohm Z° Impedance ohm <5 Standard Depth of Penetration met re V Permeability henry/meter p Resistivity microhm-centimet re 0 Conductivity siemens/meter $ Magnetic flux weber n Fill Factor dimensionless 3 Phase Lag radians Angular frequency radians/second 0 Angle between Z & R degrees I CHAPTER 1 - INTRODUCTION I 1.1 EDDY CURRENT TESTING Eddy current testing (ET) Is a nondestructive test technique based on Inducing electrical currents In the material being Inspected and observing the Interaction between those currents and the material. Eddy currents are generated by electromagnetic coils in the test probe, and monitored simultaneously by measuring probe electrical impedance. Since it's an electromagnetic induction process, direct electrical contact with the sample is not required; however, the sample material has to be conductive.

Eddy current testing is a versatile technique. It's mainly used for thin materials; in thick materials, penetration constraints limit the inspected volume to thin surface layers. In addition to flaw inspection, ET can be used to indirectly measure mechanical and metallurgical characteristics which correlate with electrical and magnetic properties. Also, geometric effects such as thickness, curvature and probe-to-material spacing influence eddy current flow and can be measured.

The large number of potentially significant variables in ET is both a strength and a weakness of the technique since effects of otherwise trivial parameters can mask important information or be misinterpreted. Virtually everything that affects eddy current flow or otherwise influences probe impedance has to be taken into account to obtain reliable results. Thus, credible eddy current testing requires a high level of operator training and awareness.

1. 2 PURPOSE OF THIS MANUAL

The purpose of this manual is to promote the development and use of eddy current testing by providing a scientifically sound training and reference manual. The selection of material presented is based on the premise that a sound understanding of basic principles is essential to obtaining valid data and interpreting it correctly. A non-rigorous approach has been used to present complex physical, phenomena in a document oriented towards application of eddy current techniques, especially for defect detection and diagnosis.

The presentation moves from theory (including a review of basic electrical concepts) to test methods and signal analysis. Simplified derivations of probe response to test parameters are presented to develop a basic understanding of eddy current test principles. Thus, eddy current signals are -2- I consistently illustrated on impedance plane diagrams (the M display used in modern eddy current test instruments) and to • aid explanation, the parameter "eddy current phase lag" is int roduced. _

Since probes play a key role in eddy current testing, ™ technical aspects of probe design are introduced with pertinent electrical impedance calculations. While knowledge • of basic electrical circuits is required for a complete | understanding of eddy current test principles, a good technical base for inspection can still be obtained if a sections of this manual requiring such a background are • skipped.

From an applications point of view, the material in this • manual provides an inspector with the necessary background to • decide: 1) what probe(s) to use, • 2) what test frequencies are suitable, £ 3) what calibration defects or standards are required for signal calibration and/or simulation, ^ 4) what tests are required to differentiate between • significant signals and false indications. ™ 5) how to estimate depth of real defects.

To supplement theory, practical examples are presented to • develop proficiency in performing inspections, and to illustrate how basic principles are applied to diagnose real m signals. •

A number of laboratory demonstrations, practical tests and multiple choice questions are included in Volume 2, "Eddy • Current Course Supplement". They are divided into groups • corresponding to the chapters in this manual. The demonstrations are intended for use in eddy current courses • to help clarify some of the more difficult concepts. The ( practical tests are to give students practice in using equipment and performing typical tests. The multiple choice & questions are intended to check students' understanding of I the course material and prepare for certification exams. *

1.3 HISTORICAL PERSPECTIVE •

Electromagnetic testing -- the interaction of magnetic fields with circulating electrical currents — had its origin in • 1831 when M. Faraday discovered electromagnetic induction. • He induced current flow in a secondary coil by switching a battery on and off. D.E. Hughes performed the first recorded ~ eddy current test in 1879. He was able to distinguish • between different metals by noting a change in excitation ™ frequency resulting from effects of test material resistivity and magnetic permeability. A I -3- I

VI—< THERE MUST BE DEFECTS IN THESE TUBES SOMEWHERE — I SAW SQU/GGLES Oh' THE EDDV CURRENT SCREEN/

Fig. 1.1:Misinterpreted Signals Initially, the extreme sensitivity to many material properties and conditions made ET difficult and unreliable. Figure 1.1 illustrates this point. It took until 1926 before the first eddy current instrument was developed to measure sample thickness. By the end of World War II further research and improved electronics made industrial inspection possible, and many practical instruments were developed. A major breakthrough came in the 1950's when Forster developed instruments with impedance plane signal displays. These made it possible to discriminate between different parameters, though the procedure was still empirical. During the 1960's progress in theoretical and practical uses of eddy current testing advanced the technology from an empirical art to an accepted engineering discipline. 4 -4- I During that time, other nondestructive test techniques such Wk as ultrasonics and radiography became well established and | eddy current testing played a secondary role, mainly in the aircraft industry. Recent requirements — particularly for m heat exchanger tube inspection in the nuclear industry — M have contributed significantly to further development of ET as a fast, accurate and reproducible nondestructive test technique. •

Until recently, eddy current testing was a technology where the basic principles were known only to researchers, and a • "black box" approach to inspection was often followed. The f authors' objective in compiling this manual is to draw upon research, laboratory and industrial inspection experience to _ bridge that gap and thereby permit the full potential of eddy B current testing to be realized; ™ I I I I I 1 I I I I f I I I -5- CHAPTER 2 - EDDY CURRENT FUNDAMENTALS

2.1 BASIC EQUIPMENT Basic eddy current test equipment consists of an alternating current source (oscillator), a probe containing a coil connected to the current source, and a voltmeter which measures the voltage change across the coil, as shown in Figure 2.1.

OSCILLATOR VOLTMETER

Fig. 2.1: Eddy Current Test Equipment The oscillator must be capable of generating a time varying (usually sinusoidal) current at frequencies ranging from about 1 kHz (1000 cycles per second) to about 2 MHz (2,000,000 cycles per second). Oscillators which operate at higher or lower frequencies, or with pulsed currents are used for specialized applications.

The coil within the probe is an insulated copper wire wound onto a suitable form. The wire diameter, the number of turns and coil dimensions are all variables which must be determined in order to obtain the desired inspection results. Coil variables are discussed in later chapters. -6- I Depending upon the type of inspection, an eddy current probe can consist of a single test coil, an excitation coil with a separate receive (sensing) coil, or an excitation coil with a I Hall-effect sensing detector, as shown in Figure 2.2. VOLTMETER VOLTMETER VOLTMETER I I I / HALL / DETECTOR I I I (A) SELF-INDUCTANCE (B) SEND-RECEIVE

The voltmeter measures changes in voltage across the coil I which result from changes in the electrical conditions and properties of the conducting material tested and/or changes in relative position between the coil and the material I tested. This voltage change consists of an amplitude variation and a phase variation relative to the current passing through the coil. The reason for amplitude and phase changes in this voltage is discussed in Chapter 3. 1 2.2 GENERATION OF EDDY CURRENTS I 2.2.1 Introduction In this section the topic of the magnetic field surrounding a I coil carrying current is introduced together with the mechanism by w.iich eddy currents are induced and how they are measured. I 2.2.2 Magnetic Field Around A Coil Oersted discovered that whenever there is an electric I current, a magnetic field exists. Consider electric current directed along a wire, a magnetic field is created in such a direction that if your right-hand thumb points in the direction of current, your curled fingers point in the I I -7-

direction of the magnetic field, This is the "right-hand rule". Associated with a magnetic field is magnetic flux density. It has the same direction as the magnetic field and its magnitude depends upon position and current. It is therefore a field vector quantity and is given the symbol B. Its units in the SI system is the tesla (T) or webers per square metre (Wb/m2).

The B-field distribution around a long straight wire is shewn in Figure 2.3(a). In Figure 2.3(b) the B-field in the axial direction of a single turn is shown as a function of radius. As more windings are added, each carrying the same current, the flux density rapidly increases and its associated distribution is altered.

(a) Straight Wire (b) Single Turn Coil CuAAent FZowing Unto page.

Fig. 2.3: Magnetic Flux Distribution

Flux density varies linearly with electric current in the coil, i.e., if coil current doubles, flux density doubles everywhere. The total magnetic flux, <|>p, contained within the loop is the product of B and area of the coil. The unit in the SI system for magnetic flux is the weber (Wb). -8- I 2.2.3 Equations Governing Generation of Eddy Currents In any electrical circuit, current flow is governed by Ohm's I Law and is equal to the driving (primary circuit) voltage divided by primary circuit impedance. I I V /Z (2.1) P P

The eddy current coil is part of the primary circuit. The I current passing through the coil normally varies sinusoldally with time and Is given by: I I sin(wt) (2.2) o I where lo is tht peak current value in the circuit and w (omega) is the frequency In radlans/s (u) equals 2irf when f is frequency in hertz). I From Oersted's discovery, a magnetic flux (p) exists around a coil carrying current (see Figure 2.4) proportional to the number of turns in the coil (Np) and the current (Ip)« I

N I (2.3) P P I I I I PROBE

(primary . circuit) II SAMPLE (secondary I circuit)

Fig. 2.4: Coil Carrying Alternating Current Adjacent To a Test Sample f II I -9-

Faraday's Law states a voltage (Vs) is created or induced in a region of space when there is a changing magnetic field. When we apply this to our coil,

V =-N ^ s p dt d<}> where ~dt *s t*ie rate °^ change in § with time. Since coil current varies sinusoidally with time, total magnetic flux in the coil also varies sinusoidally,

= $ sin(tot)

where <|>o is the magnetic flux corresponding to IQ.

The induced voltage as described by equation 2.4 results in

V = - N iii<\> cos((Ot) (2.5) s p o

which also varies periodically with time. If we bring the coil close to a test sample, Ohm's Law states that if there is a driving voltage (Vg) and the sample's impedance is finite, current will flow, Ts - VZs (2'6)

where Ig is current flowing through the sample, Vs is induced voltage and Zs is the sample's impedance or opposition to the flow of current.

These induced currents are known as eddy currents because of their circulatory paths. They, in turn, generate their own magnetic field according to Lenz's Law, which opposes the primary field,

and *E = *p - *B (2.8) where <(>£ is the equilibrium magnetic flux surrounding the coil in the presence of a test sample. The flow of eddy currents results in resistive (Ohmic) losses and a decrease in magnetic flux. This is reflected as a decrease in probe impedance. In equation form,

Z « *E (2.9)

and V = ZIp (2.10) -10-

Equation 2.9 indicates a coil's impedance is a function of the magnetic field surrounding it and in turn the magnetic field is governed by induced current in the specimen (equations 2.8 and 2.7). The relations between probe impedance and sample properties will be derived in Chapter 3.

To summarize, flux is set up by passing alternating current through the test coil. When this coil is brought close to a conductive sample, eddy currents are induced. In addition, the magnetic flux associated with the eddy currents oppose the coil's magnetic flux, thereby decreasing net flux. This results in a change in coil impedance and voltage drop. It is the opposition between the primary (coil) and secondary (eddy current) fields that provides the basis for extracting information during eddy current testing.

It should be noted that if a sample is ferromagnetic, equation 2.9 still applies but the magnetic flux is strengthened despite opposing eddy current effects. The high magnetic permeability of ferromagnetic materials distinguishes them from non-ferromagnetic materials and strongly influences eddy current test parameters.

Ferromagnetic specimen inspection is discussed in Chapter 9 and unless specified the rest of the manual is restricted to non-ferromagnetic materials. 2.3 FUNDAMENTAL PROPERTIES OF EDDY CURRENT FLOW

Eddy currents are closed loops of induced current circulating in Tlanes perpendicular to the magnetic flux. They normally travel parallel to the coil's winding and parallel to the surface. Eddy current flow is limited to the area of the inducing magnetic field.

Test frequency determines depth of penetration into the specimen; as frequency is increased, penetration decreases and the eddy current distribution becomes denser near the specimen's surface. Test frequency also affects the sensitivity to changes in material properties and defects.

Figure 2.5(a) shows the algebraic relationships and Figure 2.5(b) the oscilloscope display of eddy current and magnetic field distribution with depth into the specimen. Both the eddy currents and magnetic flux get weaker with depth because of "skin effect". In addition to this attenuation, the eddy currents lag in phase with depth. Eddy currents' phase lag is the key parameter that makes eddy current testing a useful NDT method. The parameters skin depth and phase lag ate discussed in the next section. I -11- I

C01L

(a) (b)

Fig. 2.5: Eddy Current and Magnetic Flux Distribution With Depth Into a Conductor 2.4 SKIN EFFECT

Eddy currents induced by a changing magnetic field concentrate near the surface adjacent to the excitation coil. The depth of penetration decreases with test frequency and is a function of electrical conductivity and magnetic permeability of the specimen. This phenomenon is known as the skin effect and is analogous to the situation in terrestrial heat conduction where daily surface temperature fluctuations are not appreciable below the earth's surface. Skin effect arises as follows: the eddy currents flowing in the test object at any depth produce magnetic fields which oppose the primary field, thus reducing net magnetic flux and causing a decrease in current flow as depth increases. Alternatively, eddy currents near the surface can be viewed as shielding the coil's magnetic field thereby weakening the magnetic field at greater depths and reducing induced currents.

The equation for flow of induced currents is

2 V J = 1ft (2.11)

where J is current density, is conductivity, \i is magnetic permeability and V is a differential operator of second order. -12-

For a semi-infinite (thick) conductor the solution to the above equation is (2.12a) Jx /Jo sin(u)t-B)

where Jx/J0 is the ratio of eddy current density Jx at depth x to the surface density Jo> and e - 2.718 is the base . of natural logarithms. 3 is given by x/6 where 6 = (iTfya)" ' , the standard depth of penetration (see next section).

Equation 2.12(a) can be separated into two components:

x/& j /j « e~ (2.12b) x o which describes the exponential decrease in eddy current density with depth,and sin (wt-x/S) (2.12c) denoting the increasing time or phase lag of the sinusoidal signal with depth. 2.4.1 Standard Depth of Penetration

Figure 2.6 illustrates the change in eddy current density in a semi-infinite conductor. Eddy current density decreases exponentially with depth.

(£o sin (wt)

0.2 0.4 0,6 0.B ' 1 .0

Fig. 2.6: Eddy Current and Magnetic Flux Distribution With Depth in a Thick Plate i I -13- The depth at which eddy current density has decreased to 1/e or 36.8% of the surface density is called the standard depth of I penetration. The word 'standard' denotes plane wave electromagnetic field excitation within the test sam, i.e (conditions which are rarely achieved in practice). The standard depth of penetration is given by

8 = 50/p/fU , mm (2.13a) r

or 2i/p/fyr , inches (2.13b) where p is electrical resistivity in microhm-centimetres, f is test frequency in hertz, and yr is relative magnetic permeability (dimensionless)*.

The skin depth equation is strictly true only for infinitely thick material and planar magnetic fields. Using the standard depth, 6, calculated from the above equation makes it a material/test parameter rather than a true measure of penetration. 2.4.2 Depth of Penetration in Finite Thickness Samples

Sensitivity to defects depends on eddy current density at defect location. Although eddy currents penetrate deeper than one standard depth of penetration they decrease rapidly with depth. At two standard depths of penetration (26), eddy current density has decreased to (1/e)^ or 13.5% of the surface density. At three depths (36) the eddy current density is down to only 5% of the surface density. However, one should keep in mind these values only apply to thick samples (thickness, t >5<5) and planar magnetic•excitat ion fields. Planar field conditions require large diameter probes (diameter >10t) in plate testing or long coils (length >5t) in tube testing. Real test coils will rarely meet these requirements since they would possess low defect sensitivity. For thin plate or tube samples, current density drops off less than calculated from equation 2.12(b) as shown in Figure 2.7. For solid cylinders the overriding factor is a decrease to zero at the centre resulting from geometry effects as shown in Fig. 2.7(c) and discussed in Section 7.3.1.

One should also note, that the magnetic flux is attenuated across the sample, but not completely. Although the currents are restricted to flow within specimen boundaries, the magnetic field extends into the air space beyond. This allows the inspection of multi-layer components separated by an air space.

*See Chapter 9 for a description of electrical and magnetic properties. Pr = \i^ , incremental permeability, at zero biasing magnetization flux. V -14-

-s- =0.01

•5= '•« 2.0

0 .2 .4 .6 .8 1.0

t (a) PLATE (URGE COIL. O>IOt> (U) TUBE (LONG ENCIRCLING COIL./ >St)

EQUATION 2 12 (b) Jo = EDDY CURRENT DENSITY AT SURFACE ACTUAL Jx OR Jr = EDOY CURRENT DENSITY AT LOCATION x OR r BELOW THE SURFACE

i^x^i i-^-^i i__L

Jo .4 •

0 .2 .4 .6 .8 1.0

JL TUBE AND ROD GEOMETRY r° (r, =0 FOR ROD)

(O ROD (ENCIRCLING CO IL. -C > S r0) Fig. 2.7: Eddy Current Distribution With Depth in Various Samples

The sensitivity to a subsurface defect depends on the eddy current density at that depth, it is therefore important to know the effective depth of penetration. The effective depth of penetration is arbitrarily defined as the depth at which eddy current density decreases to 5% of the surface density. For large probes and thick samples, this depth is about three standard depths of penetration. Unfortunately, for most components and practical probe sizes, this depth will be less than 35 , the eddy currents being attenuated more than predicted by the skin depth equation. The effect of probe diameter on the decrease in eddy current density or defect sensitivity with depth is discussed in Section 5.3.1.

2.4.3 Standard Phase Lag The signal produced by a flaw depends on both amplitude and phase of the currents being obstructed. A small surface defect and large internal defect can have a similar effect on the magnitude of test coil impedance. However, because of the increasing phase lag with depth, there will be a characteristic difference in the test coil impedance vector. This effect allows location and extent of a defect to be determined. I -15-

Phase lag is derived from equation 2.12(c) for infinitely thick material. It represents a phase angle lag of x/£ radians between the sinusoidal eddy currents, at the surface and those below the surface. It is denoted by the symbol 6 (beta) and is given by: = x/6 radians (2.14a)

= x/6 x 57 or (2.14b)

where x is distance below the surface in the same units as Illllllo sin l 57C 114° (3 (DEGREES)

1 .0

/3 = 4 x 57, DEGREES o

Fig. 2.8: Eddy Current Phase Lag Variation With Depth in Thick Samples When x is equal to one standard depth of penetration, phase lag is 57° or one radian. This means that the eddy currents flowing below the surface, at one standard depth of penetration, lag the surface currents by 57". At two standard depths of penetration they lag the surface currents by 114". This is illustrated in Figure 2.8. i

-16-

2.4.A Phase Lag in Finite Thickness Samples For thin samples, eddy current phase decreases slightly less rapidly with depth than stated above. See Figure 2.9(a), (b) and (c) for the plots of phase lag with depth for a plate, tube, and cylinder, respectively. The phase lag illustrated in these plots does not change significantly with coil diameter or length. For thick samples and practical probe sizes, equation 2.14 is sufficiently accurate.

0°

20° _ =0.7 40° vN 60° -

= 2,0 80° 0.8 5 S

100° i i 0 .2 .4 .6 .8 1.0 .2 .4 .6 .8 1.0 x t

(a) FLUTE (b) TUBE

D r ~i 1 L

PLATE GEOMETRY 1.0

.8 I .0 ° TUBE AND ROD (r, = 0) GEOMETRY

(3 J3 = PHASE (KITH DEPTH x, OR r,RELATIVE "' r TO SURFACE CURRENT

ACTUAL CURVES (c) ROD CALCULATED, EQUATION 2.14 (b) Fig. 2.9; Eddy Current Phase Lag in Various Samples

Phase lag can be visualized as a shift in time of the sinusoidally varying current flowing below the surface. This was illustrated in Figure 2.5. Phase lag plays a key role in the analysis of eddy current test signals. It will be used throughout the manual to link theory and observations. It should not be misinterpreted or confused with the phase angle between voltage and current in AC theory. Both the voltage and current (and magnetic field) have this phase shift or lag with depth. -17-

2.5 SUMMARY Eddy current testing is based on inducing electrical currents in the material being inspected and observing the interaction between these currents and the material.

This process occurs as follows: When a periodically varying magnetic field intersects an electrical conductor, eddy currents are induced according to Faraday's and Ohm's Laws. The induced currents (known as eddy currents because of their circulatory paths) generate their own magnetic field which opposes the excitation field. The equilibrium field is reduced resulting in a change of coil impedance. By monitoring coil impedance, the electrical, magnetic and geometric properties of the sample can be measured. Eddy currents are closed loops of induced current circulating in planes perpendicular to the magnetic flux. They normally travel parallel to the coil's winding and parallel to the surface. Eddy current flow is limited to the area of the inducing magnetic field.

Depth of penetration into a material depends on its electrical resistivity, magnetic permeability and on test frequency. The basic equation of ET is the standard depth of penetration given by

, mm (2.13a) where p is electrical resistivity, microhm-centimetres; f is test frequency, hertz; and yr is relative magnetic permeability, dimensionless.

It states that in thick materials eddy-current density decreases to 37% of the surface density at a depth of one standard depth of penetration. In most eddy current tests, especially with surface probes, the actual eddy current density (at a depth equal to that calculated by equation 2.13a) is much less than 37%.

Eddy currents also lag in phase with depth into the material. Phase lag depends on the same material properties that govern depth of penetration and is given by

B = x/6 = x , radians (2.14a) 5O/p/fUr where x is distance below surface, mm.

Phase lag is the parameter that makes it possible to determine the depth of a defect. It also allows discrimination between defect signals and false indications. It is the key parameter in eddy current testing. -18-

2.6 WORKED EXAMPLES

2.6.1 Standard Depth of Penetration and Phase Lag PROBLEM: (a) Calculate the standard depth of penetration In a thick 304 SS sample, at a test frequency of 100 kHz. (b) Determine the eddy current phase lag at a depth of 1.5 mm In 304 SS at 100 kHz.

SOLUTION: 304 SS properties: p =72 microhm -centimetres

and yr = 1 (a) from equation 2.13(a),

50^/ 72 V 100 x 103 x 1

= 50(7.2 x10 -4) =1.3 mm

Therefore the standard depth of penetration is 1.3 mm. (b) from equation 2.14(b), 6 = x/6 x 57

= —^ x 57 = 64 degrees

Therefore the phase lag is 64 degrees. I -19- I 1 CHAPTER 3 - ELECTRICAL CIRCUITS AND PROBE IMPEDANCE

3.1 INTRODUCTION Eddy current testing consists of monitoring the flow and distribution of eddy currents in test material. This is achieved indirectly by monitoring probe impedance during a test. An understanding of impedance and associated electrical quantities is therefore imperative for a fundamental appreciation of eddy current behaviour.

The first two sections review the electrical quantities important in eddy current testing. This is followed by presentation of a model of a test coil coupled to test material and its equivalent electrical circuit. The equivalent circuit approach permits derivation of simplified impedance diagrams to show the effect of test and material parameters on probe impedance in graphical form. Once the simple impedance diagram concepts of this chapter are understood, the more complex diagrams of subsequent chapters should present little difficulty.

3.2 IMPEDANCE EQUATIONS AND DEFINITIONS All information about a sample is obtained through changes in electrical characteristics of the coil/sample combination. Therefore a basic understanding of electrical quantities is important for eddy current inspection. RESISTANCE: (symbol: R, units: ohm, £2)

Opposition to the flow of electrical current is called resistance. It is constant for both direct and alternating current. The electrical component is called a resistor.

V - IR Ohm's Law (3.1) where, V is voltage drop across resistor (volt), and I is current through resistor (ampere) INDUCTANCE: (symbol: L, units: henry, H) The property of an electric circuit by virtue of which a varying current induces an electromotive force in that circuit (self) or in a neighbouring circuit (mutual) is called inductance. The electrical component is called an inductor. -20- i

total flux linkages I current through coil (3.2a) I (3.2b) (3.3) I where, N is number of coil turns

CAPACITIVE REACTANCE (symbol: Xc, units: ohm, Q) I Opposition to changes in alternating voltage across a capacitor is called capacitive reactance. I Eddy current coil capacitive reactance is normally negligible. However, capacitance can I be important when considering impedance of probe cables (Sections 5.9 and 7.2.5).

X = 3 I c 2iTf C ( «5) where, C is capacitance (farad) I IMPEDANCE: (symbol: Z, units: ohm, ft) The total opposition to alternating current flow f is called IMPEDANCE2 . For a coil, V* (3.6) f -21-

XT and 6 = Arctan -^ (3.7) where |z| is magnitude of Z, and 6 Is phase of Z (described in next section).

3.3 SINUSOIDS, PHASORS AND ELECTRICAL CIRCUITS In a direct current (DC) circuit, such as a battery and light bulb, current and voltage are described completely by their respective magnitudes, Figure 3.1(a). Analysis of alternating current (AC) circuits is more complex. Since voltage and current amplitude vary with time, the phase (or time delay) relationship between them must also be taken into account. A typical AC circuit, an inductor in series with a resistor, is presented in Figure 3.1(b). This is a simplified model of a probe assembly: the inductor is the reactive part of the assembly (coil) while the resistor models both coil wire and cable resistance. Figure 3.1(c) shows voltage across the inductor (V^) leads the current (I) by 90°, while voltage across the resistor (VR) is in phase with current. Since the current is common to both inductor and resistor, it is possible to use current as a point of reference. Hence, we deduce the voltage across the inductor leads the voltage across the resistor by 90°.

If one measures the voltage drop, Vj>, across both the inductor and resistor, we find VT leads current (or VR) by an angle lese than 90", as shown in Figure 3.1(d).

To evaluate the total voltage VT, we add vectorially the separate voltages VR and V^,

VT = VR + VL . (3.8) = I(R + jwL) (3.9a) where j ts a mathematical operator (rotates a vector CCW by 90°)

or VT = IK sin ( tot+O) + j IWL sin (wt+ir/2) (3.9b)

Representin6 voltage waveforms as in Figure 3.1(d) or aquatLon 3.9(b) can be both time consuming and confusing. A simpler voltage representation is available by means of phasor diagrams. In phasor diagrams the voltage is represented by its peak value (amplitude) and phase shift (6) relative to the current. The two terms In equation 3.9(b) both contain an amplitude and phase shift so they can be -22- I I DIRECT CURRENT I V - 1R (BATTERY) CURRENT AND VOLTAGE CAN BE I DESCRIBED BY MAGNITUDE ONLY («) DIRECT CURRENT CIRCUIT I ALTERNATING CURRENT

CURRENT MUST BE DESCRIBED BY AMPLITUDE AND PHASE I I I (M ALTERNATING CURRENT CIRCUIT I

SCSISTASCF. F. I VOLTAC-E GRAPH DISPLAY OF PHASORS !O IMPEDANCE 'jRAPH DISPLAY

Fig. 3.1; Representation of Direct Current and I Alternating Current Circuit Parameters I t -23-

represented by phasors. The first term's amplitude is IR and its phase shift is 0. The amplitude of the second term is IwL and its phase shift is IT/2 or 90°. Each phasor can be represented by an arrow starting at the origin. The phasor's amplitude is indicated by the length of the arrow OP and the phase shift by the direction of the arrow, see Figure 3.1(e). Phasors are displayed graphically with the resistive component (V^), having a phase shift 9=0, along the horizontal axis. As 6 increases the phasor rotates counter-clockwise. The reactive component (VL), having a phase shift 6 = 90°, will be represented along the vertical axis.

Current is common to both voltage components and since V=IZ, the voltage graph of Figure 3.1(e) can be converted to an impedance graph display, as in Figure 3.1(f). If this approach is applied to eddy current testing, it is found that any changes in resistance or inductive reactance will cause a change in the position of the end of the vector (point P) which represents the total impedance vector.

To obtain the reactive and resistive components from this graph requires knowledge of trigonometry.

Reactive component: XL = wL = IZI sin 9 (3.10)

Resistive component: R = |z| cos 6 (3.11)

2 22 Amplitude of impedance: |z| = -JR + XLL

Phase angle: 9 = Arctan XT/R (3.7) Note the x axis component represents pure resistance (phase shift = 0°) while the y axis component represents pure inductive reactance (phase shift = + 90°). In these calculations it is assumed coil capacitance is negligible.

3.4 MODEL OF PROBE IN PRESENCE OF TEST MATERIAL The test probe contains a coil which when placed on or close to a test sample can be considered as the primary winding of a transformer. The field created by alternating current in the coil induces eddy currents in the test sample which acts as a single turn secondary winding,Ng • 1, Figure 3.2(b)• Eddy currents align to produce a magnetic field which tends to weaken the surrounding net magnetic flux

Fig. 3.2: Model of a Coil with 'T^st Object I

There are two methods of sensing changes In the secondary current, Is. The "impedance method" of eddy current I testing consists of monitoring the voltage drop across the primary coil (V = lpZ ). The impedance Z is altered by the load of the secondary of the transformer. I Therefore, changes in secondary resistance, Rs, or inductance Ls can be measured as changes in V . The "send-receive" method of eddy current testing uses two I coils. Eddy current flow in the sample is altered by defects and these variations are detected by monitoring the voltage across a secondary receive coil, see Figure 3.2(c). I I i -25-

3.5 SIMPLIFIED IMPEDANCE DIAGRAMS 3.5.1 Derivation of Probe Impedance for Probe/Sample Combination

We now consider how changes in the test sample affect coil impedance on the impedance graph display.

From the previous section the probe and test sample can be modelled as a transformer with a multi-turn primary (coil) and single turn secondary (sample), Figure 3.3(a)r. This circuit can be simplified to an equivalent circuit where the secondary circuit load is reflected as a resistive load in parallel with the coil's inductive reactance, Figure 3.3(b). This circuit is an approximate model of a real coil adjacent to a conductor. It is assumed that all of the magnetic flux from the primary coil links the test sample; the coupling is perfect (100%). It is also assumed that there is no skin depth attenuation or phase lag across the sample thickness.

MODEL OF A COIL AND TEST SAMPLE

(l>) EQUIVALENT PARALLEL CIRCUIT

L. . I

(O EQUIVALENT SERIES CIRCUIT

Fig. 3.3 Equivalent Circuits

The equivalent circuit concept can be used to obtain simplified impedance diagrams applicable to eddy current testing. These diagrams serve as an introduction to the more detailed diagrams which include variations caused by the skin effect. The coil/sample circuit model can be transformed into the simpler series circuit by the following mathematical manipulations. The load resistance Rg can be transferred from the secondary back to.the primary winding by multiplying 2 it by the turns ratio squared, (ND/NS) , Figure 3.3(b). J -26-

The total impedance of this parallel circuit can be evaluated and transformed into an equivalent series circuit as follows:

. - Z1Z2 I P zx + z2

where Zj • I and Z2 = I where Xo= toL0, coil inductive reactance in air.

jN2R X I s Therefore Zn » „ ? ° which transforms to

NRX

| This can be viewed as a series combination, in the primary •• circuit, of resistance RL and inductive reactance X_ or

Zp = RL + jXp (3.12b) |

The series circuit in Figure 3.3(c) is therefore fully- equivalent to the parallel one of Figure 3.3(b). Rp can be t considered as coil wire and cable resistance while K. Zp=RL+jXp is the total impedance of the probe/sample combination. •

When the probe is far from the sample (probe in air), Rs is very large and by substituting Rs " «> into equation 3.12a _ results in •

RL=0, Xp=Xo and Zp-Xo I I I I I -27- The above results can be obtained by removing component NpRs from Figure 3.3(b), since Rs= <=° implies an open circuit.

One last transformation in the equation is required before impedance graphs can be obtained. Equation 3.12(a) can be simplified by setting

x G Co " o 2 where G • l/NpRg Is equivalent circuit conductance. Substitution in 3.12(a) yields

1 + C 1 +C_

Normalizing with respect to Xo, the coil's inductive reactance when far removed from the sample (coil in air) results in Z _ (3.13) X ,2 1 + C

By varying CQ) in equation 3.13, from 0 to infinity the impedance curve of Figure 3.4 is obtained. The impedance locus is that of a semi-circle with center at Xp/X0=£ and RL/XO » 0; its radius is h. With the help of equation 3.13 and Figure 3.A, impedance changes can be related to changes in the sample characteristics. C,

P (OPERATING POINT)

NORMALIZED RESISTANCE

Fig. 3.4: Impedance Graph Display I -28- I 3.5.2 Correlation Between Coil Impedance and Sample Properties The effect of test parameter variations on probe impedance can be derived from equation 3.13. Each parameter is substituted in turn into Co"X0/NpRs; if an increase in I the parameter results in an increase in Co, the operating • point (position on impedance diagram) moves DOWN the impedance curve, if Co decreases, the operating point moves UP the • impedance curve. These correlations are useful in obtaining a gj qualitative appreciation of the effect of the various test parameters. It is also useful to know that probe/sample _ effects can be derived from the simple equivalent parallel • circuit where the sample is treated as a resistor in parallel ' with an inductor (coil). The complete effect can then be obtained by adding the effect of 'phase lag', which will be H treated in later chapters. I Study of equation 3.13 reveals the following: m

1. An increase in Rs results in a decrease in CQ. Therefore an increase in resistance to eddy current flow moves the operating point, P, UP the impedance curve I (along the semi-circle), see Figure 3.5(a). 9 2. R vs where, p is electrical resistivity, I is eddy current I flow distance and A is cross-sectional area to current flow. I Therefore, p » constant x Rs An increase in electrical resistivity will move the W operating point UP the impedance curve. The opposite is true for an increase in electrical conductivity. See • Figure 3.5(a). | For thin wall tubes or plates of thickness t, _

Rs = p£/A » pirV/tw ™ and for constant probe or tube diameter, D, and coil fl width, w, |

Rs = constant/t m An increase in tube wall (or plate thickness) will move the operating point DOWN the impedance curve, see Figure 3.5(b). • I -29-

4. Co = o)L0/NpRs = constant x w

for constant sample properties.

An Increase in test frequency will move the operating point DOWN the impedance curve, see Figure 3.5(c).

5. LQ = constant x D'j probe inductance increases proportional to probe or tube diameter squared.

Also Rs = ptTD/tw » constant x D, for constant thickness, t, and coil width, w. Substituting LQ and Rs into Co -

6. In the equivalent circuit derivation perfect coupling was assumed for sake of simplification. However, it can be shown that when mutual coupling between coil and sample is decreased, the impedance point traces smaller semi-circles as Co increases from 0 to infinity, see Figure 3.5(e).

O.S R, /v_ 0

IRFftCE *„ PROBE Tr-

DECREASING FILL FACTOR OR INCREASING LIFT-OFF

0 0.5 R. -v 01 0.5 R.

Fig. 3.5: Simplified Impedance Diagrams -30- i I 3.6 SUMMARY The impedance method of eddy current testing consists of monitoring the voltage drop across a test coil. The I impedance has resistive and inductive components; the impedance magnitude is calculated from the equation l 2 |z| = */R/R2 ++ (wL)2 , ohms (3.6)

and the impedance phase is calculated from I

= Arctan ^ , degrees (3.7) I The voltage across the test coil is V = IZ where I is the current through the coil and Z is the impedance. l A sample's resistance to the flow of eddy currents is reflected as a resistive load and is equivalent to a resistance in parallel to the coil inductive reactance. This I load results in a resistive and inductive impedance change in the test coil. Coil impedance can be displayed on normalized impedance diagrams. These are two-dimensional plots with the I inductive reactance displayed on the vertical axis and resistance on the horizontal axis as in Figure 3.6. I l l

NORMALIZED OPERATING POINT \ I INDUCTANCE REACTANCE \ I -, t, f ,D l A I NORMALIZED RESISTANCE, I

Fig. 3.6: Impedance Graph Display l l I -31-

Wlth this display we can analyze the effect of sample and test parameters on coll Impedance. The equivalent circuit derivation of coil impedance is useful for a qualitative understanding of the effect of various test parameters. It is valid only for non-ferromagnetic material and for the condition of no skin depth attenuation or phase lag across the sample. (Ferromagnetic materials will be covered in Section 9.4).

Note that all test parameters result in a semicircle display as they increase or decrease. A resistance increase to the eddy current flow or increase of sample's electrical resistivity moves the operating point UP the impedance diagram, i.e., increase in coil inductance and a change in coil resistance.

An increase in a sample's electrical conductivity, thickness or tube diameter, moves the operating point DOWN the impedance curve. An increase in test frequency or probe diameter also moves the operating point DOWN the impedance curve. Although not shown in the above figure, a decrease in fill-factor or increase in lift-off results in a decrease in semicircle radius and a smaller change in coil impedance.

In some test requirements it is advantageous to operate at specific locations on the impedance diagram. By choosing the appropriate test parameters this is usually possible. 3.7 WORKED EXAMPLES 3.7.1 Probe Impedance in Air PROBLEM: An eddy current test is carried out at a test frequency of 50 kHz. Coil resistance is 15 ohms while its inductance is 60 microhenries. a) What is the inductive reactance of the test coil? b) What is the impedance of the test coil? c) What is the angle, 9 , between the total impedance vector and the resistance vector?

SOLUTION:

3 6 a) XL - 2 TTfL - (2 ir ) (50 x 10 ) (60 x 10" ) Xi * 18.8 ohms

b) zl » \R^+ (2 irfL)2 (18.8)2 - 24.1 ohms

2irfL 18.8 c) e » Arctan R = Arctan 15 = Arctan 1.253 e - 51.4 degrees I -32- I 3.7.2 Probe Impedance Adjacent to Sample PROBLEM: An eddy current test is carried out on brass using I a surface probe at 50 kHz. Coil resistance in air is 15 ohms and its inductance in air is 60 microhenries. Probe impedance with the probe on I the brass sample is measured as Zp = 2A.5 ohms and 6 " 35 degrees. I Calculate: a) Xp, inductive reactance and b) RL, resistive load SOLUTION: a) x = Z sin6 I P P = 24.5 sin 35° = 14.1 ohms I b) R = Z cosG - R,,. L p DL = 24.5 cos 35° - 15.0 = 5.1 ohms I 3.7.3 Voltage - Current Relationship PROBLEM: For the above probe impedance problem calculate I voltage drop across the probe if test current is 100 milliamperes.

SOLUTION; Probe impedance |Z| = 2 4.5 ohms I Ohm's Law states that V - I |z| I therefore, V = (0.10) (24.5) • 2.45 volts. Voltage across the probe is 2.45 volts. I I I I I I I I -33- CHAPTER 4 - INSTRUMENTATION

4.1 INTRODUCTION All the information about a test part is transmitted to the test coil through the magnetic field surrounding it. The impedance eddy current method monitors voltage drop across z the primary coil, Vp - Ip p» as coil impedance changes so will the voltage across the coil if current remains reasonably constant. The send-receive eddy current method monitors voltage developed across a sensing coil (or Hall effect detector) placed close to the excitation coil, see Figure 2.2.

In most inspections, probe impedance (or voltage) changes only slightly as the probe passes a defect, typically less than 1%. This small change is difficult to detect by measuring absolute impedance or voltage. Special instruments have been developed incorporating various methods of detecting and amplifying small impedance changes.

The main functions of an eddy current instrument are illustrated in the block diagram of Figure 4.1. A sine wave

PlHASE D.C. METER SENSITIVE AC TO DC PHASE AMPLIFIER CONVERTOR (PLUS ROTATION FILTERING) ICZ.-Z,)

X-Y MONITOR

OSCILLATOR TRANSFORMER

Fig. 4.1: Block Diagram of Eddy Current Instrument -34-

oscillator generates sinusoidal current, at a specified frequency, that passes through the test coils. Since the impedance o£ two coils is never exactly equal, balancing is required to eliminate the voltage difference between them. Most eddy current instruments achieve this through an AC bridge or by subtracting a voltage equal to the unbalance voltage. In general they can tolerate an impedance mismatch of 5%. Once balanced, the presence of a defect in the vicinity of one coil creates a small unbalance signal which is then amplified.

Since the sinusoidal unbalance voltage signal is too difficult and inefficient to analyze, it is converted to a direct current (DC) signal retaining the amplitude and phase characteristics of the AC signal. This is normally achieved by resolving the AC signal into quadrature components and then rectifying them while retaining the approximate polarity. In general purpose instruments, these signals are normally displayed on X-Y monitors. Simpler instruments, such as crack detectors, however, have a meter to display only the change in voltage amplitude. To decrease electrical instrument noise, filtering is used at the signal output; however, this decreases the frequency response and thereby restricts the inspection speed.

The most troublesome parameter in eddy current testing is lift-off (probe-to-specimen spacing). A small change in lift-off creates a large output signal. The various methods used to decrease this effect are discussed in the individual sections on specific eddy current instruments.

4.2 BRIDGE CIRCUITS Most eddy current instruments use an AC bridge to sense the slight impedance changes between the coils or between a single coil and reference impedance. In this section the important characteristics of bridge balance are discussed. 4.2.1 Simple Bridge Circuit

A common bridge circuit is shown in general form in Figure 4.2, the arms being indicated as impedance of unspecified sorts. The detector is represented by a voltmeter. Balance is secured by adjustments of one or more of the bridge arms. Balance is indicated by zero response of the detector, which means that points A and C are at the same potential (have the same instantaneous voltage). Current will flow through the detector (voltmeter) If points A and C on the.- bridge arms are at different voltage levels (there is a difference in voltage drop from B to A and B to C). Current may flow in either direction, depending on whether A or C is at higher potential. I I -35-

F1g. 4.2: Common Bridge Circuit

If the bridge is made up of four Impedance arms, having inductive reactance and resistive components, the voltage from B to A must equal the voltage from B to C in both amplitude and phase for the bridge to be balanced.

At balance,

i2z2

and from which the following relationship is obtained:

(4.1)

Equation 4.1 states that the ratio of impedances of one pair of adjacent arms must equal the ratio of impedances of the other pair of adjacent arms for bridge balance. If the bridge was made up of four resistance arms, bridge balance would occur if the magnitude of the resistors satisfies equation 4.1 (with Zj replaced with Rj, etc). However, if the impedance components are eddy current probes consisting of both inductive reactance and resistance, the magnitude and phase of the impedance vectors must satisfy equation 4.1. I -36- I In practice, this implies the ratio of inductive reactance of one pair of adjacent arms must equal the ratio of inductive reactance of the other pair of adjacent arms; the same being I true for the resistive component of impedance. Figure 4.2 and equation 4.1 can be used to illustrate the I characteristic 'figure 8' signal of a differential probe. If Zi Z3 •=-=• > -if—, point C is at a higher potential than point A. I c* p "A This implies that when Zj^ increases (i.e., coil moving across a defect) with Z2, Z3 & Z4 constant, the bridge voltage unbalance increases,and the opposite happens when I Z3 increases. It is this bridge unbalance characteristic that results in a plus-minus or 'figure 8' signal as the differential probe moves across a localized defect. This I signal occurs independent of whether the two colls are wound in opposition or in addition. 4.2.2 Typical Bridge Circuit in Eddy Current Instruments I Figure 4.3 illustrates a typical AC bridge used in eddy current instruments. It is similar to the bridge in Figure 4.2 except for two additional arms. In this bridge the probe I coils are placed in parallel with variable resistors. The balancing, or matching of voltage vector phase and amplitude, is achieved by varying these resistors until a null is achieved. Potentiometer Rj balances the reactive component I of the coils to make the phase angle of each coil circuit equal. Potentiometer R^ balances the resultant voltage with an equal voltage amplitude to null the instantaneous I voltage between Rj and R2« I

TEST COIL I I REFERENCE I COIL I Fig. 4.3; Typical Bridge Circuit Used in Eddy Current Instruments I I 7 I -37-

The Inductive voltage drop across each coil is equalized by controlling the current passing through the coils. This is done by varying potentiometer R.2« However, when the test coil inductance differs significantly from reference coil inductance, potentiometer R2 will have to be rotated to one extremity. This means less current passes through one coil making it less sensitive than the other coil. When this occurs, a distorted (unsymmetrical) signal results if a differential probe is used. In addition, the common cable lead carries the unbalanced current, resulting in cable noise, especially if the cable is not properly shielded and grounded.

In the Figure 4.3 circuit, the output voltage for large (>10%) off-null (off-balance) conditions is a nonlinear function of the change in coil impedance. However, for defect detection, close to balance, this discrepancy is small. 4.2.3 Bridge Circuit in Crack Detectors

Portable eddy current instruments are often used to inspect for surface defects. A typical crack detector circuit is shown in Figure 4.4. An oscillator supplies an alternating current to an AC Bridge, containing a single eddy current probe coil as one arm of the bridge. A capacitor is connected in parallel with the coil so the L-C (inductance- capacitance) circuit is near resonance. When the coil is placed on a test sample, the bridge is unbalanced and the pointer on the meter swings off-scale. The bridge can be balanced by adjusting potentiometer Rj.

Fig. 4.4: Simplified Circuit of Crack Detector I -38- I 4.3 RESONANCE CIRCUIT AND EQUATIONS

Probe-cable resonance must be considered when operating at I high test frequencies and/or using long probe cables. In addition, crack detectors are purposely designed to operate close to resonance. This section contains basic information 1 about resonant (tuned) circuits. If a capacitor is connected in parallel with the test coil I (inductor), there is a unique frequency at which the inductance-capacitance (L-C) circuit resonates. At this frequency the circuit is said to be tuned. Under this condition the output voltage, for a given measurement, is 1 maximum. A capacitor in parallel with the eddy current probe converts the circuit of Figure 3.3(c) to that of Figure 4.5. I I I I I Fig. 4.5: Parallel LC Circuit I I I I I I T I -39- r At resonance,

2 RXK X (4.2) R2 +(X -X )2 P c hence Z - «> when R - 0

If resistance, R, is negligible compared to Xp and Xc resonance occurs when

X = X or wL = 1/toC (4.3a) P c

or a) = 1/v^LC (4.3b)

Since a) = 2irf, resonant frequency is

f - i— (4.4a) 2irAc where L is coil inductance in henries and C is cable capacitance in farads. When resistance, R, is significant,

!L fr — - -3- <*•**)

X where Q = r-E- , quality factor. R

The resonant frequency of a practical parallel resonant circuit (R ^ 0) is the frequency at which the reactive power of the inductance and capacitance are equal, or the total impedance appears as pure resistance. i -40- I 4.4 EDDY CURRENT INSTRUMENTS General instrument functions were described using the block i diagram of Figure 4.1. In this section specific instruments are covered. It answers the questions: What is the test frequency? How is lift-off compensated for? How is 1 balancing achieved? What type of outputs do they have? 4.4.1 General Purpose Instrument (Impedance Method) I Figure 4.6 shows a typical eddy current instrument with various control functions. FREQUENCY control sets the desired test frequency. Frequency is selected by continuous 1 control or in discrete steps from about 1 kHz to 2 MHz. The coils' impedances are normally balanced using an AC bridge circuit. These bridges require two coils on adjacent bridge arms such as arms No. 2 and No. 4 in Figure 4.3. Coil I impedance must be compatible with internal bridge impedance. I FERR1TE CARBON STEEL I MONEL I S.S, TYPE 304 LEAD 1 BRASS ALUMINIUM COPPER STORAGE MONITOR BALANCING I I OUTPUT O I I Fig. 4.6: Typical Eddy Current Instrument With Storage Monitor I Most bridges can tolerate a coil impedance between 10 and 200 ohms. The BALANCING controls, labelled X and R in some instruments, are potentiometers R^ and R£ in Figure 4.3. I They match coil impedance to achieve a null when the probe is in a defect free location on the test sample. Some instruments have automatic balancing. I 1 -41-

The bridge output signal amplitude is controlled by the GAIN control. In some instruments it is labelled as SENSITIVITY. It controls the amplifier of the bridge output signal, shown in Figure 4.1,and therefore does not affect current g Lng through the probe. However, some instruments control amplification by varying current through the coils. This is undesirable because it could cause coil heating, and when testing ferromagnetic materials the magnetization level changes, resulting in signal distortion and non-repaatable signals.

Following amplification of the bridge unbalance signal, the signal is converted to direct current signals. Since the AC signal has both amplitude and phase it is converted into QUADRATURE X and Y components. The quadrature components of the bridge output are generated' by sampling the sinusoidal signal at two positions 90° apart (one-quarter wave) on the waveform (or by using electronic multipliers). The DC voltage values (amplitudes) constitute the X and Y quadrature components. If phase is taken relative to the resistive voltage component, then the X quadrature component is Rj^ (or Vg) and the Y component, X^ (or V^Jjin equation 3.12(b) or Figure 3.4. We now have an efficient way of analyzing bridge unbalance signals.

Eddy current instruments do not have a phase reference. To compensate for this, they have a PHASE SHIFT control (phase- discrimination control). Normal impedance diagram orientation with inductive reactance displayed vertically (+Y) and resistive horizontally (+X) can be obtained experimentally. This is achieved by adjusting the PHASE control until the signal from a probe approaching a ferrite sample (high yand very high P) displays a vertical (+Y) signal indicating an increase in probe inductive reactance, see Section 5.5.6 for examples. PHASE) control can also be used to minimize the effect of extraneous signals such as lift-off. The X-Y signal pattern is rotated until the lift-off signal is horizontal (X). Thus any vertical (Y) channel signal indicates defects, thickness variations, etc., with little effect from probe wobble.

The output signal is normally filtered internally to decrease instrument or system noise. This decreases frequency response of the instrument and reduces the maximum inspection speed; at faster inspection speeds signal distortion results. Instruments can have a frequency response of 30 to 1000 Hz, although 100 to 300 Hz is most common. At 300 Hz, the maximum attainable tube inspection speed, to detect an abrupt defect without signal distortion, is about 0.25 m/s.

Signals are commonly displayed on X-Y storage monitors wich the X component on the horizontal axis and the Y component on the vertical axis. The writing speed or frequency responsu is greater than 1 kHz for a storage CRT. -42-

Analysis of recorded signals is normally done visually. The storage monitor display in Figure 4.6 shows the change in coil impedance as a surface probe was placed en various test samples illustrating the effects of resistivity, permeability and lift-off. In the "impedance" method of eddy current testing, the flow of eddy currents is monitored by observing the effect of their associated electromagnetic fields on the electrical impedance of the inspection coil(s). This impedance includes coil wire and cable resistance, Z " V(RL + R

Coil wire and cable resistance increase linearly with temperature according to

R = R0(14oiAT)

where a is temperature coefficient of resistance and AT is change in temperature. If the probe and/or cable experience a change in temperature during inspection, the output signal from the eddy current instrument changes; this is normally referred to as temperature drift. 4.4.2 Crack Detectors A typical crack detector circuit was shown in Figure 4.4. I Crack detector probes contain only one coil, with a fixed value capacitor in parallel with the coil to form a resonant m circuit. At this condition the output voltage, for a given I change in coil impedance, Is maximum. The coil's inductive reactance, X^, must be close to the capacitive reactance, ^ Xc. In most crack detectors this is in the range of 20 to M 100 ohms. • Crack detectors that operate at or close to resonance do not flj have selectable test frequencies. Crack detectors for | non-ferromagnetic, high electrical resistivity materials such as Type 304 stainless steel typically operate between 1 and 3 £ MHz: those for low resistivity materials (aluminum alloys, I brasses) operate at lower frequency, normally in the 10 to 100 kHz range. Some crack detectors for high resistivity materials can also be used to inspect ferromagnetic I materials, such as carbon steel, for surface defects. W Normally a different probe is required; however, coil impedance and test frequency change very little. • -43-

PROBE WITH LIFT-OFF=O.l mm

PROBE WITH LIFT-OFF =0 mm METER

OUTPUT

SAMPLE WITH DEFECT

0.8 0.9 1.0 1.1 1.2

OSCILLATOR FREQUENCY, _L f

Fig. A.7: Meter Output with Varying Oscillator Frequency Crack detectors have a meter output and three basic controls: balance, lift-off, and sensitivity. BALANCING control is performed by adjusting the potentiometer on the adjacent bridge arm, until bridge output is zero (or close to zero). GAIN control (sensitivity) adjustment occurs at the bridge output. The signal is then rectified and displayed on a METER. Because the signal is filtered, in addition to the mechanical inertia of the pointer, the frequency response of a meter is very low (less than 10 Hz). LIFT-OFF CONTROL adjusts the test frequency (by less than 25£) to operate slightly off resonance. In crack detectors the test frequency is chosen to minimize the effect of probe wobble (lift-off), not to change the skin depth or phase lag. The set-up to compensate for probe wobble can be described with the help of Figure 4.7. Frequency is adjusted by trial-and-error to obtain the same output signal on the meter with the probe touching the sample and at some specified lift-off (normally 0.1 mm). At this frequency a deep surface defect will give a different reading on the meter, ar- shown in Figure 4.7.

However,the meter output is a complex function of signal phase and amplitude, and cannot be used to reliably measure depth of real defects. Nor can they be used to distinguish between real and false indications such as ferromagnetic inclusions. -44- 1 4.4.3 Material Sorting and Conductivity Instruments Material sorting,or conductivity instruments,have a I precalibrated meter output and have a unique way of compensating for lift-off. Instruments for sorting of high resistivity materials (Type 304 stainless steel) use a fixed, I high test frequency, normally between 200 and 500 kHz,and those for low resistivity materials (aluminum alloys), a low test frequency, between 20 and 100 kHz. They incorporate AC bridges and normally have two coils (one as reference). Coil impedance I is in the range of 20 to 100 ohms. They either have bridge balancing or a zeiroing control, to keep the signal on scale. GAIN CONTROL or sensitivity adjustment occurs at the bridge I output. The signal is then rectified and displayed on a METER. LIFT-OFF compensation is normally pre-set. Figure 4.8 I explains how the probe-wobble (lift-off) signal is eliminated. The bridge is purposely unbalanced (by pre-set internal adjustment)* such that the unbalance point, P, is at the centre of curvature of the lift-off impedance locus, AB. I The instrument meter reads a voltage proportional to the distance, PB' or PA1, from the chosen unbalance point to the impedance curves. The amplitude of this voltage remains I constant with probe wobble but changes significantly for wall thickness (and resistivity) variations. In fact any signal that traces an impedance locus different from lift-off will change meter output. I

PRESET UNBALANCE I

METER READING, / I

AIR WALL THICKNESS / / 1 VN — t = 0 .5 mm JFAV. INDUCTIVE OFF\ REACTANCE V^ y — t * 1 ram I I I RESISTIVITY I 0 RESISTANCE Fig. 4.8: Unbalanced Bridge Method Showing Selection I of Operating Point

*This is achieved by subtracting a signal equal to OP from the I signal 0A. I I I -45- With this type of instrument only the magnitude of the impedance change is measured. This instrument is effective for conductivity and wall thickness measurement (and deep defects) and is Simple to operate. It has only two basic controls: balance and sensitivity. 4.5 SEND-RECEIVE EDDY CURRENT SYSTEMS

The "send-receive" eddy current method eliminates the temperature drift sensed by general purpose instruments. The flow of eddy currents is monitored by observing the effect of their associated electromagnetic fields on the voltage induced in an independent coil(s), r"gure 4.9. The excitation or primary coil is driven with sinusoidal current with constant peak-to-peak anp1 .ude to obtain a constant magnetomotive force,

N I sin cot (2.3) P P

RECEIVE CO I LS

777777Z77, TEST ARTICLE

'. 4.9: Send-Receive Circuit -46-

This makes the excitation magnetic flux independent of primary coll resistance* The secondary or receive coil(s) is connected to a high input impedance amplifier, hence the induced voltage Vs is not affected by receive coil resistance.

COS wt (2.5) P It

The wire resistance of both the excitation and receive coils can change, because of temperature, without affecting the output signals; temperature drift has thus been eliminated. Temperature independence makes this method useful for measuring resistivity, wall thickness and spacing between components. It has no significant advantage over the impedance method for defect detection, except in the through-wall transmission system discussed in Section 5.A. 4.5.1 Hall-Effect Detector

Most send-receive circuits consist of one excitation (or driver) coil and one or more receive (or pick-up) coils.

However, the induced magnetic flux can be measured with a Hall-effect detector rather than by monitoring the induced voltage Vs across a pick-up coil, see Figures 2.2b and 2.2c.

I I I Fig. 4.10: Hall Detector Circuit I 1 I I -47- I Th e induced voltage in a pick-up coil is proportional to the time rate of change of the magnetic flux and therefore is I proportional to the test frequency, V cc f r Pick-up The Hall detector instead responds to the instantaneous i magnitude of the magnetic flux, 4>o • This means the output voltage is independent of test frequency, making it useful for low frequency inspection (especially if the detector has to be small).

The Hall detector works as follows: When direct current is passed through a Hall element, voltage (electric potential) is produced, perpendicular to current flow, see Figure 4.10. This voltage is proportional to the component of magnetic flux perpendicular to the element and the element surface area. This voltage is NOT from a change in element resistance. Hall elements as small as 1 mm square are commercially available.

4.5.2 Send-Receive Coils and Lift-Off Compensation General purpose "send-receive" instruments are similar to "impedance" instruments, as described in Section 4.4.1. The main difference is the method of balancing because of the different coil configuration. Most send-receive circuits consist of one excitation coil and two receive coils positioned symmetrically inside or outside the excitation coil. They can either be differential where both coils sense the test specimen or absolute where only one coil senses the test specimen, as shown in Figure 4.9. Although coil impedance is not important in send-receive instruments, the induced voltage is a function of number of windings and test frequency. Therefore their inductive reactance tend to be similar to coils used in impedance instruments.

The sensing coils are wound in opposition so the excitation field induces no net voltage in the receive coils when they both sense the same material. In the presence of a defect, the voltage changes as each coil moves over it. Figure 4.9 illustrates a surface reflection type probe where both excitation and pick-up coils are on the same side of the test sample. However, the excitation coil and pick-up coils can be placed on opposite sides of the sample; this method is referred to as through-wall transmission. The two methods are compared in Section 5.4.

The output signals in most send-receive instruments are the quadrature components of the secondary voltage. However, in some special purpose instruments, one output signal is proportional to amplitude and the other to phase of the secondary voltage (relative to primary voltage). They -48-

compensate for LIFT-OFF as follows: if coil-to-sample spacing varies there is a large change in amplitude of the secondary voltage but little change in phase. The phase shift between the secondary and primary sinusoidal voltages is measured at a voltage level Vo slightly larger than zero, Figure 4.11. At this voltage the sinusoidal voltages have the same phase shift for zero lift-off as for maximum (perhaps 0.1 mm) lift-off. The voltage discriminator in these phase-shift measuring eddy current instruments trigger on the Vo voltage pointfand thereforetthe output signal for lift-off between 0 and 0.1 mm is minimized. Measurement of resistivity, wall thickness or deep defects can be made without lift-off noise.

V(t) PROBE SIGNAL. LIFT- OFF =0

PROBE SIGNAL, LI FT - OFF = 0.i mm

PROBE SIGNAL. DEFECT IN TEST ARTICLE

Fig. 4.11: Secondary Voltage Waveform for Various Test Conditions

4.6 MULTIFREQOENCY EQUIPMENT The eddy current NDT method is sensitive to many test parameters, making it very versatile. However, one is usually only interested in a single parameter such as defects. Insignificant parameters such as changes in electrical or magnetic properties, the presence of dents or support plates in tube inspection and lift-off in surface probe inspection can mask defect signals. The multifrequency eddy current method was developed to eliminate the effect of undesirable parameters. I - 49 - I The response to various anomalies changes with test frequency. This allows a means of discriminating against I unimportant changes. In multifrequency instruments, two or more frequencies are used simultaneously (through the ame coil(s)). Coil current consists of two or more superimposed I frequencies, i.e., the coil(s) is excited with more than one test frequency simultaneously. A three-frequency multifrequency instrument acts the same way as three separate (single-frequency) eddy current instruments. Band-pass I filters separate the signals at each frequency. The discrimination or elimination process is accomplished by combining the output signals (DC signals) from individual frequencies in a manner similar to simultaneous solution of multiple equations. The elimination of extraneous signals is achieved by matching the signal at two test frequencies and subtracting. This process is continued for other unwanted signals using other test frequencies until the final output consists of only the defect signal. A discussion of inspection results with multi-frequency is covered in Section 8.4.

Multifrequency instruments have the same controls and functions as general purpose "impedance" type instruments, described in Section 4.4.1, with the addition of mixing modules. These modules are used to combine or subtract the output signals from each combination of frequencies.

4.7 PULSED EDDY CURRENT EQUIPMENT

Faraday's Law states that eddy currents are induced in a conductor by a varying magnetic field. This magnetic field can be generated by passing sinusoidally varying current through a coil. However, the current can be of other waveforms such as a train of pulses. This method works only on the send-receive principle where the flow of eddy currents is monitored by observing the effect of their associated electromagnetic fields on the induced voltage of the receive coil(s). The voltage pulse is analyzed by observing its amplitude with time, Figure 4.12.

To compensate for LIFT-OFF, the voltage is sampled at a preset time, tj. When the waveform is triggered (measured) at time t^, the voltage for zero lift-off and maximum lift-off is the same, whereas the voltage waveform in the presence of a defect is different. This method is quite similar to the send-receive method described in Section 4.5.3. Therefore, by measuring the voltage at the appropriate crossing point, lift-off effects can be drastically decreased. -50-

V(t) OEFECT IN TEST ARTICLE

LIFT - 0FF = 0.1 mm

Fig« 4.12; Voltage Across a Pulsed Eddy Current Pick-Up Coil as a Function of Time The pulsed eddy current method offers another advantage. The pulsed driving current produces an inherently wideband frequency spectrum, permitting extraction of more selective information than can be determined from the test specimen by a single frequency method. Unfortunately, there is at present no commercially available instrument that operates on this principle.

4.8 SPECIAL TECHNIQUES

Two old methods used to measure large coil impedance variations (greater than 5%) are the ELLIPSE and SLIT methods. These methods analyse the AC signal directly on an oscilloscope (without converting it to DC). They were mainly used for material sorting. They are obsolete methods and a detailed description is not warranted in this manual; a full description is contained in Reference 5. Another technique, MODULATION ANALYSIS, is also described in Reference 5. It works on the same principle as "frequency spectrum analysis" where a discrete frequency component of a waveform can be analysed without interference from lower or higher frequency noise. The inspection must be performed at constant speed (In fact it only works if there is relative motion between coil and sample). It is used in production- line testing at speeds up to 2 m/s or higher. It is a very specialized and complicated method and a detailed description is not warranted in this manual. -51-

4.9 RECORDING EQUIPMENT

During inspection, eddy current instruments and recording equipment are typically connected as in Figure 4.13. The eddy current signal is monitored on a storage CRT (cathode ray tube) and recorded on X-Y and two-channel recorders. Recording on an FM tape recorder for subsequent playback is also common. The important characteristic of these recording instruments is FREQUENCY RESPONSE, or speed response, which limits inspection speed. Section 4.4.1 indicated general eddy current instruments have a frequency response of 100 to 300 Hz, limiting the inspection speed to 0.25 m/s. To be compatible, recording instruments must have the same or higher frequency response.

X-Y STORAGE MONITOR o EDDY CURRENT INSTRUMENT X? ?Y PROBE

o Tl 6 X i v 2-CHANNEL FM TAPE CHART RECORDER RECORDER

Fig. 4.13: Block Diagram of Eddy Current Monitoring Equipment -52-

X-Y Recorders Signal analysis for signal discrimination and defect depth estimation is normally done on X-Y signal patterns. The CRT storage monitors have a frequency response of at least 1 kHz and therefore do not restrict maximum inspection speed. However, to obtain a permanent visual record of the signal, it must be recorded on X-Y recorders. The fastest recorders have a speed of response of 8 Hz for small signals. This drastically limits inspection speed if used on-line. It is therefore only used in the laboratory or to record playback from tape recorders (this is done by recording at the highest tape speed and playing back at the lowest, a factor of 8:1 for most tape recorder). One solution to on-line recording of X-Y signals is to photograph the CRT display; however, this is not practical for recording many signals.

Another solution is to use storage monitors with hard copy (paper output) capability. These exist commercially but require custom-made control units. They have a frequency response of 1 kHz or higher.

Strip Chart Recorders Recording X and Y signal components against time is useful in locating defects and determining their length. Common two channel ink-pen strip chart recorders have a speed response of approximately 100 Hz. At maximum inspection speed (0.2S m/s) the recorded signal will decrease in amplitude and be slightly distorted.

Ink-ejection strip chart recorders have a speed response of 1 kHz. These recorders are not readily available in North America and use a lot of paper. Ultraviolet light recorders have a speed response higher than 1 kHz, but require special paper. These recorders are rarely used in eddy current testing.

FM Tape Recorders Tape recorders allow storage of eddy current signals (on magnetic tape) for subsequent retrieval. They have a frequency response proportional to recording speed. The lowest recording speed is 24 mm/sec (15/16 ips)' giving a frequency response of 300 Hz, and the fastest,, 380 mm/s (15 ips), will respond to 4.8 kHz. I I -53- 4.9.1 Frequency Response Eddy current instruments and recording instrumentation have limited frequency response. This means they require finite time to respond to an input signal. ' Frequency respo.de, sometimes called speed of response, is defined as the frequency at which the output signal falls to 0.707 (-3 dB) of the maximum input signal.

A test coil with an effective sensing width w passing over a localized defect at a speed s will sense the point defect for a duration of w/s seconds. This signal is approximately equal to one wavelength with a frequency f • s/w hertz (4.6)

where s is speed in mm/s and w is width in mm. For example, at a probe speed of 0.5 m/s and probe sensing width of 2 mm, f = 250 hertz. If the instrumentation has a frequency response of 250 hertz, the output signal is reduced to 0.707 the input signal and the X-Y signal is distorted. If the instrumentation frequency response is 500 hertz, the output signal decreases only slightly. For this example, the eddy current instrument should have a frequency response equal to or greater than 500 hertz to obtain undistorted signals. Or inversely, if the instrument frequency response is only 250 hertz, the maximum inspection speed should be reduced to 0.25 m/s

4.10 SUMMARY Basic eddy current equipment consists of an alternating current source (oscillator), voltmeter and probe. When the probe is brought close to a conductor or moved past a defect, the voltage across the coil changes and this is read off the voltmeter. The oscillator sets the test frequency and the probe governs coupling and sensitivity to defects.

For effective purchase or use of an eddy current instrument, the following information is needed: (a) type of instrument: impedance, send-receive, crack detector, etc. (b) type of outputs: single (meter) or quadrature (X-Y) component outputs (c) test frequency (d) type of lift-off compensation. Most eddy current instruments use an AC bridge for balancing but use various methods for lift-off compensation. Send-receive instrument should be used for accurate absolute measurements in the presence of temperature fluctuations. Multifrequency instruments can be used to simplify defect signals in the presence of extraneous signals. 1 -54-

Eddy current Instruments and recording equipment have a finite frequency response limiting the inspection speed to i normally 0.25 m/s. Most Instruments tolerate probe impedance between 10 and 200 I ohms.

Crack detectors operate close to coil-cable resonance. The resonant test frequency is given by ! 1/2TT./LC (4.Aa) I where L is coil inductance in henries and C is cable capacitance in farads. The lift-off signal is minimized by adjusting the frequency (slightly off resonance) until zero and a small probe lift-off gives zero output signal. High test I frequencies are normally used to inspect for shallow defects in high resistivity or ferromagnetic materials. Low test frequencies are used for detecting deep defects or inspecting i good conductors. Crack detectors have a meter output, and cannot be used to reliably measure defect depth. I 4.11 WORKED EXAMPLES 4.11.1 Impedance at Resonance i PROBLEM: In a parallel L-C circuit, inductance is 80 x 10 henries, capacitance is 5 x 10~9 farads and resistance is negligible. Calculate (a) resonant I frequency, (b) inductive reactance and (c) capacitive reactance. i SOLUTION:

(a) f (4.4a) t

252 kHz I 27T x10"6)(5 x10~9) (3.4b) (b) Inductive Reactance, XL = 2irfL t 2TT x 252 x 103 x 80 x 10~6 126.5 ohms I (c) Capacitive Reactance, = l/2irfC (3.5) I 1 x = 126.5 ohms c - 3 9 2TT x 252 x 10 x 5 x 10" n -55-

CHAPTER 5 - TESTING WITH SURFACE PROBES

5.1 INTRODUCTION The goal of this chapter Is to present a practical approach to eddy current inspections using surface probes. The emphasis is on test variables such as test frequency, probe size and type; these are normally the only variables an inspector has at his control. These selections are usually determined by skin depth considerations, defect size, and probe size.

Impedance graphs and the Characteristic Parameter are included because they are tools that an inspector should not be without. A thorough understanding of impedance graphs is essential to manipulate test conditions to minimize and/or to •cope with undesirable test variables. Erroneous conclusions are often made by persons who do not have a working knowledge of impedance graphs. The scope of the approach to an eddy current inspection can be very broad; a successful outcome usually depends on the proper approach. When planning an inspection the first questions that must be answered before proceeding are; For what type of defects is the inspection being conducted? If the expected defects are cracks, how big are they? Do they have directional properties? What is the minimum acceptable defect size? Does the material have ferromagnetic properties? Other variables will, of course, influence the test but these questions must be answered in order to select an appropriate probe size and test frequency.

5.2 SURFACE PROBES The eddy current probe plays two important roles: it induces eddy currents, and senses the distortion of their flow caused by defects. Sensitivity to defects and other variables in the test article can be affected by probe design. This is achieved by controlling direction of eddy current flow, by controlling the coil's magnetic field, and by selecting an appropriate coil size. The effects of undesirable material variations and/or variations in probe to test article coupling (lift-off) can often be decreased by using multiple coils.

A surface probe, as the name implies, is used for inspecting surfaces, flat or contoured, for defects or material properties. Defects can be either surface or subsurface. (Surface defects are those that break through, or originate at the surface - typically cracks, voids, or inclusions: a subsurface defect does not break the surface and is therefore not visible). -56-

5.2.1 Probe Types

Simple Probes Surface probe designs can vary from a simple, single coil attached to lead wires, to complex arrangements, as shown in Figure 5.1. Most eddy current instruments require two

tis

FERR1TC CORE /, TEST COIt

ZIRCWIUU TEST 1HIIUE

TEST COU ' 21OCONIUM «LU»

Fig. 5.1; Surface Probes

similar coils to satisfy their AC bridge network as discussed in Chapter 4. If only one coil senses the test material, it is an .absolute probe; if both coils sense the test material, it is a differential probe. The simple probe in Figure 5.1(a) is therefore undesirable because a second coil or electrical device with similar impedance will be necessary for bridge nulling. An exception would be in the use of Crack Detectors; these instruments operate with an internal balancing circuit (see Section 4.2.3).

A better arrangement is shown in the pencil probe of Figure 5.1(b). This probe incorporates a second coil (reference) mounted far enough from the test article that it will not be influenced by it. The two coils have the same impedance when the probe is balanced in air, but will change relative to each other when the test coil is coupled to a sample. However, the degree of coupling is usually small because of the inherent small size of pencil probes so the coils still match well enough for most instruments over a reasonable frequency range. The probe shown has ferrite cores; ferrite is used for three reasons: I -57-

1* higher Inductance from a given coil size, 2. small surface area in contact with the material, 3. the coil can be further from the contact surface providing greater wear protection. A further improvement in reference coil arrangement is shown in Figure 5.1(c); it is attached to a disc whose properties are similar to the test material. With this arrangement the relative impedance of the two coils will not be affected by test frequency.

The probe shown in Figure 5.1(d) is a spring loaded type designed to minimize lift-off. The shoe provides a broad area for squarely positioning the probe on a flat surface, while the spring maintains probe contact at constant force.

Figure 5.1(e) shows a probe used for inspecting large diameter tubing. The probe can be rotated and/or moved axially. The design shown incorporates a replaceable wear cap.

Other Probe Designs A multi-coil array as shown in Figure 5.2(a) is useful for inspecting tubes. This type of probe could detect defects

-SURFACE COILS TEST TUBE / .TORROIDAL REFERENCE COIL - PROBE CENTERING DISCS

TEST COILS

(a) DIFFERENTIAL SURFACE PROBE MULTI SURFACE -COIL PROBE

. FERROMAGNETIC CORE

'COILS COMPENSATING COIL

/y/ "^MAGNETIC FIELD SENSING COIL

(b) (d) GAP PROBE LIFT -OFF COMPENSATING PROBE

Fig. 5.2: Special Surface Probes -58-

that would not be detected by a conventional circumferential coil (discussed in Section 7.5). A gap probe, Figure 5.2(b), uses ferromagnetic material to shape the magnetic field. The field is confined by the core causing eddy currents to flow in circular loops perpendicular to the flux lines. A differential configuration is shown in Figure 5.2(c); the two coils are placed side-by-side. Both coils have high sensitivity to localized variations but tend to cancel out the effect of lift-off, gradual material variations, or ambient temperature changes.

A lift-off compensating probe is shown in Figure 5.2(d); this probe combines the signals from two coils to effectively rotate the defect signal relative to the lift-off signal, therefore, even on "rough" surfaces, shallow defects can be detected.

SEND TEST ARTICLE COIL (DRIVER COIL) RECEIVER COIL

PIGK-UP COILS (WOUND OPPOSING EACH OTHER)

ELECTRICAL CONNECTIONS

(c)

5.3: Send-Receive Probes i -59-

Send-Receive Probes Figure 5.3(a) shows a through-transmission probe arrangement. Current flowing in the SEND coil produces a magnetic field, part of which is transmitted through the test article. The field is detected by the RECEIVER coil, inducing a voltage. There will be no signal variation from the receiver coil when a defect-free test article is moved anywhere between the two coils as long as the coil-to-coil spacing remains constant.

Figure 5.3(b) shows a reflection-type probe arrangement. The probe consists of a large send coil which generates a field, and two small receiver coils wound in opposite directions, as mirror images to one another, as shown in Figure 5.3(c). With the probe in air, net output is zero. However, if one end is placed near a test article, the field differs at the two ends, and a net voltage appears across the two coils. 5.2.2 Directional Properties

Eddy currents are closed loops of induced current circulating in a plane perpendicular to the direction of magnetic flux. Their normal direction of travel is parallel to the coil winding and parallel to the surface. See Figure 5.4. Pancake type surface probes are therefore insensitive to poor bonding of coatings and flaws parallel to the surface of a sample.

SURFACE CRACK EDDV CURRENTS LAMINAR CRACK rTEST PLATE EDDY CURRENT FLOWS PARALLEL TO COIL WINDINGS • POOR SENSITIVITY TO LAMINATIONS

ZERO SENSITIVITY L0» SENSITIVITY MAXIMUM SENSITIVITY AT CENTRE OF COIL PARALLEL TO WINDINGS ACROSS WINDINGS

Fig. 5.4: Directional Properties of a Surface Probe -60-

When testing for flaws such as cracks, it is essential that the eddy current flow be at a large angle (preferably perpendicular) to the crack to obtain maximum response. If eddy current flow is parallel to the defect there will be little or no disruption of currents and hence no coil impedance change. When testing for flaws parallel to the surface, such as laminations, a horseshoe shaped probe (a gap probe with a very large gap) has reasonable sensitivity. 5.2.2.1 Sensitivity at Centre of a Coil Probe impedance changes with coil diameter, as will be discussed further in Section 5.5. A simplified derivation of this diameter effect is derived below, for the case of no skin depth attenuation or phase lag and long coils. From Faraday's Law,

V - - N^ s dt

The magnetic flux density, B, is approximately constant across a coil's diameter, hence 4) = BA

- (B)(irr2)

where r is radial distance from centre of probe;

therefore, V = - mr2 g s dt

r ° V a r 2 s

-i r LJ u Ac -61-

Resistance to flow of current is proportional to flow path length and resistivity and inversely proportional to cross- sectional area, Ac,

. 27rrp unit depth x unit width

Since I = V /Z by Ohm's Law s s

and Z = V?JR 7+ (toL) = R , at low test frequency s s and no skin depth effect, therefore, V 2 s r

1 cc r or s

cc - I from Lenz's Law, it follows s 1 nee •. s that * cc r

Therefore, eddy current flow and its associated magnetic flux are proportional to radial distance from the centre of a coil. Hence no current flows in the centre (r = 0) and there is no sensitivity to defects at the centre of a coil. 5.2.3 Probe Inductance The factor governing coupling and induced voltage in test material is the magnetic flux surrounding the coil. The total magnetic flux ( _ ) is proportional to probe inductance (L) and current (I), i.e., <* LI. In most eddy current instruments excitation current is kept reasonably constant (in the milliaiapere range) but probe inductance could vary by a factor of one thousand. The most important aspect of inductance is that probe 'impedance, which is a function of inductance, must be compatible with the instrument and signal cable, 2 2 ^L R + X and 0 = Arctan — Li R

where X^ " 2 TfL. when f is in hertz, L in henries and R is coil wire resistance in ohms. - 62 -

TABLE 5.1 SURFACE COIL IMPEDANCE

Do = 1.6 mm = 3.2 mm D = 6.3 mm D = 12.7mm D = 25.4 .mm 0 0 0

L = 0.27 yH L • 0.54 pH L = 1.1 UH L = 2.1 pH L = 4.3 pH R = 0.2 a R = 0.1J! R = 0.05 fi R = O.02C2 R - o.oin N = 21 40 AWG 34 AWG 28 AWG 22 AWG 16 AWG (0 .080 mm) (0 .16 mm) (0 .32 mm) (0 .64 mm) (1 . 3 mm) L = 1.5 L = 3.0 L = 6.1 L = 12 L = 24 R = 1 R = 0.5 R = 0.3 R = 0.1 R = 0.06 N = 50 43 AWG 37 AWG 31 AWG 25 AWG 19 AWG (0 .056 mm) (0 .11 mm) (0 .23 mm) (0 .45 mm) (0 .91 mm)

L * 5.8 L = 12 L = 23 L = 47 L = 94 R = 4 R = 2 R = 1 R = 0.5 R = 0.3 N = 98 46 AWG 40 AWG 34 AWG 28 AWG 22 AWG (0 .040 mm) (0.080 mm) (0.16 mm) (0 .32 mm) (0 .64 mm)

L = 11 L = 23 L = 45 L = 90 L = 180 R = 9 R = 3 R = 2 R = 0.9 R = 0.5 N = 136 48 AWG 41 AWG 36 AWG 29 AWG 23 AWG (0.031 mm) (0 .071 mm) (0 .13 mm) (0.29 mm) (0 .57 mm)

L = 24 L = 49 L = 97 L = 195 L = 390 R = 17 R = 8 R = 4 R = 2 R = 1 N = 200 49 AWG 43 AWG 37 AWG 31 AWG 25 AWG (0.028 mm) (0.056 mm) (.0.11 mm) (0 .23 mm) (0.45 mm)

->. p

-E = D, =0.2 Dr t -63-

The self-inductance of a long coil (solenoid) can be calculated from the equation

10 2 LQ = 4TT x 10~ yr N A/£ (5.1a)

LQ is self-inductance in henries where Vr is relative permeability of core (normally =1.0) A is coil's planar surface area, millimetres^ I is coil length, millimetres.

This formula is a good approximation for coils of length/diameter ratio greater than 10. For a short coil, end effects will reduce inductance because of lower flux at coil ends. The N^ term remains since N enters in N p (total number of flux linkages) and again since 4>p itself is proportional to N. The following approximate equation can be used to calculate inductance of short coils:

L = 4iry r N2(£n ^- - 2) 10~10 (5.1b) or K.

D +D where -r is mean coil radius =• o; i , mm

and K - 0.112 (2£ + D + D. ) , mm

Most eddy current instruments will operate over a fairly broad range of probe impedance (and probe inductance) without substantial reduction in signal-to-noise ratio and signal amplitude. An instrument input impedance of 100 ohms is typical, although any impedance between 20 a ad 200 ohms is generally acceptable, unless test frequency is too close to probe-cable resonance; see Section 5.9. Exact probe inductance calculations are therefore not essential. To facilitate impedance calculations, Table 5.1 has been prepared. This table lists coil inductance and resistance (with probe away from test material) for various outside diameters and number of coil turns, keeping both the inside diameter and coil length equal to 0.2 times the outside diameter. Wire diameter is chosen to fill available coil cross-sectional space. Using this table and the knowledge that inductance,

L « N2D2 (5.2) where N is number of turns of wire and D is average coil diameter, one can usually make a reasonable estimate of wire size and number of turns required to achieve a particular inductance. NORMALIZED DEFECT SIGNAL AMPLITUDE NORMALIZED DEFECT SfGNAL AMPLITUDE

vx/vx=0

o c3 —• o o c C3 o — "3 ro co •>» OJ a> O ro < cn a> o 00 o II C3 <3 cn II o m cn f cn

Ull U .^ / V C3 o r a S n> / r> cn cn f / / to ro ro CO as / / ID CO f o "•* II -n / —i I ro cn / ro 3 y 3 CJI cn To w m cn / n> CO a / STANC E (mi r

1/ OEFEC T "• " ea ro o r / 3 a / CO / a m CO o cn a m \v cn C r m m ~t vsN i / O y \ \\^

12. 5 m C3 \ ro I t / cn t / ] cn V \ cn / 1 3 cn \ en /

] .ON G ( {_ o o a CW cn cn CJ) DEPTH_^^ ro o . LIF T-OFF- CO u o> *- 3 3 I -65-

I 5.3 PARAMETERS AFFECTING SENSITIVITY TO DEFECTS During eddy current inspection one must be aware of the limitations of the technique and should take maximum advantage of its potential. Although sensitivity to deep ( surface defects is excellent, sensitivity to deep sub-surface defects is very poor. A subsurface defect only 5 mm from the surface is considered very deep for eddy current test f purposes. I There are two factors that contribute to this limitation. The skin depth effect causes eddy currents to attenuate with depth depending on the material properties and test frequency. This effect is normally minor and can be controlled (within limits) by reducing test frequency. The ( predominant effect (rarely mentioned) is the decrease in magnetic flux, and consequently eddy current density, with I depth because of the small diameter of most practical probes. One can increase penetration by increasing probe diameter, but as a consequence sensitivity to short defects decreases. I One could optimize sensitivity if defect length is known; however,the maximum depth of detectability is still very small. Unlike ultrasonic inspection where a defect is detected many transducer diameters away, eddy current testing I is limited to detecting defects at a depth of less than one probe diameter. It is this effect of probe diameter that limits most volumetric eddy current inspection to materials less than 5 mm thick. In following subsections, limitations I There is a decrease in sensitivity to defects as a coil is moveare ddiscusse away frod man dth eempirica surfacel . exampleThis si s presentedcaused by. the decrease 5.3.1 Sensitivitin magnetiyc witfluxh Lift-Ofdensity f witanhd Defecdistanct eDept resultinh g from finite I probe diameter. Figure 5.5(a) shows the extent of this decrease for three probes of different diameters.. Note,for example, the sensitivity of the smallest probe (5 mm diameter) decreases a factor of four when moved about 1 mm F from the surface.

This loss of sensitivity with distance will also apply to i defects in a solid, in addition there will be a decrease due to skin depth attenuation. I Figure 5.5(b) illustrates the decrease in signal amplitude with subsurface defect depth without skin depth attenuation (solid lines) and with skin depth attenuation (dashed lines). With large skin depths (low test frequency) the decrease in subsurface defect sensitivity with depth is similar to the decrease in sensitivity with distance for surface defects shown in Figure 5.5(a). This implies magnetic flux density decreases with distance from the coil in air as in a solid (without skin depth attenuation). 1 -66- I At a typical test frequency, where one skin depth equals • defect depth (6= 2 mm for the dashed lines in Figure 5.5(b)), I a further decrease, by about a factor of three, in signal amplitude at x = 2 mm is attributed to skin depth attenuation. This occurs since at one skin depth eddy I current density is 37% of surface eddy current density. •-•*

The decrease in defect sensitivity with depth in a finite 1 thickness sample, without skin depth attenuation, is { approximately the same as in an infinitely thick sample. However, with skin depth attenuation, defect sensitivity ... decreases less rapidly than the dashed lines in Figure ! 5.5(b); the curve would fall somewhere in between the dashed and solid lines. '] In general, the decrease in defect sensitivity with depth is ..; determined by probe size rather than skin depth attenuation. Since most defects are not much longer than sample thickness, one cannot use probes with coil diameter much larger than [ sample thickness (because of loss in sensitivity with defect length, Figure 5.6). Eddy current testing with surface probe is therefore normally limited to thicknesses less than 5 mm.

5.3.2 Effect of Defect Length

Eddy current flow is limited to the area of the inducing magnetic field which is a function of coil geometry; defect sensitivity is proportional to coil diameter in a surface probe, and to gap width in a horseshoe probe. As a general rule, probe diameter should be equal to or less than the expected defect length. The effect of probe diameter and defect length is shown in Figure 5.6. For example, when defect length equals probe diameter, the signal amplitude ranges from one-third to two-thirds of the amplitude for an infinitely long crack depending on probe diameter and test : f requency.

The sensing area of a probe is the area under the coil plus an extended area due to the magnetic field spread. The effective diameter, Dejf.of a probe is approximately equal to the coil diameter, Dc> plus four skin depths,

At high frequencies the 46 term will be small and the sensing diameter can be assumed to be about equal to coil diameter, but at low test frequencies the magnetic field spread can be significant. In this case it is common to use ferrite cups to contain the field. This results in a concentrated field without affecting depth of penetration. -67-

1008

7 mm PROBE DIAMETER

1.3 mm PROBE OlAHETfcR

s1 MHz = 0.36 mm 8l0OKHz = 1.16 n

10 12 14 22

EDM NOTCH LENGTH, mm

Fig. 5.6: Effect of Defect Length

5.4 COMPARISON BETWEEN SURFACE AND THROUGH-WALL INSPECTION The major limitation of conventional eddy current methods is lack of penetration. Figure 5.7(a) illustrates typical results obtained with the conventional eddy current method, where the probe is placed on one side only of a 4 mm thick, 100 mm diameter tube. Test frequency is 30 kHz and skin depth, 6»1.7 mm. Note the drastic decrease In signal amplitude and the significant phase rotation of the defect signals with depth. A defect has to be long and very deep before it can be picked up from the opposite side of the tube wall. This decrease in sensitivity with depth is due to both finite probe size and skin depth attenuation.

Figure 5.7(b) illustrates typical results obtained with through-wall transmission equipment where excitation and receive coils were located directly opposite each other across the wall. The probes were conventional absolute pancake type surface probes. The output signal appears as a 'figure 8' because the signal was filtered (differentiated). i -68- 1 I 252 FROM SURFACE I I CD, SURFACE 25S 507 75? GROOVE l.D. SURFACE GROOVE *HHI TUBE OF DEFECT SIGNAL, Y COMPONENT I

1 VOLT [ l 1 50% 75% l.D. GROOVE 0.8 ™ DEEP mm mln HOLES. 0.8 D1A, 13 LONG 13 mm LONG I X-Y DISPLAY OF DEFECT SIGNALS (a) Conventional Surface Probe Results I I I I I x25l 50% 75%, 1 O-D. ~V ' i,D. GROOVE HOLES GROOVE 0.8 mm DEEP 0.8 mm DIA 0.8 mm DEEP I AMPLITUDE OF DEFECT SIGNALS, Y COMPONENT X-Y DISPLAY OF DEFECT SIGNALS (FILTERED) I (b) Through-Wall Transmission Results I

Fig. 5.7: Comparing Conventional and Through-Wall Transmission Techniques I I I I -69- The Y-amplitude presentation in Figure 5.7(b) shows defect signal amplitude does not change significantly with defect depth. It is important to note the phase of the signals does not change with defect depth when using the send-receive raethod as shown in the X-Y display.

5.5 IMPEDANCE GRAPH DISPLAY

Impedance graphs are an indispensable aid in eddy current inspections. An understanding of these graphs provides an operator a clear picture of all variables and the ability for appropriate action to minimize effects of adverse conditions.

All information about the test article is transmitted to the test coil via the magnetic field. The variation of the magnetic flux, , with time induces a voltage, V, across the test coil which, by Faraday's Law, depends on the magnitude and rate of change of and on the number of turns in the coil, N

V=-N| (2.4) = - Ldl/dt since = LI/N.

The variation in amplitude and phase of this voltage vector indicates the condition of the test article. The voltage vector can be resolved into the two quadratures, in-phase VQ, and out-of-phase VgQ . Since V = IZ and I is kept approximately constant, the voltage graph can be replaced with the impedance graph, as discussed in Section 3.3.

Impedance depends not only on test article variables but also on probe parameters. The probe parameters are coil diameter, number of turns, length, and core material. The instrument parameter that affects impedance is test frequency (since f « d

The inductive reactance component is normalized by dividing by the product of frequency and coil inductance (GJLO) when the probe is away from test material (in air).

XL U)L X 0)L o o where w is angular frequency, radians/second L is inductance, henries Lo is inductance of coil in air, henries XL is reactance, ohms Xo is reactance of coil in air, ohms -70-

r 1 r Ixl IX ixi • ixi AIR TEST ARTICLE INDUCTIVE REACTANCE

luL — — AIR

TEST ARTICLE

RESISTANCE

(a) BEFORE NORMALIZATION (b) AFTER NORMALIZATION

Fig. 5.1 Coll Impedance Display

The resistive component Is normalized by subtracting coil wire and cable resistance, R and then dividing by U)L DC 0

" RDC (A)L where R^ is coll resistive load due to eddy currents in test mate ria1.

The normalized components X^/XQ and RL/X0 are diraen- sionless and independent of both coil inductance and coil wire and cable resistance. Changes in the normalized parameters indicate variations in eddy current flow ir>to the test article only. Figure 5.8 displays probe impedance before and after normalization. Changes in the test article are reflected by a change in impedance point P. Figures 5.9 to 5.11 are normalized coil impedance graphs, produced by computer simulation, showing the change in the point P for the following sample variables: electrical resistivity, permeability, and thickness. Figures 5.12 and 5.13 show effects of test frequency and coil diameter. NORMALIZED REACTANCE NORMALIZED REACTANCE

I I

\= 1 c111

NORMALIZED REACTANCE NORMALIZED RE0CI/1NCE o o c -72-

5.5.1 Effect of Resistivity Figure 5.9 shows the effect of electrical resistivity for a range of conducting materials. The impedance point moves up the curve with increasing resistivity. Impedance points for step changes in coil to test article spacing between zero and infinity are also included. Note that a small increase in spacing (lift-off) produces a large impedance change. This results from decreased magnetic flux coupling to the sample. There would be a relatively larger effect on the impedance of a small coil than on the impedance of a large coil for the same change in spacing.

5.5.2 Effect of Permeability Note in Figure 5.10 there is a large impedance increase for a small increase in permeability. Small permeability changes can obscure other test variables. 5.5.3 Effect of Thickness Figure 5.11 traces the impedance point path as sample thickness decreases from infinity to zero. As test material becomes thinner, causing increased resistance to eddy currents, the impedance point moves up the curve. This was also the case in the resistivity graph, Figure 5.9. This implies that any condition causing an increase in resistance to flow of eddy currents, cracks, thinning, alloying elements, temperature, etc., will basically move the impedance point up the curve towards the probe impedance in air, XL/XO»1. The impedance curve in Figure 5.11, from the knee down, makes a reversal swirl as the probe moves across a conductor with increasing thickness. This is due to skin depth and phase lag effects which overshadow all basic movements of the impedance point. 5.5.A Effect of Frequency

Figure 5.12 shows the effect of test frequency (an instrument parameter). As frequency is increased, eddy currents are sampling a thinner layer close to the surface (skin depth effect, discussed in Chapter 2). When frequency is decreased eddy currents penetrate deeper into the material and the impedance point moves up the curve. Towards the upper end of the curve, impedance is mainly composed of resistance which has a great dependency on temperature, both in the test article and in coil wire resistance (although the latter does not appear on this normalized curve). It is therefore desirable, when possible, to operate near the knee of the curve say, 20 to 200 kHz in this example. -73-

5.5.5 Effect of Probe Diameter

Figure 5.13 shows effect of coil diameter (a probe parameter). Note increasing coil diameter moves the impedance point down the curve, similar to increasing frequency. When test conditions dictate use of a low frequency, the operating point can often be brought down the curve to the desired knee region by increasing coil diameter (provided test conditions will permit a large probe).

LIFT-OFF .^3

0, = 0.2 Do

I = 0.2 Do

Frequency =50 kHz

NORMALIZED RESISTANCE

^ 5.13; Impedance Graph-Surface Coil Diameter Effect

5.5.6 Comparison of Experimental and Computer Impedance Diagrams

The impedance graphs shown in Figure 5.9 to 5.12, produced by computer simulation, can be verified using a standard eddy current instrument. Figure 5.14 shows probe response to various test variables: resistivity, permeability, lift-off, and test frequency. The solid lines are output voltage traces generated by varying probe-to-test article spacing (lift-off) from infinity to contact with various conducting samples, while keeping test frequency constant at 10 kHz, and again at 100 kHz. The dashed lines, connecting the points when the probe was in contact with the samples, were sketched in to show the similarity between these graphs and the normalized impedance graphs in the preceding section. Note that the points move down the curve with increasing conductivity and also with increased frequency. For example, the operating point for 304 SS moved from the top of the impedance diagram at 10 kHz to near the knee at 100 kHz. -74-

LIFT-OFF L FERRITE T SAMPLE (p.fi) IRON MONEL 400

MONEL 400

INDUCTIVE INDUCTIVE REACTANCE REACTANCE 304 SS

f =100 kHz f =10 KHz

RESISTANCE RESISTANCE

(a)

Fig 5.14; Probe Response to Various Test Parameters at Two Frequencies 5.6 CHARACTERISTIC PARAMETER In Section 5.5 impedance graphs were normalized to make test article parameters independent of probe properties such as inductance. Another method, proposed by W.E. Deeds, C.V. Dodd and co-workers, combines frequency and probe diameter with test material parameters, to form one characteristic parameter(2).

r (5.4)

where r is mean coil radius a) is angular frequency Ur is relative magnetic permeability (=1.0 for nonmagnetic materials) and a is electrical conductivity. -75-

Using this characteristic parameter, one impedance graph can be plotted to describe four test parameters with Pc as the only variable.

z El

i 5 — r wtj.o- = CONSTANT - —LIFT-OFF CONSTANT r = COIL MEAN RADIUS t, = LIFT-OFF/r ANGULAR FREQUENCY MAGNETIC PERMEABILITY ELECTRICAL - CONDUCTIVITY

0 05 0.10 0.15 0.20 0.25 0.30 0.35 NORMALIZED RESISTANCE

Fig. 5.15: Impedance Diagram with Characteristic Parameter, Pr Consider Figure 5.15. The solid lines are generated by starting with Pc equal to zero and increasing the value to iniinity (while holding coil to test article spacing constant). The dashed lines are generated by starting with the coil infinitely far away from the test article and bringing the coil closer until it contacts (while holding Pc constant). Note the similarity between these curves and the impedance graphs shown in preceding sections (the horizontal scale is twice the vertical scale). The usefulness of the characteristic parameter is that it provides a modelling parameter. Conditions of similarity are met when

£• = —Z. v 1 u)1 i ir lo 1 T 2 Lo2 ur 2o 2 or rl UiyrlP2 = r2

Test 1 Test 2 i -76- I I I I

STORAGE OSCILLOSCOPE I DISPLAY I

NOMENCLATURE I V - VOLTAGE - CURRENT - ANGULAR FREQUENCY I (a. = 2irf) - PROBE INDUCTANCE IN AIR I - PROBE WIRE I CABLE DC RESISTANCE R3 -SFECIMEN AC RESISTANCE I SUBSCRIPTS: T - TOTAL I L - INDUCTANCE R - RESISTANCE P - PRIMARY S - SECONDARY I I I Fig. 5.16: Coil Impedance/Voltage Display I I I I -77-

Test conditions with the same Pc value have the same operating point on the normalized impedance graph. If, for f example, test article resistivity measurements were required (for checking consistency of alloying elements for intance), the best accuracy would be achieved by operating near the I knee of the curve where there is good discrimination against lift-off. (Equation 5.4 does not include skin depth effects, which may be an overriding consideration). To operate at the knee position in Figure 5.15 a probe diameter and frequency combination are selected such that Pc£10. The value of Pc in equation 5.4 is given in SI units; we can use the following version using more familiar units.

4 2 Pc = 7.9 x 10" 7 f/p (5.5)

where r is the mean radius, mm f is frequency, Hz p is electrical resistivity, microhm-centimetre ( yr = 1 for nonferromagnetic material)

It should be noted that the characteristic parameter, Pc, must be used in conjunction with Figure 5.15 (obtained analytically); it cannot be used to obtain Figure 5.15.

5.7 DEFINITION OF "PHASE" TERMINOLOGY This section attempts to clarify the concept of phase. the voltage/impedance graphs, presented in Section 5.5, are used as a link between impedance diagrams and the display on an eddy current instrument monitor. In eddy current work the most confusing and often incorrectly used term is PHASE. Part of the problem arises because of the existence of two eddy current methods, coil impedance and send-receive. In this section an attempt is made to clarify some of the multiple uses of the word.

Figure 5.16 shows the impedance of a probe touching test material. The two axes represent the quadrature components, VL and VJJ, of voltage across a coil. In the absence of real numbers, the axes can also be considered as the normalized parameters wL/wL and RL/ wL0. The following list summarizes uses of the term PHASE. One or more of these are often used without adequate explanation because the term will have a colloquial meaning. -78- i I 1. Q]_t 0j - Arctan g— , anglaglee between total voltagvoltae vctovectorr I« and resistive voltage vector. •

NOTE: An impedance bridge measures amplitude of the impedance vector Z and the angle 9 , where the I resistance includes K-c . This vector could therefore not be shown on Figure 5.16. (It is shown on the B impedance diagram in Figure 5.8(a)). f 2. AS.., Change in phase of normalized resultant voltage vector as probe is moved over a defect. 3. 0_, Phase between secondary voltage (induced voltage) and I primary voltage (excitation voltage). Send-receive B instruments measure secondary voltage. 4. AQ2, Change in phase of secondary voltage as probe is moved over a defect. This is approximately the phase measured by some send-receive eddy current instruments I without X-Y outputs. 5. 0 , Phase between the voltage signals obtained from m J LIFT-OFF and a crack or void. It is related to PHASE • LAG 0 , explained below. (03 is about double the phase lag.) 03 is used to estimate defect depth during ET. • 6. |3f PHASE LAG (not shown in Figure 5.16) of eddy currents flj below the surface relative to those at the surface. It was derived in the eddy current density equation Chapter 2, i.e.g = x/6for semi-infinite plates, where x is the • distance below the surface and 0 is in radians. fl 7. 0,, Many eddy current instruments have a PHASE knob by which the entire impedance voltage plane display can be rotated. It is common practice to rotate the display • to make LIFT-OFF horizontal. (On an eddy current I instrument display, absolute orientation of inductive and resistive axes may be unknown). m 8. Q , Phase between inductive voltage and current in a I circuit; 0 = 90° • 5.8 SELECTION OF TEST FREQUENCY I 5.8.1 Inspecting for Defects The first question that must be answered before proceeding • with an inspection is: For what type of defects is the | inspection being done? If the defects are cracks: What is the smallest defect that must be detected? Are the cracks A surface or subsurface? Are they likely to be laminar cracks I or normal to the test surface? A single general inspection M, procedure to verify the absence of any and all types of defects often has little merit. Inspections often require two or more » test frequencies and/or different probes to accurately identify I defects. * Test frequency can be selected without knowledge of the characteristic parameter, Pc, or the operating point on the impedance graph. It should be chosen for good discrimination I between defects and other variables. The most troublesome variable is LIFT-OFF variations, so separation of defects from lift-off is the foremost consideration. f ; I I -79- Only the skin depth equation has to be used,

mm (2.13a)

A test frequency where <5 is about equal to the expected defect depth provides good phase separation between lift-off and defect signals. Figure 5.17 illustrates the display on

COIL LIFT-OFF SURFACE CRACK SUBSURFACE VOID (A) SUBSURFACE VOID (B) INCREASING

SUBSURFACE VOID (A) SUBSURFACE X -Y DEFECT SIGNALS SURFACE VOID (B) CRACK

(a) (b)

Fig. 5.17: Typical Response Signals for Two Types of Defects

an eddy current instrument monitor as a probe passes over surface and subsurface defects. Test frequency is such that <5 equals depth of deepest defect, and instrument controls are selected such that a signal from lift-off is horizontal. Note the difference in signal amplitude and angle relative to lift-off of subsurface voids A and B. Thin results from skin depth attenuation and phase lag. If, during inspection, a signal indicating a defect is observed, test frequency may be altered to verify whether the signal represents a real defect or the effect of another variable. This discussion is expanded in the next chapter under Signal Analysis. -80-

5.8.2 Measuring Resistivity Resistivity can be measured at small localized areas or by sampling a larger volume of a test article to determine bulk resistivity. The volume of material interrogated depends on probe size and test frequency. For bulk measurements a large probe would be used and a low frequency to maximize penetration. The skin depth equation is again used to estimate depth of penetration at the test frequency.

Electrical resistivity measurement is a comparative technique; reference samples of known resistivity must be used for calibration. Variables that affect the accuracy of resistivity measurement are lift-off, temperature, and changes in the flow of eddy currents in test articles not related to electrical resistivity (such as cracks, thickness and surface geometry).

For best discrimination between resistivity and other variables the operating point on an impedance graph should be considered. Figure 5.12 illustrated the effect of test frequency on normalized probe impedance. At the top of the graph the angle, between lift-off variations and the resistivity curve, is small. Moving down the curve the angle, separating the two variables, increases towards the knee with no appreciable change beyond that. However,small lift-off variations, at the bottom of the curve, produce a large impedance change. The best operating point is somewhere between the two extremes, near the knee of the impedance curve.

REFERENCE SAMPLE " IMPEDANCE POINT ~ s.

iff IMP1E ANC1E POINT 'S r F UNKNOWN INCREAS ING —.— FF REFERENCE" SAMPLE \ MONITOR 1 [ DISPLAY

RESISTANCE

(a) IMPEDANCE GRAPH • RESISTIVITY EFFECT

Fig. 5.18: Resistivity Measurement and the Impedance Graph I -81- Figure 5.18 shows the method of manipulating test conditions I to best deal with lift-off. Figure 5.18(a) shows the resistivity impedance curve with a frequency and probe selected to operate near the knee. Figure 5.18(b) i: an enlarged section of the curve rotated so lift-off signals are approximately horizontal. This is the view on an eddy current instrument monitor.

•i Next consider temperature effects. First, test article resistivity will be a function of temperature so test sample * and standards should be at uniform temperature. A greater ' potential error is in probe wire resistance, RJJC • The coil wire resistance Is a part of the probe impedance circuit, so variations in temperature which affect coil resistance will appear as an impedance change. For greatest accuracy, the inductive reactance, XL, should be large compared to coil wire resistance; XL/IL > 50 is desirable. Obviously this condition is not easily satisfied at low test frequencies where inductive reactance is low. One solution is to use a large diameter probe cupped in ferrite. The large diameter and ferrite cup will both increase XL/R^

Another solution is to use a Send-Receive instrument. Such an instrument has a high input impedance, sensing only voltage changes in the receive coil. Coil wire resistance is insignificantly small in comparison to instrument impedance and therefore has no effect.

Consider next the effect of changes in eddy current path not related to electrical resistivity. If the test is supposed to be a measurement of electrical resistivity, thickness should not influence the signal. The skin depth equation must again be used. Test article thickness should be equal to or greater than three skin depths, t > 3 § ,

t >3s

•f >12|00 p , H2

where t is thickness, p is resistivity in microhm- centimetres, and f is frequency.

Other sources of signals are edge effects and surface geometry. When the test article's edge is within the probe's magnetic field, an increase in resistance to eddy current flow will be detected. Edge effect can be reduced by probe design, such as a ferrite cupped probe, or by increasing test frequency. I -82- I If the surface of the test article is contoured, the magnetic flux coupling will differ from that of a flat surface and a correction factcr nay be required. I Cracks or voids are usually less of a problem. The signal from a crack will be very localized whereas resistivity variations are usually more gradual. The best procedure to I determine if a localized signal is from a change in resistivity is to rescan with a smaller probe at higher and lower frequency (at least three times and one third the test I frequency). The angle between the signals from lift-off and resistivity should vary only slightly whereas the angle between lift-oft and defect signals will increase with t requency. I

An example of resistivity variations in a zirconium alloy, due to a change in oxygen concentration, is shown in I Figure 5.19.

TEST ARTICLE WIDTH I I I I I X,VOLTS

(a) X-Y DISPLAY OF COIL IMPEDANCE FROM I CHANGE IN ELECTRICAL RESISTIVITY I I I

(b) MODIFIED C-SCAN DISPLAYING Y-COMPONENT OF COIL IMPEDANCE VECTOR FROM A Ci.ANGE I IN ELECTRICAL RESISTIVITY Fig. 5.19; Eddy Current Signals from a Change in Electric a .1 Resistivity on the Surface of a Zr-Nb Test Article. Test Frequency = 300 kHz. I -83-

5.8.3 Measuring Thickness Test frequency should be chosen so 'lift-off and 'change in thickness1 signals are separated by a 90° phase angle, see Figure 5.20(a). This frequency can be calculated using the skin depth equation. A reasonable approximation for thin sections is when obtained when

t/6 = 0.8 (5.6)

which converts to 1.6 p/t: kHz (5.7a) where o" is skin depth, mm t is test article thickness, mm P is electrical resistivity, microhm-centimetres f is frequency, kHz y is relative permeability (Pr = 1 for non- ferromagnetic material). In testing thick material, this equation can similarly be used to choose a test frequency to separate lift-ofi and subsurface defect signals by 90°. Equation 5.7(a) can be used by replacing t with x,

f • 1.6 p/x2 kHz (5.7) where x is depth of subsurface defect.

INCREASING RESISTIVITY [HICKNESS

DFft< LIFT-OFF 1

.BALANC POINT 1 FOF NOI INAL KK ASING -THI CKNI SS-I Ti IICKNESS

(b) EDDV CURRENT INSTRUMENT MONITOR DISPLAY

RESISTANCE

(a) IMPEDANCE GRAPH - RESISTIVITY AND THICKNESS EFFECT Fig. 5.20: Thickness Measurement and the Impedance Graph -84-

Conventional thickness measurement is to display the lift-off signal horizontal (along the X-axis) and use the vertical signal (along the Y axis) to measure thickness, see Figure 5.20(b). It the signal on the instrument monitor is set to move from right to left as the probe is moved away from the test article, a vertical movement up or down denotes decreasing and increasing thickness respectively.

5.8.4 Measuring Thickness of a Non-Conducting Layer on a Conductor

An insulating layer will not conduct eddy currents so measurement ot its thickness is essentially a lift-off measurement (provided it is non-ferromagnetic), i.e. the distance between the coil and test article. At high test trequency a small variation in lift-off produces a large change in probe impedance as shown in the impedance graph of Figure 5.9.

To minimize the signal from variations in the base material, the test should therefore be done at the highest practical frequency. The maximum frequency would be limited by probe-to-instrument impedance matching, cable resonance problems and cable noise.

The measurement is a comparative technique so standard reference thicknesses must be used for calibration.

5.8.5 Measuring Thickness of a Conducting Layer on a Conductor

Measurement ot the thickness of a conducting layer on a conducting test article can be done provided there is a ditterence in electrical resistivity (Ap) between the two. The measurement is essentially the same as the thickness measurement described in Section 5.8.3. There is one important difference; variables in the base plate, in addition to the variables in the layer, will affect the signal.

Figure 5.21(a) shows a computer simulation of a layer thickness measurement. The model shows the magnitude and direction ot variables when attempting to measure a layer (clad X),-nominally 0.75 mm thick, with resistivity p = 3 pfi.cm on a base (clad 2) with resistivity 5 liSl.cm. The plot is part of a normalized impedance graph. In addition to material property variables, the parameter of space (gap) between the layers is shown as well as the effect of an increase in test coil temperature. At 10 kHz, t/6 is 0.8 and, as predicted, the angle separating signals from -85-

EDDY CURRENT IHPEDANCE PLANE

1 i 1 1 1 1 1 1 1 1 .B94

5 ••20-. .892 -

=_8OsI .BSD ~3 • 10- < >< .8BB I i 1 L> r % RESISTIVIT Y - ,11 -7SJ! 0 TO B2S .775\ /^ RESISTIVITY 3 • 20. cm } i t AIR GAP n TO 37 .EBB — •"AIR G»P — ) RESIST IVITY 3 = 5 r 2D-. p il - cm •50..^JK »,c ( Y~ -20 .BB4 - • 2C .4C- - -*%* -RES STIVITY 3

.8B2 RANGE OF VARIABLES SHDIN IN CDHPUTDR PLOTS ; / 0 1 .BBQ

FREQUENCY = ID kHz .8TB _

,876 i 1 1 1 1 iii~ .052D .0540 .0560 .0590 .0600 .0620 .0640 .B560 .OEBO -O7D0

NDRMLIZED RESISTANCE. -^L

Fig. 5.21; Computer Simulation of a Multi-Layer Sample

lift-off and layer (clad 1) thickness is about 90°. Unfortunately, so are the signals from test coil temperature, gap, and resistivity of the base (clad 3). Some of these parameters can be discriminated against at higher and/or lower test frequencies.

5.9 PROBE-CABLE RESONANCE

Probe-cable resonance must be considered when operating at high test frequencies and/or using long signal cables, e.g., frequencies greater than 100 kHz and cables longer than 30 m. Host general purpose eddy current instruments cannot operate at or close to resonance.

frobe-cable resonance can be modelled as shown in Figure A.5. Tn simple terms, resonance occurs when inductive reactance of the coil equals capacitive reactance of the cable, i.e. when

(i)L = l/u)C

is wnere is angular frequency, in radians/second. L coil inductance in henries and C is total cable capacitance in farads. -86-

Transfortning this equation and substituting w=2irf shows resonance occurs when frequency is

fr = 1/21T /Tc (4.6a) This approach is sufficiently accurate for most practical applications. A more rigorous approach to resonance is presented in Section 4.3.

Resonance is apparent when a probe and cable combination, which balances at a low frequency, will not balance as frequency is increased. At the approach of resonance, the balance lines on the eddy current storage conitor will not converge to a null. The two balancing (X and R) controls will produce nearly parallel lines rather than the normal perpendicular traces, on the storage monitor. A number of steps can be taken to avoid resonance:

1. Operate at a test frequency below resonance, such that f is less than 0.8fr< 2. Select a probe with lower inductance. (Since fr is proportional to 1/ /L, inductance must be decreased by a factor of four to double resonant frequency)• 3. Reduce cable length or use a cable with lower capacitance per unit length (such as multi-coax cables). This will raise the resonance frequency since capacitance is proportional to cable length and fr is proportional to 1/ /cT 4. Operate at a test frequency above resonance, such that f is greater than 1.2fr. However, above resonance the sensitivity of all eddy current instruments decreases rapidly with increasing frequency because capacitive reactance (Xc=l/ (OC) decreases, and current short circuits across the cable, rather than passing through the coil.

5.10 SUMMARY

Test probes induce eddy currents and also sense the distortion of their flow caused by defects. Surface probes contain a coil mounted with its axis perpendicular to the test specimen. Because it induces eddy currents to flow in a circular path it can be used to sense all defects independent of orientation, as long as they have a component perpendicular to the surface. It cannot be used to detect laminar defects.

For good sensitivity to short defects, a small probe should be used; probe diameter should be approximately equal or less than the expected defect length. Sensitivity to short subsurface defects decreases drastically with depth; even a 'thin' 5 mm sample is considered very thick for eddy current testing. -87-

The analysis of eddy current signals is the most important ana unfortunately the most difficult task in a successful inspection. A thorough understanding of impedance graphs is essential to manipulate test conditions to minimize undesirable test variables. The characteristic parameter for surface probes is used to locate the operating point on the impedance diagram. It is given by

P = 7.9 x 10~4 r2 f/p (5.5) c where r is mean radius, mm; f is test frequency, Hz; and p is electrical resistivity, microhm-centimeters.

The criterion for defect detection with impedance plane instruments is phase discrimination between lift-off noise and defect signals. Test frequency is chosen such that 'lift-off* and "change in wall thickness1 signals are separated by a 90° phase angle. This can be derived from the following equation:

f = 1.6 p/t2 , kHz (5.7) where t is sample thickness, mm.

If inspection is performed at high test frequencies and/or with long cables, it is desirable to operate below probe-cable resonance frequency. This is normally achieved by using a probe ot sufficiently low inductance.

To optimize test results, the inspector has control over probe size and test frequency. In choosing probe diameter the following must be considered:

(a) operating point on impedance diagram (b) probe inductance and resistance (c) sensing area (d) sensitivity to defect length (e) sensitivity to defect depth (f) sensitivity to iltt-off (g) sensitivity changes across coil diameter (zero at centre) (h) sensitivity changes with ferrite core or cup.

Choice of test frequency depends on:

(a) depth of penetration (b) phase lag (c) operating point on impedance diagram (d) inductive reactance (e) probe-cable resonance. -38-

5.11 WORKED EXAMPLES

5.11.1 Effective Probe Diameter

PROBLEM: Determine sensing diameter of a 5 mm probe when (a) testing 316 stainless steel ( p = 72 microhm- centimetres) at 2 MHz,

and (b) testing brass ( P = 6.2 microhm-cm) at 10 kHz,

SOLUTION: (a)

(2.13a)

0.30 mm 2 x 10 x 1

Deff = Dc '2 = 6#2 mm

(b) 6 = 5oJ~ = 1.25 mm

10 mm eff

5.11.2 Characteristic Parameter

PROBLEM: If an available probe had coil dimensions of 10 mm outer diameter and 4 mm inner diameter, determine the best frequency for resistivity measurements of a zirconium aiioy ( P = 50 microhm-cm).

SOLUTION: The best frequency for resistivity measurements is when the operating point is at the knee location on the impedance diagram. This occurs when the characteristic parameter Pc=10. Using equation 5.5, 2 /lO.O -f- 4.0 \ 7.9 x 10-4 f/50 = 10

therefore, f = 50 kHz.

(This calculation places no emphasis on skin depth effect, which may be an overriding consideration). -89-

CHAPTER 6 - SURFACE PROBE SIGNAL ANALYSIS

6.1 INTRODUCTION

Manufacturing and preventive maintenance inspection of "flat" components with surface probes is one of the oldest and most important applications of eddy current testing. Manufacturing inspection of small steel components for defects and hardness is almost exclusively performed by eddy current methods. For safety reasons and preventive maintenance (savings on replacement costs and downtime) inspection of aircraft components for cracks and heat treatment effects has been performed since commercial aircraft first went into service. Eddy current testing is one of the most effective NDT methods for the above applications because it doesn't need couplants, it is fast, and 100% volumetric inspection is often possible.

This chapter describes how to maximize signal-to-noise by proper choice of test frequency and minimizing "lift-off" noise- Emphasis is given to signal analysis and how to recognize and discriminate between defect signals and false indications. An attempt is made throughout this chapter to illustrate discussion with real or simulated eddy current s ignals.

6.2 EDDY CURRENT SIGNAL CHARACTERISTICS

6.2.1 Defect Signal Amplitude

A defect, which disrupts eddy current flow, changes test coil impedance as the coil is scanned past a defect. This condition is shown pictorially in Figure 6.1 which portrays eddy currents induced by a surface probe in a defective plate. Eddy currents flow in closed loops as illustrated in Figure 6.1(a). When a defect interferes with the normal path, current is forced to flow around or under it or is interrupted completely. The increased distance of the distorted path increases the resistance to current just as a long length of wire has more resistance than a short length.

Eddy currents always take the path of least resistance; if a defect is very deep but short, current will flow around the ends; conversely, if a defect is very long (compared to the coil diameter) but shallow, the current will flow underneath. In summary, defect length and depth (and width to soae degree) increase resistance to eddy current flow and this, in turn, changes coil impedance. (The effect of defect si'.e on flow resistance in tube testing is derived in Section 8.2.1) . -90-

COIL BOUNDARIES SURFACE COIL EDDY CURRENTS WINDINGS TEST PLATE TEST PLATE EDDV CURRENT DISTORTION AT CRACK

CRACK EDDV CURRENTS TAKE THE PATH OF LEAST RESISTANCE UNDER DR AROUND A DEFECT

(a) EDDT CURRENTS FLO! IN CLOSED PATHS. A DEFECT INTERFERES IITH THE NORMAL PATH

Fig. 6.1: Eddy Currents In a Defective Plate

In terms of the equivalent coil circuit of a resistor in parallel with an inductor and its associated semi-circular impedance diagram (Section 3.5), a defect moves the operating point up the impedance diagram. Increasing resistance in a test article changes both probe inductance and resistance.

In the preceding discussion the defect was considered to disrupt the surface currents closest to the coil. Consider the difference between surface and subsurface defects. When a surface probe is placed over a deep crack of infinite length, the surface currents must pass underneath the defect if they are to form a closed loop, see Figure 6.2(a). This is not the case with subsurface defects as shown in Figure 6.2 (b). Although the void in this picture is not as far from the surface as the bottom of the crack, the void may not be detected. Eddy currents concentrate near the surface of a conductor,and therefore, tests are more sensitive to surface defects than internal defects.

The skin depth equation helps in the understanding of this phenomenon. In Chapter 2 it was shown that current density decreased with distance from the surface in the following proportions: - 63% of the current flows in a layer equivalent in thickness to one skin depth, 5 , - 87% flows in a layer equivalent to two skin depths, 2 6 , - 95% flows in a layer equivalent to three skin depths, 3 6 . -91-

SURFACE COIL TEST PLATE CRACK

(a) EODV CURRENT FLOK UNDER A CRACK (b) EDDY CURRENT FLOW AROUND A SUBSURFACE VOID

Fig. 6.2: Eddy Current Flow in the Presence of (a) Surface and (b) Subsurface Defect

Since only 5% of the current flows at depths greater than the 3 6 , there is no practical way to detect a subsurface defect at this distance from the surface. But in the case of a long surface defect 3 6 or greater in equivalent depth, most of the current is flowing under the defect. Surface cracks will be detected and depth can be estimated even if eddy current penetration is a small fraction of the defect depth. Once eddy currents are generated in a metal surface, they will follow the contour of a crack because a potential is set-up about the crack.

6.2.2 Defect Signal Phase

From the above description one cannot predict a defect signal in detail, only its relative amplitude and direction on the impedance diagram. A more complete explanation requires inclusion of phase lag. Consider the cross section of a surface probe as shown in Figure 6.3(a). This pictorial view shows the distribution of magnetic field magnitude and phase around a coil as derived by Dodd(2^ - The solid lines are contours of constant magnetic field strength; the dashed lines represent constant phase. Since the magnetic field and induced eddy currents have approximately the same phase, the dashed lines will also represent the phase (B) of the eddy currents. Amplitude drops off exponentially with distance and eddy current flow increasingly lags in phase (relative to eddy currents adjacent to the coil) both with depth and with axial distance from the coil. Skin depth effect occurs in both radial and axial directions.

Figure 6.3(a) permits an approximate derivation of eddy current signals for the shallow surface, subsurface and deep surface defects illustrated. One needs to establish a -92-

CONSUNT JHPUITUDE

SH«LLOI OEFECT

5US5UBFHCE OEFECT-

OEFECT POSITIDK

Fig. 6.3: Derivation of Eddy Current Signal Appearance for Three Types of Defects -93-

reference phase direction as starting point; the LIFT-OFF direction is convenient and can be defined as the signal resulting from increasing the space between the coil and test article, starting from the point when the space is minimum.

The signal or effect of defects can be imagined as the absence of eddy currents which were flowing in the area before the defect existed at this location. As the defects approach the coil from positions 0 to 5 in Figure 6.3(a), the signal on the eddy current storage monitor moves from point 0 to 5, tracing the curves illustrated in Figure 6.3(b). This procedure is reasonably straight forward for shallow surface and subsurface defects since they are localized and only intersect one phase and amplitude contour at any given position. For the deep defect one has to divide the defect into sections and determine weighted average values for amplitude and phase at each position.

The shallow surface defect in Figure 6.3(b) has a large component in the lift-off direction; primarily its approach signal makes it distinguishable from lift-off. As defect depth increases, signals rotate clockwise due to increasing phase angle. The angle indicated in Figure 6.3(b) is not the value calculated from the phase lag equation,

3 = x/6 (2.14)

where 3 is phase lag (radians), x is distance of defect below the surface (mm) and 5 is skin depth (mm).

The angle between lift-off and defect signals is about 2 3- Although probably not strictly true, one can imagine defect phase angle as the sum of a lag from the coil to the defect and the same lag back to the coil.

The foregoing discussion assumes that the defect is a total barrier to the flow of current. Although this assumption is valid for most cracks or discontinuities, some cracks are partial conductors. Fatigue cracks, formed when the test article is under a tensile stress, can become tightly closed when stress is released. The result is that some fraction of eddy currents could be conducted across the crack interface and the magnitude of the coil impedance change due to the defect will be less. The phase lag argument is still valid; a deep crack will still be distinguishable from a shallow crack by the shape of the eddy current signal, but the sensitivity to such a :^;V will be reduced because of smaller amplitude.

6.3 EFFECT OF MATERIAL VARIATIONS AND DEFECTS IN A FINITE THICKNESS

For each test, one must decide on the test frequency to use and on the phase setting. The conventional way of setting -94-

phase on an eddy current instrument is to display the "lift-off" signal horizontally (on the X-axis) with the impedance point moving from right-to-left as the probe is raised. All material variables will then display an eddy current signal at an angle clockwise to the lift-off signal.

7 mm

LIFT-OFF 1.5 mm 2.0 mm

— -At

LIFT-OFF LIFT-OFF LIFT-OFF

FREQUENCY = 10 kHz FREQUENCY = 50 kHz FREQUENCY = 200 kHz

Fig. 6.4; Probe Response to Various Test Parameters at Three Frequencies

Discrimination between defects and other variables is accomplished through pattern recognition and varying test frequency. Figure 6.4 displays the change in coil impedance loci for various parameters at different test frequencies. The electrical resistivity (Ap) signal angle, relative to lift-off, increases only slightly as frequency is increased, whereas a change in plate thickness ( At) signal angle continually increases with frequency. The angle, between the signal from lift-off and plate thickness change, equals about twice the phase lag across the plate thickness. The signal from a change in magnetic permeability (Ay) of the plate is approximately 90° to the lift-off signal at low frequency and decreases only slightly with increasing frequency.

Figure 6.5(a) illustrates a computer simulation of coil response to various test parameters. The simulation is based on the same probe and test sample used in the previous figure. Comparison of these two figures reveals computer simulation gives very realistic results. -95-

LIFT-OFF 0.25 mm '••-.. I 2 mm

\ \V

p - 72 ^ A/> - -25 At > 25

\ 0.25 mm\

200 kHz

(a)

\ -.

" - |- 1 ' -5

0.25 Mi *•* 10 kHz

0.7 >" ^ \ \ \ \ \ p = 72 „] I \ \

aM = .25. \ \ u '1.0 \ \

0.25 • 0.4

0.1 0.2 0.3 0.4 0.5 (b)

Fig. 6.5; Computer Simulation of Probe Response to Various Test Parameters -96-

Note at 50 kHz the increase in magnetic permeability signal (Ay) is to the right of the electrical resistivity signal for the 7 mm probe. For the 25 mm probe at 50 kHz it is to the left of the Ap signal. As the operating point moves down the impedance curve with increasing probe diameter, a raslstivity signal rotates CW relative to a permeability signal. Note also that the permeability signal is not perfectly parallel to the inductive reactance axis. This is due to the skin depth and phase lag changing with permeability, rotating the signal CW.

During general inspection for all parameters in a thin plate test frequency is normally chosen such that 'lift-off' and 'change in plate thickness" signals are separated by 90° on the impedance plane. This frequency is empirically derived by setting ratio between plate thickness and skin depth equal to approximately 0.8,

t/6 = 0.8 (5#6)

Substituting in equation 2.13 yields

f = 1.6 p/t2 , kHz (5-7> where p is electrical resistivity (microhm-centimetres), and t is plate thickness (mm).

This frequency has been proven in practice on various conductivity samples and various probe diameters. The 90° phase angle increases only slightly with increasing probe diameter, see Figure 6.5(b). All defect signals (from surface or subsurface defects) will fall inside this 90° band. Shallow defects, cracks or pits, on the opposite side of the plate will produce a signal whose angle approaches that of wall thickness, i.e. 90°. Shallow defects on the surface nearest the probe will produce a signal whose angle is close to that of lift-off.

The two methods of discriminating between defects and other variables, pattern recognition and varying test frequency, complement each other. Consider signal pattern behaviour due to nominal wall thickness and resistivity variations. These variables normally change gradually along a sample. Whereas cracks, pits, and subsurface voids or inclusions exhibit a step change. Discrimination between these variables is enhanced by analyzing their behaviour at different test frequencies, as shown in Figures 6.4 and 6.5. An extremely important point to remember is that ail defects will fall between the 'lift-off signal angle and the'decrease-in- wall-thickness ' signal angle regardless of frequency. (For practical applications this statement is valid; however, the signal from a shallow defect with length greater than a probe diameter may dip slightly below the lift-off signal). -97-

REAL CALIBRATION CRACK/ CRflCKS

5 mm 7I1 2 mm SAMPLE : p = 50 jifl, • cm

/ir = 1.00

CRACK CRACK 2 mm DEEP NOTCH 2 mm DEEP NOTCH

LIFT-OFF LIFT-OFF 0.5 mm DEEP NOTCH 0.5 mm DEEP NOTCH

FREQUENCY = 50 kHz FREQUENCY = 300 kHz

Fig. 6.6; X-Y Display of Coil Impedance Vector from Calibration Grooves and a Real Crack. Estimated Depth=1.3 mm,

6.4 COIL IMPEDANCE CHANGES WITH DEFECTS

6.4.1 Surface Defect Measurement

Figure 6.6 illustrates the method used to predict depth of surface defects. Pattern recognition is used where coil impedance response from the defect is compared with calibration defects. To estimate defect depth by pattern recognition, the real and calibration defect signals must be comparable in amplitude. This can be achieved by changing the gain of the display (normally by decreasing the calibration defect signals). Defect depth is estimated by interpolation.

Amplitude of defect signals is not a reliable parameter for estimating defect depth. Amplitude is affected by length and the degree of contact across the two interfaces (e.g., crack closure). Whereas the coil impedance locus (the X-Y display of coil impedance) depends mainly on the integrated response with depth of the eddy current phase lag.

6.4.2 Subsurface Defect Measurement

Signals from subsurface defects, Figure 6.10(b), have an average phase angle relative to lift-off of approximately 28 where B is the phase lag of the eddy currents at depth x. This signal is similar to a change in wall thickness signal and its phase was denoted by O3 in Figure 5.16. -98-

6.5 COIL IMPEDANCE CHANGES WITH OTHER VARIABLES

6.5.1 Ferromagnetic Indications

In eddy current testing the test coil is sensitive to many test parameters. One variable that often causes problems is magnetic permeability. At common test frequencies one can easily mistake a signal due to increased permeability (ferromagnetic indication) for a serious defect. The following discussion briefly outlines the problem and shows how one can differentiate between defects and ferromagnetic indications•

It is generally recognised that magnetic saturation is required for eddy current testing of ferromagnetic alloys. Conversely, saturation is not usually employed when testing "non-magnetic" alloys such as austenitic stainless steels and nickel base alloys. Unfortunately, these alloys and any alloys containing iron, nickel or cobalt can display variations in magnetic permeability. This is caused by the strong dependence of magnetic properties on metallurgical variables such as composition, grain size, thermal processing, cold work, contamination and segregation.

The following are examples of ferromagnetic indications in nominally nonmagnetic alloys which have been encountered:

- Ferromagnetism associated with manufacturing defects in Inconel 600 extrusions (possibly from chromium depletion at the surface). - Ferromagnetism associated with EDM calibration grooves in Type 304 stainless steel. - Permeability variations occurring in austenitic stainless steel castings probably due to segregation (or possibly contamination). - Ferromagnetic inclusions in zirconium alloys resulting from pick-up during forming. - Magnetite (Fe3O^) deposits on heat exchanger tubes due to steel corrosion somewhere else in the cooling system.

The first rwo types of defects would have made defect depth predictions seriously inaccurate, and the last three types of ferromagnetic Indications could have been mistaken for defects such as cracks or pitting. Some of the anomalous ferromagnetic indications listed above could be suppressed by saturating the test area with a permanent magnet possessing a flux density of a few kilogauss. If saturation is not possible (or incomplete) there is another way to determine if an indication is due to a defect or a magnetic effect. The method involves retesting at a much lower frequency. It is illustrated in Figure 6.7 for the case of a surface probe passing over defects and a ferromagnetic inclusion.

At typical test frequencies (100-500 kHz) there is little phase separation between the signals frosn defects and magnetic inclusions. As test frequency is reduced, the operating -99-

FERROMAGNETIC / INCLUSION FERROMAGNETIC INCLUSION

PROBE D1A = 7 mm SMIPLE p =

2 mm DEEP

0.5 nu DEEP 0 0.D5 0.10 0.15 0.20 0.25

NORMALIZED RESISTANCE Ri

Fig. 6.7; Coll Impedance/Voltage Display at Three Frequencies point moves up the impedance curve and defect signals rotate as shown. The important point to note is that relative to lift-off, defect signals rotate CCW whereas the magnetic inclusion signal rotates CW and approaches 90° at low frequency (approximately 10 kHz or lower for the above probe and sample). On the impedance diagram of Figure 6.7 the direction of the ferromagnetic signal would not vary appreciably with frequency; increased permeability primarily increases coil inductance.

When a magnetic inclusion is not on the surface - if it is subsurface or on the opposite side of a thin test plate — there is the added complication that the angle of the signal will be rotated relative to the angle of a ferromagnetic indication on the surface adjacent to the coil. This arises from phase lag across the plate thickness. The previous approach of retesting at reduced frequency will also serve to distinguish between defects and magnetic inclusions. If the phase of the signal from the indication increases to 90° relative to 'lift-off, it is a ferromagnetic anomaly; if it decreases to nearly 0", it Is a defect. -100-

To summarize: (a) Many nominally "non-magnetic" alloys can exhibit ferromagnetic properties and almost any alloy can pick up magnetic inclusions or contamination during manufacture or service. (b) At normal eddy current test frequencies magnetic indications will often appear similar to defects. (c) Magnetic indications can be distinguished from defects by retesting at a reduced test frequency.

6.5.2 Electrical Resistivity

Electrical resistivity is a material parameter which, unlike a defect, usually varies over a significant area. However, if it is localized, and the eddy current signal is small, it could be mistaken for a small defect. The best means of distinguishing the two is to rescan with a smaller probe at the same test frequency, at three times the test frequency, and at one third the test frequency. Unlike a defect signal, the angle between resistivity and lift-off changes little with frequency. See impedance graph in Figure 5.9.

As with the detection of any signal source, resistivity is affected by skin depth. At high frequency, when skin depth is small, there will be greater sensitivity to surface resistivity variations. At lower test frequency, eddy currents penetrate deeper into the material so the measurement will represent a larger volume.

6.5.3 Signals from Changes in Sample Surface Geometry

Abrupt changes in surface curvature result in eddy current signals as probes traverse them. It causes changes in coupling creating a large lift-off signal and the curvature also changes eddy current flow distribution creating an effective resistance change, yielding a signal at an angle to the lift-off direction. The combined effect may be a complicated signal, as shown in Figure 6.8. The appearance of this type of signal will not change significantly when rescanned at higher and lower test frequency.

Such signals can be difficult to analyze because they depend on how well the probe follows complicated surface curvatures. Basically the direction of the impedance change obeys the following rules when using surface probes:

- decreasing radius of curvature on an external surface, e.g., ridge, produces a change in the direction of increasing resistivity, - decreasing radius of curvature of an internal surface, e.g., groove, produces a change in the direction of decreasing resistivity. -101-

Figure 6.8(a) illustrates the signal as a probe traverses a shallow groove (decrease in surface radius) on the internal surface of a 100 mm tube. Figure 6.8(b) shows the signal as a probe traverses a flat (increase in surface radius). The test was done with a 9 mm diameter probe at a test frequency of 300 kHz.

1 VOLT

(a) WIDE SHALLOW GROOVE (b) LOCAL FLAT SPOT

Fig. 6.8: X-Y Display of Surface Coil Impedance for Internal Surface Variations in a 100 mm Diameter Tube

6.6 CALIBRATION DEFECTS

Analysis of eddy current signals is, for the most part, a comparative technique. Calibration standards are necessary for comparing signal amplitude and phase (shape) of unknown defects to known calibration defects. Calibration signals are also used for standardizing instrument settings, i.e., sensitivity and phase rotation.

Existing national specifications and standards only supply broad guidelines in choice of test parameters. They cannot be used to establish reliable ET procedures for most inspections. Figure 6.9 shows a calibration plate proposed by the authors for general application. The effect of the following can be established using this plate:

1. Varying Electrical Resistivity 2. Varying Thickness 3. Surface Geometry (Curvature) 4. Defect Length for Constant Depth 5. Defect Depth for Constant Length 6. Increasing Subsurface Defect Size for Constant Defect Depth 7. Increasing Distance of Subsurface Defects from the Surface with Constant Defect Size 8. Varying Thickness of a Non-conducting Layer (lift-off) 9. Varying Thickness of a Conducting Layer 10. Ferromagnetic Inclusions - 102 -

i i i i i r

ION-CONDUCT ING COPPER CHROUIUU UYER LAYER PLATE

0.2 mm 1.0 mm 0.1 mm

0 1 ran 0.5 mm .05 mm

0 05 mil 0.1 mm .01 mm

(b) BACK SIDE

/ / / / / / / / / /// ERRITE|/>=120 f/>= 70 |c=50 [ P=2i | P- 1 \ P = 4 |/>=1.7 ~" ** 2 mm - E r /tfl - cm in 1.5 mm — \ 1 1 | 0.7 mm - —0.12 0 25 0.5 1 0 2.0 4 0 d = 1 mm DEPTH, mm

3 • __ 0.5 mm 1 I 1 3d = 2 mm i: 1. 2. 4. 10 LENGTH, mm ll = 1 mm 3d d=2mm CONSTANT DEPTH = 0 .5 m 25 \ ^•9 COPPER • O > d = 0.5 mm t=0 1 mm IRON VOID \

R0=50mm R.=5Q R. =20 R^S Ro = 5 RO=IO Ro=25

(a) FRONT SIDE

Fig. 6.9; Calibration Standard -103-

More than one calibration plate would be required to cover a complete range of materials. A group of three would normally suffice, comprising base materials: aluminum alloy, p=p 4 yyn.cm; bronze, p 25 yO . cm; and Type 316 stainless steel,p =74 Figure 6.10(a) illustrates eddy current signals obtained with an absolute surface probe from some of the calibration block, defects. Figure 6.10(b) illustrates signals from the same defects using a differential surface probe, similar to that in Figure 5.2(c) .

0.5 mm DEEP 4 mm DEEP 4 mm DEEP

1 mm DEEP

0.5 mm DEEP

0.7 mm DEEP

1.5 mm DEEP

LirT-OFF LirT-OFF

SUBSURFACE DEFECTS

(a) (b)

Fig, 6.10: Eddy Current Signals With (a) Absolute and (b) Differential Surface Probes -104-

6.7 SUMMARY

Defect signal amplitude is a function of defect length, depth and closure (if a crack). Signal phase is primarily a function of defect depth. For volumetric Inspection of thin material the following test frequency should be used:

f = 1.6 p/t2 , kHz (5.7)

where p is electrical resistivity, microhm-centimetre, and t is wall thickness, mm.

At this frequency there is good discrimination between defects and lift-off signals but not between defects and ferromagnetic signals. Magnetic indications can be distinguished from defects by retesting at reduced frequency. Defect signals rotate CCW (approaching 0") whereas ferromagnetic signals rotate CW (approaching 90°) relative to lift-off signals.

There are few national standards governing eddy current inspections with surface probes. For effective inspection, a calibration block should simulate the test piece and contain appropriate surface and subsurface defects along with ferromagnetic inclusions. Basic knowledge of phase lag and impedance diagrams is also required for reliable analysis of eddy current indications. -105-

CHAPTER 7 TESTING OF TUBES AND CYLINDRICAL COMPONENTS

7.1 INTRODUCTION

Tubes or rods up to about 50 mm diameter can be inspected for defects with encircling coils. Defect sensitivity in larger diameter components decreases because the inspected volume increases while defect "volume" remains the same for a given defect. For larger diameters, surface probes should be used to obtain higher defect sensitivity, see Chapter 5.

The components can be in the form of wire, bars or tubes and round, square, rectangular or hexagonal in shape, as long as appropriate coil shapes are used. Inspection is fast and efficient since an encircling coil samples the complete circumference of the component, allowing 100% inspection in one pass.

Defect detectability depends on disruption of eddy current flow. Therefore, the best probe is the one which induces highest possible eddy current density in the region of material to be inspected, and perpendicular to the defect.

When planning an inspection, the following questions must first be answered: - For what type of defects is the inspection to be performed? - If cracks are expected, do they have directional properties? - Does the material or components in close proximity have ferromagnetic properties? Once these questions have been answered one can decide on suitable probe design, test frequency and calibration standards. With the proper procedures one can discriminate between defect signals and false indications as well as determine depth once a defect is located. These procedures are based on a knowledge of impedance diagrams and phase lag.

7.2 PROBES FOR TUBES AND CYLINDRICAL COMPONENTS

7.2.1 Probe Types

Four common probe types for testing round materials are illustrated in Figure 7.1: (b) and (d) are differential probes, (a) and (c) show absolute probes. Each type contains two separate coils to satisfy AC bridge circuit requirements, which Is the typical mode of operation of most eddy current instruments, see Chapter 4. These bridges require matching coils on two separate legs of the bridge to balance, thus permitting amplification of the small impedance differences between the two coils. If the two coils are placed side-by-side, both equally sensing the test material, the probe is "differential". If one coil senses the test article, the other acting only as a reference, the probe is absolute. -106-

Figure 7.1(a) and (c) show effective designs for absolute probes; the piggy-back reference coil is separated from the test article by the test coil and therefore couples only slightly to the test article (fill factor<

CENTERING DISCS

TES' "OIL

REFERENCE COIL

(A> ENCIRCLING PROBE. ABSOLUTE (c) INTERNAL PROBE. ABSOLUTE (PIGGY-BACK REFERENCE) (PIGGY-BACK REFERENCE)

(D) INTERNAL PROBE, DIFFERENTIAL (B) ENCIRCLING PROBE, DIFFERENTIAL

Fig. 7.1: Tube Probe Types

Coll Size

The best compromise between resolution and signal amplitude is obtained when coil length and thickness equal defect depth. See Figure 7.2 for a labelled diagram of a probe cross section.

As a general guideline for tube inspection, coil length and depth should approximately equal wall thickness. However, to improve coupling,a rectangular cross section with thickness reduced to one-half the length can be used. For greater sensitivity to small near surface defects, coil length and thickness can both be reduced further. Unfortunately this will result in a decrease in sensitivity to external (far surface) defects.

Coil spacing, In differential probes, should approximately equal defect depth or wall thickness for general inspections. -107-

COIL SPACING *— COIL WIDTH '//A////

COIL THICKNESS _J D (AVERAGE COIL DIAMETER) TUBE-COIL CLEARANCE

Fig. 7.2 Probe Coll nomenclature

For increased sensitivity to near surface defects, spacing can be reduced at the expense of a reduction in sensitivity with distance from the coil.

Probe-to-tube clearance or gap should be as small as possible. In most internal tube inspections, a gap equal to half the wall thickness is common. A larger gap (smaller fill-factor or coupling) results in a small decrease in near surface defect resolution and a large decrease in signal amplitude for all types of defects.

7.2.2 Comparing Differential and Absolute Probes

Absolute probes with a fixed reference coil are essential to basic understanding. They enable study of all physical properties of a test article by plotting characteristic impedance loci. When an absolute coil signal is plotted as a function of distance (as the probe travels along a tube axis) dimensional variations and discontinuities can be separated. See the example of Figure 7.3(b). The signal is a function of effective cross-sectional area of eddy current flow, i.e., wall thickness in the case of tubes, and can be analyzed like a surface roughness trace with the extra advantage that subsurface flaws can be sensed. -108-

In tube testing with an internal coil, absolute probe signals from defects and supports are simple and uivdistorted; signals from multiple defects and defects under support plates are often vectorially additive. Differential probes have two active coils usually wound in opposition (although they could be wound in addition with similar results). When the two coils are over a flaw-free area of test sample, there is no differential signal developed between the coils since they are both inspecting identical material. However, when first one and then the other of the two coils passes over a flaw, a differential signal is produced. They have the advantage of being insensitive to slowly varying properties such as gradual dimensional variations and temperature: the signals from two adjacent sections of a test article continuously cancel. Probe wobble signals are also reduced with this probe type. However, there are disadvantages; the signals may be difficult to interpret, even to the extent of being misleading. Defect signals under support plates can be extremely complicated. The signal from a defect is displayed twice: once as the first coil approaches the defect and again for the second coil. The two signals form a mirror image and the signal direction from the first coil must be noted. If a flaw is longer than the spacing between the two coils only the leading and trailing edges will be detected due to signal cancellation when both coils sense the flaw equally.

i i SUPPORT PLATE POSITION

SECTION THROUGH TUBE SHOWING CORRODED AREA

DIFFERENTIAL COILS ABSOLUTE COIL (a)

B TRACE «ITH ABSOLUTE PROBE

KALI LOSS 1 COMPONENT

TRACE WITH DIFFERENTIAL PROBE WALL LOSS TI 1" COMPONENT

Fig. 7.3: Eddy Current Y-Channnel Recordings from a Brass Heat Exchanger Tube OP =26.9 mm, t=l.lmm, fgp =21 kHz -109-

An even more serious situation occurs with differential probes when the ends of a flaw vary gradually; the defect may not be observed at all. An example of this is shown in Figure 7.3; this brass heat exchanger tube suffered g neral corrosion as well as localized corrosion on either side of a support plate. The gradual upward trend of the Y-DISTANCE recording in Figure 7.3(b) shows the pronounced grooves at A and B are superimposed on an area of general wall thinning in the vicinity of the support plate. Note the response of a differential probe to the same defect in Figure 7.3(c). The differential probe senses the localized grooves but the Y-DISTANCE recording shows no indication of the gradual wall thinning which was apparent in Figure 7.3(b).

Table 7.1 compares advantages and disadvantages of the two probe types.

TABLE 7.1 COMPARISON OF ABSOLUTE AND DIFFERENTIAL PROBES

ADVANTAGES: DISADVANTAGES:

ABSOLUTE PROBES respond to both sudden and gradual - prone to drift from changes in properties and dimensions temperature instability combined signals are usually easy to - more sensitive to probe separate (simple interpretation) wobble than a differential show total length of defects probe

DIFFERENTIAL PROBES

not sensitive to gradual changes - not sensitive to gradual in properties or dimensions changes (may miss long immune to drift from temperature gradual defects entirely) changes - will only detect ends of less sensitive to probe wobble long defects than an absolute probe - may yield signals difficult to interpret

7.2.3 Directional Properties

When inspecting for defects, it is essential that flow of eddy currents be as perpendicular as possible to defects to obtain maximum response. If eddy currents flow parallel to a defect there will be little distortion of the eddy currents and hence little change in probe impedance.

The eddy current flow characteristics of circumferential internal or external probes are listed and illustrated in Figure 7.4. 110-

EDDT CURRENTS

EDDY CURRENTS FLOI IN CLOSED PATHS tOm CURRENT FLDHS PARALLEL TO EDDY CURRENT FLO* DIMINISHES TO LIMITED TO CONDUCTING MATERIAL COIL WINDINGS - NOT SENSITIVE ZERO AT THE CENTRE OF > SOLID ROD TO PURELY CIRCUMFERENTIAL CRACKS NO SENSITIVITY «7 CENTRE

/COIL

EOOY CURRENT FLOHS PARALLEL E!IO» CURRENTS CONCENTRATE NEAR THE TO TUBE SURFACE - NOT SENSITIVE SURFACE CLOSE TO THE COIL - DEPTH TO LAMINAR SEPARATIONS. OF PENETRATION IS CONTROLLED BY TEST FREOUENCI' Fig. 7.4; Directional Properties of Eddy Currents in Cylindrical Test Articles

In addition to considerations of eddy current flow direction, the following are important: Magnetic flux is not bounded by the tube wall but will induce eddy currents in adjacent conducting material, e.g. tube support plates in heat exchangers- Eddy current coils are sensitive to ferromagnetic material introduced into a coil's magnetic field. The ferromagnetic material need not be an electrical conductor nor need it form a closed path for eddy currents. - Eddy current coils are sensitive to all material variations that affect conductivity or permeability. 7.2.4 Probe Inductance The equations quoted in Section 5.2.3 to calculate inductance for surface probes are also used to calculate inductance of probes for testing tubes and cylinders. The important aspect of inductance is that probe impedance, which is a function of inductance, must be compatible with the eddy current instrument and signal cables, Jprobe = V- i where XL = 2 TT f L whan f is in hertz and L in henries and R is coil wire resistance in ohms. -111-

TABLE 7.2 ENCIRCLING OR INTERNAL COIL IMPEDANCE

D - 8.9 mm D - 12. 7mm D -15.9 mm D -19.1 mm D -22 ,2mm Hire Size 0 0 o 0 0

L - 6.1 uH L - 11 UH L - 15 UH L - 20 uH L - 25 uH 31 AWG N - 25 (0.23 mm) R - 0.3 £2 R - 0.4 n R - 0.5 a R - o.6 a R - o.7 a

L - 23 L - 42 L - 59 L * 77 L * 96 34 AWG N - 49 (0.16 mm) R = 1 R - 1.5 R - 2 R « 2 R ' 3

L - 64 L » 110 L - 160 L - 210 L - 260 37 AWG N - 81 (0.11 mm) R - 3 R - 5 R - 6 R = 8 R = 9

L »• 200 L = 360 L - 510 L = 660 L = 830 39 AWG N - 144 (0 .089 mm R = 9 R - 14 R - 18 R = 22 R = 26

L - 490 L - 880 L =•1.24 mH L = 1.62 mH L - 2.0 2 mH 41 AWG N = 22? (0.071 mm) R = 24 R - 35 R = 45 R * 55 R = 64

Most eddy current instruments will operate over a fairly broad range of probe impedance without a substantial reduction in signal-to-noise ratio or signal amplitude. An instrument input impedance of 100 ohms is typical, although a probe impedance between 20 and 200 ohms is normally acceptable, unless the test frequency is too close to probe-cable resonance frequency, see Section 7.2.5. Exact probe inductance calculations are therefore not essential.

To facilitate impedance calculations Table 7.2 has been prepared. This table lists coil inductance and resistance (with probe in air) for various diameters and wire sizes while keeping coil cross section constant at 1.2 mm x 1.2 mm. (These dimensions are fairly typical of tube wall thickness in heat exchangers). With the aid of this table, and knowledge that inductance is proportional to the square of number of turns and the square of mean coil diameter (L a N^ ),one can usually make a reasonable estimate of wire size and number of turns for a particular probe. -112-

7.2.5 Probe-Cable Resonance Probe-cable resonance must be considered when operating at high test frequencies and/or using long signal cables, e.g. frequencies over 100 kHz or cables longer than 30 m. Most general purpose eddy current instruments cannot operate at or close to resonance.

Probe-cable resonance can be modelled as shown in Figure 4.5. In simple terms, resonance occurs when inductive reactance of the coil equals capacitive reactance of the cable, i.e. when coL « 1/wC

where to is angular frequency, radians/second L is coil inductance,henries C is total cable capacitance, farads

Transposing this equation and substituting

This approach is sufficiently accurate for most practical applications. A more rigorous approach to resonance is presented in Section A.3.

Resonance is apparent when a probe and cable combination, which balances at a low frequency, will not balance as frequency is increased. At the approach of resonance, the balance lines on the eddy current storage monitor will not converge to a null. The two balancing (X and R) controls will produce nearly parallel lines, rather than the normal perpendicular traces, on the storage monitor. A number of steps can be taken to avoid resonance:

1. Operate at a test frequency below resonance, such that ffest is less than 0.8 fr . 2. Select a probe with_lower inductance. (Since fr is proportional to 1//L , inductance must be decreased a factor of four to double the resonant frequency). 3. Reduce cable length or use a cable with lower capacitance per unit length (such as multi-coax cables). This will raise the resonance frequency since capacitance is proportional to cable length and f is proportional to 1/-TC , r 4. Operate at a test frequency above resonance, such that ftest is greater than 1.2 fr. However, above resonance the sensitivity of all eddy current instruments decreases rapidly with increasing frequency because capac^tive reactance (X;. = 1/ :•' C) decreases, and current short circuits across the cable rather than passing through the coil. -113-

7.3 IMPEDANCE PLANE DIAGRAMS Eddy current probes for testing cylindrical components differ mechanically from those for plate testing, but coil impedance can be treated similarly for both test coil configuraLions. The impedance display treatment introduced in Chapter 5 applies for internal and external circumferential coils with the following changes:

i) Lift-off becomes "fill-factor". Fill factor is a measure of coupling between the coll and test object. In general, it is the fraction of magnetic field that crosses the test object; for a long coil, this is the fraction of the test coil area filled with test material. Fill-factor, r\ (eta), is the ratio

for an encircling coil,

2 and n = D /D* (7.1b) for a bobbin type internal coil,

where I)o is cylinder diameter D is average coil diameter and Dt is tube internal diameter Fill-factor is always a quantity less than or equal to one (n < 1.0). For a coil inside a tube the impedance change due to decreasing ri is the same as an increase in Di (with constant wall thickness). For a coil around a tube or cylinder, decreasing r; is the same as decreasing DQ. ii) Probe diameter in plate testing is replaced by tube or cylinder diameter in ET of cylindrical components. They have a similar effect on the operating point on the impedance diagram.

Figure 7.5 summarizes the effect of test and material variables on a simple semicircular impedance diagram. Note the similarity of changes in resistivity, test frequency, diameter and fill-factor with the surface probe results of Figures 5.9 to 5.13. -114-

DECREASING FILL-FACTOR

INCREASING RESISTIVITY (p) COIL THIN -HALL TUBE

INCREASING FREQUENCY (f) and DIAMETER

0.2

0.2 0.4 0.6 NORMALIZED RESISTANCE

Fig. 7.5: Simplified Impedance Diagram of a Long Coil Around a Non-magnetic Thin-wall Tube Showing Effect of Test and Material Variables

Impedance diagrams presented in the literature are often only strictly valid for long coils (much longer than material thickness), coil lengths for inspection are normally only a fraction of material diameter. Decreasing coil length has an effect similar to decreasing fill-factor, it causes the impedance diagram to be smaller than expected (but similar in shape) fron coil and test material geometry. Following sections will present impedance diagrams for tubes and solid cylinders. For simplicity a fill-factor of unity will be used. -115-

7.3.1 Solid Cylinders

The impedance diagram for a solid cylinder (diameter, Do) inside a long coil is shown in Figure 7.6. As in Figure 7.5 an increase in test frequency or diameter moves the operating point (the point on the impedance diagram that specifies the normalized inductive reactance and resistance of the test coil) down the curve while an increase in resistivity moves it up the curve. This diagram applies to both wires and round bars.

COIL

INCREASING RESISTIVITY

DECREASING FILL-FACTOR

INCREASING FREQUENCY

NORMALIZED RESISTANCE

Fig. 7.6: Impedance Diagram for a Solid Cylinder

The shape of impedance diagrams for cylinders differ markedly from a semicircle, particularly at higher test frequencies. The shape difference is due to skin effect and phase lag, factors which were not included in arriving at the semicircular shape in Chapter 3. At high test frequencies the curve approaches the X and V axes at 45°. In testing cylinders with an encircling coil it should be recognized that sensitivity to defects at the centre of bar or wire is zero, regardless of test frequency. The reason for this is illustrated schematically in Figure 7.7 which -116-

LOH FREQUENCY 8 > 3. 4

INTERMEDIATE FREQUENCY S = — 4

Fig. 7.7: Schematic of Eddy Current Distribution in a Cylinder Surrounded by an Encircling Coil

shows plots of eddy current density across a cylinder. Defects have to disrupt eddy current flow in order to affect probe impedance. It is apparent from Figure 7.7 that eddy current density is always zero at the centre of a cylinder resulting in no sensitivity to defects.

7.3.1.1 Sensitivity in Centre of a Cylinder It was stated in the previous section that eddy current density in the centre of a cylinder is zero and hence there is no sensitivity to defects. The relationship of current flow with depth Into a cylinder is derived (very approximately) below, for the case of no skin depth attenuation and long coils. From Faraday's Law,

s dt

The magnetic flux density, B, is approximately constant inside a long coil, hence

= BA = (B)(TTr2)

where r is' radial distance from centre of cylinder; -117-

theref ore,

Resistance to flow of current Is proportional to path length and resistivity and inversely proportional to cross-sectional area, Ac, 2irrp _ 2frrp R unit length x unit depth

Vs by Ohm1s Law Since Z

2 fi (

Therefore, eddy current flow is proportional to radial distance from centre of a cylinder. Hence no current flows at the centre (at r*0) and there is no sensitivity to defects. -118-

7.3.2 Tubes The impedance diagram for an extremely thin-wall tube with either an internal or external circumferential coil is a semicircle. This shape is only obtained when wall thickness, t, is much less than skin depth (t<<6 ), i.e. skin effect and phase lag are negligible. This situation will rarely be encountered in practice, especially at intermediate and high test frequencies, but the concept is useful since it defines one of the coil impedance limits. With an external coil the other limit is defined by the impedance curve for a solid cylinder (maximum possible wall thickness). The impedance diagram for any tube tested with an external coi1,hence,has to lie between the two broken curves in Figure 7.8, for example the solid line applies to

M.ENCIRCLING COIL

CYLINDER (0; = 0)

TUBE

THIN »*LL (DJKD,,)

DECREASING WALL THICKNESS

NORMALIZED RESISTANCE

Fig. 7.8: Impedance Diagram for a Tube with Encircling Coil Showing Effect of Decreasing Wall Thickness

a tube with internal diameter 80% of the outside diameter i.e., D^/DQ = 0.8. Tubes with D^/VQ greater than 0.8 would lie to the right of the solid line. The dotted lines in Figure 7.8 trace the shift in operating point as wall thickness decreases (DQ constant, D^ increasing). Note the spiral shape of the wall thickness locus. The thick wall end of the curve deviates from a semicircle locus. -119-

This is attributed to phase lag across the tube wall and forms the basis for eddy current signal analysis which will be treated in detail in Chapter 8. Figure 7.8 also illustrates the dependence of the terms "thick-wall" and "thin-wall" on test frequency. Near the top of the diagram (low frequency) a tube with Dj/DQ = 0.8 qualifies as thin wall, there is no phase lag across the tube wall, t <<6. Near the bottom (high frequency) the same tube becomes thick-wall because thickness becomes much greater than skin depth, for eddy current purposes the tube now appears as a solid cylinder.

When a tube is tested with an internal circumferential coil the impedance diagram for a thin-wall tube remains semicircular but that for a thick-wall tube differs markedly from a solid cylinder; compare Figures 7.8 and 7.9. The

THICK WALL TUBE (Dj«D0)

TUBE (0|/D0 = O.B)

TUBE (0/D0 = 0.9)

THIN HALL (DiS!00)

DECREASING WALL THICKNESS

NORMALIZED RESISTANCE

Fig. 7.9: Impedance Diagram for a Tube With Internal Coil Showing Effect of Decreasing Wall Thickness impedance locus for any given tube will again fall between the dashed curves at intermediate frequencies and approach the thin-wall curve at low frequency and the thick-wall curve at high frequency as shown for tubes with D^/Do = 0.8 and 0.9. As in the previous figure, a change in wall thickness produces a coil impedance change along the dotted lines tracing a spiral shaped curve. Again, this departure from a semicircle is attributed to phase lag across the tube wall. -120-

7.3.3 Characteristic Frequency for Tubes Section 5.6 described how the Characteristic Parameter Pc= "r^wya , introduced by Deeds and Dodd, enabled presentation of the effects of changes in T, a) , y and a on a single impedance diagram. This allowed test coil impedance to be specified in terms of a single quantity rather than four independent variables. One could use this parameter in testing cylinders and tubes. However, most eddy current literature refers to a similar variable, the characteristic or limit frequency, fg usually attributed to Forster. It differs from P because probe radius, F, is replaced with tube or cylinder dimensions.

By definition, fg is the frequency for which the Bessel function solution, to Maxwell's magnetic field equations for a finite test object, equals one. (Bessel functions are similar to, but more complex than trigonometric sine and cosine functions). For a solid cylinder or thick-wall tube tested with an encircling coil,

f - 5.07p g n2 , kHz prDo (7-2a)

with P in microhm-centimetres and Do in millimetres. For a thick-wall tube with an internal coil, f = 5..- °lP- , kHz

For a thin-wall tube with internal or external circumferential coils,

kHz (7.2c)

The ratio f/f„ defines the operating point on impedance diagrams. For non-magnetic materials (nr=l), frequency ratio for cylinders and thick-wall tubes tested with external coils is given by

f/f = fD2/5.07p 8 ° (7.3a)

where f is test frequency in kilohertz. -121-

For a thick-wall tube tested with an internal coil, f/f = fD7/5.O7p (7.3b)

For thin-wall tubes tested with internal or external coils,

f/f = fD t/5.O7p ± (7.3c)

i.o

i o\ THICK-HALL TUBE (INTERNAL COIL)

f/tg = fO,V 5 07^5 2 o\ 16.0 \ 3.0 4 D SOLID CYLINDER (EXTERNAL COIL) ! f/le = fD0 /5 07p 5 0 /9.0 Via

0.4 - THIN-MLL TUBE "7 (INTERNAL ( EXTERNAL COILS) f/fE = fOjt /5.D7/O

0.2 - I" r

4 0

0.2 0.4 0.G

NORMALIZED RESISTANCE

Fig. 7.10: Impedance Diagrams for Tubes and Rods with Long Coils and Unity Fill-factor Showing Variation of Along Impedance Loci Figure 7.10 shows impedance diagrams for thin-wall tubes, solid cylinders and thick-wall tubes with values of f/fg (from 0 to infinity) on the curves. The impedance plots are both different Jn shape and have drastically different f/f_ ratios. For example, at the "knee" in the curves a i ^ ^« _ 1. t___ fir* ^ i* __ _ _ -m * * r M W S" • thin-wall tube has f/fg =1, for a cylinder f/fg-a and a = thi^k-wall tube has f/rg *• These differences originate 2 in the defining equations which contain Do , D/* and Dit. To find the operating point on an impedance diagram using frequency ratio one has to know the geometry (tube or cylinder). For tubes which do not satisfy the conditions for -122-

either thin or thick wall, calculation of f/fg is not possible except near the top and bottom of impedance diagrams where curves for intermediate wall tubes converge with the thin-and thick-wall curves, respectively.

In addition to defining operating point, frequency ratio can also be used for extrapolation or scale modelling; using the similarity condition. This condition states if two objects have the same f/fg then eddy current distribution is identical in each. Hence if test frequency f^ meets test requirements for article No. 1, one can calculate f2 for article No. 2 from the following:

For cylinders, 2 flDolp2 =

for thin-wall tubes,

and for thick-wall tubes (internal inspection),

f D P £ D2 l U 2 " 2 i2Pl

7.3.4 Computer Generated Impedance Diagrams As indicated in the previous section, exact analytical solutions (Bessel function solutions) for impedance loci of test coils around or inside tubes are only possible for limiting cases. These solutions have the additional drawback that they are only strictly true for long coils. An alternative was made available by C.V. Dodd and his co-workers(j^) at Oak Ridge National Laboratories. Th°y developed computer programs to calculate coil impedance. These are valid for all coil lengths, internal and external coils and all tube wall thicknesses. Such computer programs permit paper experiments to define operating point as well at the effect of variations in coil size and shape, resistivity, wall thickness and test frequency.

Figure 7.11 is an example of computer generated impedance display for a short internal coil in an Inconel 600 tube at various test frequencies. Fill-factor and the effects of small changes in resistivity (Ap), wall thickness (At) and magnetic permeability (Ap) were examined at each frequency. Note the similarity with the impedance plots of Figure 6.5 obtained for a surface probe. The angular (phase) separation between fill-factor, Ap , At and A]J provides the basis for eddy current signal analysis which will be treated in Chapter 8. -123-

i.ook-

i.04 0 OB Q.1

N0RU1U2EO RESISTKNCE

Fig. 7.11: Computer Simulation of Probe Response to Various Test Parameters 7 .4 CHOICE OF TEST FREQUENCY

Test frequency is often the only variable over which the inspector has appreciable control. Material properties and geometry are normally fixed and probe choice is often dictated by test material geometry and probe availability. Choice of a suitable test frequency depends on the type of inspection. Testing for diameter variations normally requires maximum response to fill-factor which occurs at high frequencies. Testing for defects requires penetration to possible defect locations; surface defects can be detected at higher frequencies than subsurface defects. Maximum penetration requires a low frequency which still permits clear discrimination between signals from harmless variations in material properties and serious defects. The above factors show choice of test frequency is usually a compromise.

7.H . i Test Frequency for Solid Cylinders As discussed in Section 7.3.1, the sensitivity at the centre of a cylinder, with an encircling coil, is zero at all test frequencies. Therefore, there is no advantage in using a very low test frequency to increase penetration. Maximum test sensitivity is obtained when the impedance diagram operating point is near the knee of the curve. This -124-

= condition occurs when f/fR 6. At this point balanced sensitivity to defects, resistivity and dimensions is obtained. At this test frequency, Do/<5 f^3.5. Increasing the frequency ratio f/fg to 15 or 20 improves discrimination between surface defects and fill-factor variations (probe wobble), at the expense of reduced sensitivity to subsurface defects. Maximum sensitivity to diameter variations is obtained at higher test frequencies, f/fg = 100 or more.

A frequency ratio lower than 6 will result in a decrease in phase lag and therefore less phase discrimination between defects and fill factor. To distinguish between ferromagnetic variations (or inclusions) and defects, the operating point should be on the top quadrant of the impedance diagram. A frequency ratio of approximately two (f/f_ = 2) would achieve this.

7.4.2 Test Frequency for Tubes When inspecting tubes for defects, the criterion to satisfy is (a) phase discrimination between defect signals and other indications and (b) good phase separation between internal and external defect signals. A test frequency, proven in practice on many types and sizes of tubes, is the frequency f OQ which yields 90° phase separation between fill-factor variations (and internal defect signals) and external defect signals. The frequency f 90 is empirically derived from the ratio between thickness and skin depth, slightly larger than one, t/6 = 1.1

and converts to 2 f90 = 3p/t kilohertz (7.4)

where p is resistivity in microhm-centimetres and t is tube wall thickness in millimetres. This equation is valid for both internal and external coil inspection and is roughly independent of tube diameter. At f 90 , there is good sensitivity to internal and external defects and little sensitivity to magnetite deposit and ferromagnetic support plates.

The characteristic frequency ratio f/fg cannot be used to satisfy the criterion of phase discrimination, because the fg equation is not a function of phase lag. It would also be wrong to use it for defect detection because it is a function of tube diameter. The latter would require different test frequencies for different diameter tubes to keep f/fg constant. -125-

If one desires to distinguish ferromagnetic signals from other indications, the operating point should be on the top quadrant of the impedance diagram for thin-wall tubing, Figure 7.10. This point is located by calculating th test frequency to make the characteristic frequency ratio equal to or less than 0.5 (f/fg <0.5). Inspection Standards and Specifications

A number of industrial codes cover eddy current tube inspection. The various ASTM specifications are E-215 (aluminum alloys), E-243 (copper and copper alloys), E-426 (stainless steels.) and E-571 (nickel alloys). None of the ASTM standards specify test frequencies, they sometimes present normal ranges such as 1 to 125 kHz for aluminum alloys. Such numbers are of little use in deciding on a suitable test frequency for a particular test. The ASME Boiler and Pressure Vessel Code, Section V, Article 8 (1980) specifies test frequency in terms of the angle between through-wall and external defect indications from a calibration tube. The procedure specified will normally yield a frequency higher than fo,Q , perhaps as high as

Most calibration tubes consist of drilled holes of various diameters and/or various depths from the external surface. Some calibration tubes have EDM (electric discharge machining) notches in the circumferential and axial directions and on both internal and external surfaces.

7.5 PROBES FOR DETECTING CIRCUMFERENTIAL CRACKS A conventional internal circumferential (bobbin) probe induces a flow of eddy currents parallel to the coil windings and therefore circumferential in direction (Figure 7.4). To sense a defect, coil impedance must change; this will occur only if the eddy current flow path is disturbed. Circumferential defects parallel to this current, which present no area perpendicular to this path, will therefore not be sensed.

(a) (b)

Fig. 7.12: (a) Probe No. 1- Multi-pancake Coil Probe (b) Probe No. 2 - Zig-zag Coil Probe -126-

To detect circumferential defects the coil must induce currents at an angle to the cracks. Two possible types of probes are (a) surface probes and (b) zig-zag probes. Figure 7.12 shows examples of such probes. The surface probe induces currents in a circular pattern whereas the zig-zag probe induces currents to follow the 30° coil angle. The probes shown in Figure 7.12 are differential. In the surface probe configuration a multi-coil array is used; the four surface coils in each row are connected in series and the two rows are connected differentially. A single absolute surface coil can also be used, provided the probe maintains contact with the tube surface by spring force or other means (otherwise lift-off noise would be intolerable). See Figure 7.13 for the cross section of a typical spring-loaded internal probe for tube testing.

TEST COIL PLASTIC BODY

REFERENCE SPRING COIL

Fig. 7.13: Spring Loaded Internal Surface Probe for Tube Inspections

A single surface probe is unquestionably the easiest to use; signal analysis is discussed in Chapter 6. The main disadvantage is the partial circumferential coverage; multiple passes or helical scanning are necessary for 100% coverage. Another disadvantage of the surface probe configuration (single or multiple) is the loss of sensitivity with distance from the coil. If surface coils are small, as will be the case for most tube inspections, the reduction in sensitivity with distance from the surface will be greater than with circumferential coils, see Section 5.3.1. The sensitivity to small localized defects originating from the outside surface could be as much as 10 times lower than the sensitivity to internal defects. A zig-zag coil has less attenuation to outside defects, it falls into the circumferential class in this respect. Neither zig-zag nor surface coil probes will give uniform sensitivity around the'r circumference. There will be peaks of maximum and minimum sensitivity depending on the angle between eddy -127-

current path and defect orientation. This can best be visualized by considering a short circumferential crack passing over the coils: there will be areas, such as at the peaks of the zig-zag, where eddy current flow is almost parallel to the crack, resulting in poor sensitivity.

Figure 7.14 shows examples of signal response to real circumferential fatigue cracks with the probes discussed above.

(a) MULT I -PANCAKE COIL PROBE

(b) ZIG-ZAG COIL PROBE WMr

-/vv'-J'vr—-—*"~'s-

Fig. 7.14: Eddy Current Scans of Circumferential Cracks in Inconel Tubing (Signal Amplitude Normalized to a 1.6 mm Diameter Through Hole), x - 400 kHz. -128-

7.6 SUMMARY

Test coils induce eddy currents and also sense the distortion of their flow caused by defects. Encircling or bobbin probes have test coil(s) mounted with their axes parallel to the tube or rod axis. Since the coils are wound circuraferentially the induced eddy currents also flow circumferentially. They cannot be used to detect circumferential cracks, laminar defects, nor defects in the center of a rod.

As a general guideline for tube inspection, probe coil length, depth, and spacing (If differential) should approximately equal wall thickness. An absolute bobbin probe (single test coil) should be used for general in-service heat exchanger inspection. However, for short localized defects, differential probes (two test coils side-by-side) are normally preferred. Analysis of eddy current signals is the most important and unfortunately the most difficult task in a successful inspection. A thorough understanding of impedance diagrams and effect of phase lag is needed to manipulate test conditions to minimize undesirable test variables. The Characteristic Frequency for tube inspection is used to locate the operating point on the impedance diagram. It is given by

f = 5.07p/Dt kHz (7.2c) g where p is electrical resistivity and D is tube internal diameter (for bobbin probe) and external diameter (for encircling probe); t is tube wall thickness.

One needs to know the operating point on the impedance diagram to determine effects of fill-factor, electrical resistivity, and magnetic permeability. The optimum sensitivity to fill-factor is near the bottom of the impedance diagram, in the middle for electrical resistivity and at the top for magnetic permeability. When inspecting tubes for defects, criteria to satisfy are (a) phase discrimination between defect signals and other -129-

indications and (b) good phase separation between internal and external defect signals. For general purpose testing the frequency given by

3p/f kHz (7.4)

is used where t is wall thickness in mm. This frequency yields 90° phase separation between internal and external defect signals and little sensitivity to magnetic deposits and ferromagnetic support plates.

Special probes are needed to inspect for circumferential cracks or defects close to tubesheets. Single, spring loaded, surface probes are effective. 7.7 WORKED EXAMPLES

7.7.1 PROBLEM: Calculate frequency to operate at the knee location of the impedance diagram for a cylinder 5 mm in diameter and electrical resistivity p =• 10 microhm-centimetres.

f D SOLUTION: o 5.07P = 6 (7.3a)

6 x 5.07 x 10 therefore f = 52 12 kHz

7.7.2 (a) Calculate the test frequency to inspect Inconel PROBLEM: 600 tubing with Dj = 10.2 mm, t = 1.1 mm and p = 98 microhm-centimetres.

SOLUTION: Best test results are obtained when there is sufficient phase separation between internal and external defect signals. A phase separation of 90° allows good discrimination between the two and reasonable defect depth estimates. To achieve 90° phase separation, the test frequency is determined by

(7.4) 90 t2 (derived from t/6 =1.1)

= 3 X 9* = 245 kHz (1.1)

Therefore 245 kHz is the required frequency. -130-

7.7-2 (b) Determine the approximate operating point on the PROBLEM impedance diagram, for problem (a). SOLUTION: Since t/<5 = 1.1 this tube cannot be considered thick or thin walled. Therefore, neither equation 7.2(b) nor 7.2(c) is strictly valid. However, for t/6 > 1, equation 7.2(c) for thick-wall tubing yield an approximate solution.

f/fg = fD^/5.07 (7.3c) = 245 x (10.2) /5.07-x 98 = 51.3 This would place the operating point on the lower quadrant (much lower than the knee location) of the thick-wall curve of Figure 7.10.

7.7.2 (c) Calculate a test frequency for the above tube PROBLEM suitable for discriminating between ferromagnetic inclusions and defects, when testing with an internal probe.

SOLUTION: The operating point should be on the top quadrant of the impedance diagram for thin-wall tubing, Figure 7.10. This point is located by calculating the test frequency to make the ratio of Forster's characteristic frequency equal to or less than 0.5.

f/f (7.3b)

0.5

therefore

f = (0.5)(5.07p)/Dit

0.5 x 5.07 x 98/10.2 x 1.1 = 22 kHz

Therefore, at 22 kHz (9% of f9Q ), it should be possible to discriminate between defects and ferromagnetic indications. -131-

CHAPTER 8 - TUBE TESTING - SIGNAL ANALYSIS

8.1 INTRODUCTION Manufacturing and in-service inspection of tubes is one of the most important applications of eddy current testing. For in-service inspection of small-bore tubing in particular, eddy current is by far the most frequently used method. Access is usually limited to tube ends which makes other NDT techniques difficult or impossible to apply.

This chapter emphasizes in-service testing of tubes using internal probes. This approach is taken because testing of solid cylinders and tubes with external coils (manufacturing inspection) is generally less complicated. If the reader understands in-service inspection he should encounter no problems applying similar principles to other test s ituations.

Reasons for the appearance of impedance plane eddy current signals are presented first. Repetition from previous chapters is intentional, it was desired to keep this chapter as independent as possible without excessive cross- referencing. Discussion of simple defect indications is followed by superimposed signals which are frequently encountered during in-service inspection such as defects at baffle plates and tubesheets. A section dealing with surface probe internal tube inspection is included, difficult test situations have been resolved with this technique. Signals which could be mistaken for real defects (anomalous indications) are the subject of another section. The chapter concludes with a discussion of multifrequency testing, including its advantages and limitations.

An attempt is made throughout this chapter to illustrate discussion with real or simulated eddy current defect signals. 8.2 EDDY i. RRENT SIGNALS 8.2.1 Defect Signal Characteristics

A defect, which disrupts eddy current flow, changes test coil impedance as the coil is scanned past the defect. A non- rigorous derivation of this effect can be obtained using Figure 8.1 which portrays eddy currents induced in a tube with either an internal or external coil. Consider a unit length of tube as being the secondary winding of a transformer (similar to treatment in Chapter 3). The resistance of a conductor of length &, cross-sectional area A and resistivity p is R = £p/A, ohms -132-

Without a defect, resistance around this tube is

2irrp/t (8.1a)

Introduction of a long defect, of depth h, which constricts eddy current flow over the distance A0 (in radians) , increases total resistance to

R = 2frrp/t + A0hrp/t(t-h) (8.1b) or R Ro (defect free resistance) + AR (due to defect)

Fig. 8.1: Schematic Illustration of Eddy Currant Distribution Around a Defect in a Tube

A short defect will also increase resistance but by a smaller AR since current can flow both under and around it. Note that it is width of affected zone, A6 , rather than actual defect width which determines effect of the defect on resistance. In summary, the above argument illustrates that defect length, depth and width (to some extent) all increase resistance to current flow and hence defect signal amplitude. In terms of the equivalent coil circuit of a resistor in parallel with an inductor and its associated semicircular impedance diagram (Chapter 3), a defect moves the operating point up the impedance diagram. Increasing resistanc in a specimen changes both probe inductance and resistance.

The above discussion does not predict a defect signal in detail, only its approximate amplitude and direction on the impedance diagram. A more complete explanation requires inclusion of phase lag. Consider an absolute coll around a cylindrical sample as in Figure 8.2(a). (The treatment for a differential coil would be similar but more complicated because the twin coil configuration generates two mirror image signals and cross-coupling between the two coils causes further complications). Figure 8.2(a) shows the distribution of magnetic field amplitude and phase around a coil as derived by Dodd(^). The solid lines are contours of constant magnetic field strength; the dashed lines are constant phase. Since magnetic field and induced eddy currents have about the same phase, the dashed lines also represent the phase of the eddy currents. Similar diagrams could be derived for coils inside or around tubes. Amplitude drops off exponentially with distance and eddy current flow increasingly lags in phase (relative to eddy currents adjacent to the coil) both with depth and with axial distance from the coil. Skin depth effect occurs in both radial and axial directions.

Figure 8.2(a) permits derivation of eddy current signals for the surface, subsurface and deep defects illustrated. One needs to establish a reference phase direction as starting point, the fill-factor direction is convenient and can be defined as the signal resulting from a very shallow surface defect which only decreases coupling without changing phase lag distribution. Hence choosing the phase contour which just touches the surface under the coil as the 0° contour fixes fill-factor direction as in Figure 8.2(b). The signal or effect of defects can be imagined as the absence of eddy currents which were flowing In the area before the defect existed at this location. On moving the coil (or defects past the coil) from positions 0 to 5 in Figure 8.2(a), one observes the change in amplitude and phase sketched in Figure 8.2(b). This procedure is reasonably straight forward for the surface and subsurface defects since they are localized and only intersect one phase and amplitude contour at any given position. For the deep defect, one has to divide the defect into sections and determine weighted average values for amplitude and phase at each position. -

The surface defect in Figure 8.2(b) has a large fill-factor component, primarily its approach signal makes it distinguishable from fill-factor. As defect depth increases, signals rotate clockwise due to increasing phase angle. -134-

CWSTIHI HIPLITUOE

SUBSURFACE DEFECT 2 3 » 5

DEFECT POSITION (a)

SUBSURFACE DEFECT (X2> (b)

FILL -FACTOR

Fig. 8.2: Derivation of Eddy Current Signal Appearance for Three Types of Defects -135-

The angle between fill-factor and defect signals in Figure 8.2(b) is about 2 3 , where 3 = x/<5. Although probably not strictly true, one can imagine defect signal phase angle as the sum of a lag of & from the coil to the defect and the same lag back to the coil.

8.2.2 Effect of Test Frequency We can now combine Figure 8.2 results with impedance diagrams from Chapter 7 to illustrate the effect of test frequency on defect signal appearance. Figure 8.3(a) shows part of Figure 7.9, the impedance diagram for a tube with D^/Do = 0.8 tested with a short internal coil. The dotted lines trace the impedance change with decreasing DQ. An external defect (0D defect) in a tube is essentially a decrease i n DQ with Dj held constant, therefore the dotted lines trace the change in impedance as a coil is scanned past an 0D defect. Note the similarity between the subsurface defect in Figure 8.2(b) and the 0D defect at 2 f90 in Figure 8.3(a). The display is normally rotated counter-clockwise to make a signal from fill-factor approximately horizontal. This is achieved by rotating the phase control knob on the eddy current instrument.

NORMALIZED RESISTANCE

Fig. 8.3(a): Relation Between Impedance Diagram and Defect Signal Appearance -136-

With this phase setting and at fgo an OD defect shows wall loss (+Y) in a tube without a change in fill-factor as in Figure 8.3(b). An ID defect consist- of wall loss (+Y component) as well as a large fill-factor (-X component) because of decreased coil/tube coupling. The through-wall defect (hole) signal contains elements of both ID and OD defects and hence yields a signal which falls between the two. Note that all defect signals must fall between decreasing fill-factor and OD defect signals.

OD DEFECT THROUGH-WALL -X -*• +x DEFECT 10 DEFECT

DECREASING FILL FACTOR -Y

Fig. 8.3(b); Defect Signal Appearance at

Figures 8.3(a) and 8.4 show what happens to defect signals with changing test frequency. Reduced frequency results in rotation of defect signals towards the fill-factor direction. At very low frequencies (less than f go/4) signals from different types of defects become difficult to distinguish due to small phase angle separation. -137-

Increasing test frequency increases phase separation between ID and OD defect signals as predicted by phase lag. At fgQ the ID and OD defect signals are separated by about 90° with low sensitivity to tube supports and external deposits. At higher test frequencies, 2 fgo and above, higher sensitivity to probe wobble and dents is obtained and the increased •angular separation of defect signals makes it difficult to discriminate between OD defects and probe wobble or fill- factor variations, see Figure 8.4(c).

c i fi. 8 . i Appearance of Calibration Defect Signals at Different Test Frequencies -138-

8.2.3 Calibration Tubes and Simple Defects Both manufacturing and in-service inspection require calibration tubes with artificial defects for initial instrument set-up and subsequent signal analysis and interpretation. These tubes should be identical in material and size to tubes to be tested. Minimum calibration requirements include ID, OD and through-wall defects (see also the ASTM and ASME codes cited in Section 7.4.2). For in-service inspection, expected signal sources such as baffle plates, magnetite deposits and dents are useful and often essential for reliable signal analysis. Figure 8.5 shows typical signals, at fgo , from a calibration tube suitable for in-service heat exchanger inspection. Both absolute and differential probe signals are shown. The 90° phase separation between ID and OD defects also exists for differential probes. Note the similarity with the signals derived in the previous section.

STEEL SUPPORT PLATE

OUTSIDE INSIDE THROUGH GROOVE GROOVE HOLE u • " f" i m 12.7 mm

P - 98 ufl- cm

H0,LE OUTSIDE

DECREASING FILL FACTOR

ABSOLUTE DIFFERENTIAL

Fig. 8.5: Eddy Current Signals from a Typical Calibration Tube. Test Frequency fqn - 250 kHz. Qualitative reasons for the appearance of ID, OD and through- wall defects were presented in Section 8.2.2. The other signals in Figure 8.5 can be explained in a similar fashion. Magnetite is a ferromagnetic non-conductor, its signal is due -139-

to its high permeability. As indicated in Figure 7.11 increasing permeability of tube material yields a signal which falls between OD and through-wall defects. The magnetite signal in Figure 8.5(b) is essentially such a signal rotated about 90° clockwise because of phase lag across the tube wall. A dent places tube material in closer proximity to the coil resulting in improved coupling (increased fill-factor) and hence yields a signal opposite to decreasing fill-factor. Probe wobble yields a signal very close to the fill-factor direction because radial displacement of the coil reduces the coupling to the tube. The reason for baffle plate signal appearance is due to a combination of factors. For carbon steel baffles, the effects of high magnetic permeability and intermediate resistivity partially cancel resulting in small signal amplitude. Phase lag across the tube wall rotates this signal clockwise.

5« ID GROOVE I0S OD GROOVE 1.6 mm 0.25 mm 2.5 m WIDE 2,5 mm WIDE HOLE DENT CARBON STEEL J SUPPORT

V CHANNEL A \r

DISTANCE

I CHANNEL

Fig. 8.6: Appearance of Quadrature Components or. a Chart Recording for a Calibration Tube

In eddy current tube testing one normally records the quadrature components (vertical, Y; horizontal, X) of coil impedance on a two-channel strip chart recorder as shown in Figure 8.6. With phase adjusted as shown, any real defect will exhibit a Y component. The X-channel information is required for detailed signal analysis to decide type and depth of defects which -140-

can only be performed reliably through phase analysis. Accurate phase analysis can be done on-line by monitoring the signals on an eddy current instrument storage monitor. Alternatively an X-Y recorder or similar device permits hard- copy storage of quadrature signals. A flaw indication on an X-Y monitor is normally a curved locus; it does not have a simple and unique phase angle. If an absolute probe is used the significant angle to measure is the tangent angle at the defect signal tip, see Figure 8.7(b). If a differential probe is used, the phase angle is the slope of the straight line joining the end points of the "figure-8" signal, see Figure 8.7(c). Figure 8.7(a) illustrates the change in phase angle with defect depth. This curve should be used only as a guide since defect signal phase angle can change with defect and probe geometry.

I.D. O.D. O.D. DEFECT DEFECTS DEFECTS THROUGH HOLE

1.0. OEFECT

(b) ABSOLUTE THROUGH H0LE UJ o O.D. DEFECT

10 (c) DIFFERENTIAL SIGNAL PATTERN PHASE ANGLE (0), DEGREES

(a)

Fig. 8.7: Eddy Current Phase Angle/Defect Depth Calibration Curve at -141-

When an eddy current signal source is located it is often useful to retest at other frequencies to confirm a defect exists and/or to improve depth estimate. Defect depth is estimated from signal pattern recognition and verifier' by comparison with calibration defect signals at various test frequencies. Normally, frequencies of one-half and twice are sufficient. However, to check for magnetic deposits or inclusions a frequency of one-tenth fgg or less may be required (see Sections 7.4.2 and 8.3.1). Figure 8.4 shows effect of changes in frequency on calibration signals. Increasing test frequency increases phase separation between ID and OD defects as predicted by phase lag. It also increases sensitivity to probe wobble and dents but lowers sensitivity to tube supports and external deposits. One night question the validity of comparing machined holes and grooves in calibration tubes with real defects to estimate type and depth. The following examples justify this app roach.

Figure 8.8 shows external corrosion in a copper tube. Attack is general but non-uniform with localized severe pitting. An absolute internal probe was used to obtain signals from artificial defects and three of the localized pits. The phase angle of the first two corrosion indications shows they are OD defects, comparison with the calibration defect led to a depth estimate of 25 to 50X. Independent mechanical measurement found deepest penetration to be 50% for both defects. The third defect has a noticeably different phase angle from the first two. It approaches the angle for a through-wall hole, hence its depth was estimated to be 50 to 75% (actual measurement yielded 75%).

1.6 urn "" OD CALIBRATION HOLE r.ROOVE" ° DEFECTS ,„. ,0 ^ ' ' ECCENTRIC GROOVE

CORROSION DEFECTS

Fig. 8.8: External Corrosion in a Copper Tube (Do =15.9 mm, t=1.0 mm, f9Q = 5.3 kHz) -142-

An example of stress corrosion cracking (SCC) in Type 316 stainless steel, from a heavy water plant heat exchanger, is shown in Figure 8.9. The crack extends nearly half way around the tube. Phase angle of the crack signal shows it extends through the tube wall. Since the eddy currents flow parallel to coil windings, circumferentially, the large crack signal is due entirely to the component of the crack along the tube axis. The intergranular, branching nature of SCC generally permits their detection. Since a defect must disrupt eddy current flow to be detectable, if circumferential cracks are suspected, fatigue cracks for example, special probes are required, see Section 7.5 and 8.2.5.

30 -.10 PL-

50% 01) CONCENTRIC GROOVE CRACK SIGNAL

CALIBRATION DEFECTS

Fig. 8.9: Stress Corrosion Cracking in Type 316 Stainless Steel Tubing (Dn =19.1 mm, t=1.8 mm, fqp =68 kHz)

8.2.4 Vectorial Addition and Defects at Baffle Plates

During in-service inspection of tubes in heat exchangers, tube supports (baffle plates) are frequently defect prone regions. Inspection for defects at baffles is possible because eddy current signals are often vectorially additive. This permits analysis of superimposed signals; the signals can be (mentally or graphically) subtracted from the total indication with resultant separated signals appearing similar to calibration defects. Vectorial addition provides the basis for mult ifrequency eddy current testing (Section 8.4). -143-

Figure 3.10 illustrates how signals from a steel baffle plate and an external groove are added to obtain a superimposed indication. The difference between the end points of the baffle plate and baffle and groove signals equals the indication obtained from the groove by itself.

0D GROOVE

CARBON STEEL BAFFLE

Fig. 8.10: Vectorial Addition of Eddy Current Signals

Figure 8.11(a) shows a section of stainless steel tube removed from a power plant heat exchanger with part of the carbon steel support plate still in place. The support shows considerable corrosion; originally there was about 0.25 mm clearance between the tube and the hole in the plate. Corrosion products have completely filled the gap leading to crevice corrosion evident in Figure 8.11(b) which is a similar tube with the plate removed. Calibration signals are presented in Figure 8.11(c). The eddy current signal from the baffle plate region of Figure 8.11(a) is shown in Figure 8.11(d). This seemingly simple signal is actually quite complex. The upward component is due to external pitting similar to that in Figure 8.11(b). The presence of a support plate should result in -X, -Y signal components; in fact a + X deflection is observed. This is the result of denting of the tube. Denting is circumferential constriction of tubes due to compressive stresses exerted by baffle plate corrosion -144-

products such as magnetite. The presence of magnetite can also contribute to signal distortion particularly at low test frequencies- Tube denting is of concern because, in addition to complicating eddy current signal analysis, it can lead to further tube damage such as stress corrosion cracking or thermal fatigue because tubes are no longer free to expand and contract during thermal cycling.

(a) (b)

OD GROOVE

DEFECT (c) (d)

Fig. 8.11; Corrosion and Denting Under a Steel Baffle Plate (Do =15.9 mm, t=1.25 mm, fop = 80kHz)

Another example of defects near a carbon steel tube support is shown in Figure 8.12. These were obtained from a brass, thermal power plant condenser tube which suffered erosion/corrosion on either side of supports. This is the same tube as in Figure 7.3. Defect signals from the baffle plate vicinity are so large the support signal is obscured. The main point of this example is the advantage of using phase angle, rather than amplitude, to judge defect severity. Defect B with both differential and absolute probes has a phase angle approaching that of a through-wall hole, i.e., it probably extends at least 75% through' the wall. Defect A on the other hand is vertical and hence is probably no deeper than 50% even though it exhibits greater amplitude than B. -145-

DEFECT SIGNALS

OD GROOVE •D GROOVE

HAM-Lt \

(a) COL I BRAT I OK DEFECT SIGNALS

ABSOLUTE DIFFERENTIAL

Fig. 8.12; Quadrature Eddy Current Signals from the Brass Tube In Figure 7.3

To this point we have only considered ferromagnetic tube supports, carbon steel is the material used in most heat exchangers. With magnetic baffle plates vectorial addition appears to apply for all types of defects. Unfortunately deteriorating water quality, denting problems and longer service life requirements have made it necessary to construct some heat exchangers with non-ferromagnetic support plates. Vectorial addition of eddy current signals involving nonmagnetic supports is generally not valid. Several factors contribute to this situation, nonmagnetic supports yield much larger signals than magnetic supports. The large signal from nonmagnetic baffle plates effectively reduces signal-to-noise making small defects more difficult to detect.

Possibly the most difficult defects to detect under nonmagnetic supports are those of the same width as the plate, e.g., fretting wear from tube vibration.

Figure 8.13(a) illustrates such a situation, a brass baffle plate with a copper-nickel tube containing simulated 50% deep fretting wear. The same defect with a magnetic baffle plate is shown in Figure 8.13(b) for comparison.

Problems in detecting defects at non-magnetic supports can not be overcome by employing a multifrequency eddy current technique. The multifrequency approach relies on vectorial -146-

MAXIMUM GAP

50? 0D "ECCENTRIC GROOVE

OD GROOVE

BAFFLE WITH MAXIMUM (SAP

BRASS BAFFLE IN CONTACT

MAGNETIC BAFFLE (b) Fig. 8.13; Wear Under (a) Non-Ferromagnetic and (b) Ferromagnetic Baffle Plates addition being valid (Section 8.4). Sensitivity can be improved by employing special probes as will be shown in Section 8.2.6. 8.2.5 Tube Inspection at Tubesheets Heat exchanger tubesheets are usually made of carbon steel, eddy current response should therefore appear similar to a baffle signal. In addition, a large fill-factor (tube expansion) signal is also obtained as a result of tubes being rolled into tubesheets. Rolling eliminates corrosion prone crevices and also helps hold tubes in the tubesheet. With carbon steel tubesheets, expansion usually yields the largest signal component, the tubesheet only contributes appreciably at test frequencies below f90• Figure 8.14 shows tube configuration at a tubesheet and typical eddy current s ignals.

Occasionally one, may encounter a tubesheet clad with a corrosion resistant alloy such as stainless steel or Inconel. If the cladding is non-magnetic the same complications arise as with non-magnetic baffle plates (Section 8.2.4). Fortunately, most tubesheets are only clad on the primary side (near tube ends) where service related defects rarely occur. -147-

TUBESHEET x , N END OF Nx x J ROLLED JOINT IKi [ 153 f

EXPANSION SIGNAL

Fig. 8.14; Schematic of Tube Geometry at Rolled Joint in Tubesheet and Associated Eddy Current Signals

The end of the rolled joint at the inboarr. edge of a tubesheet is a defect prone area because of high residual and service stresses and also because deposits tend to accumulate at this location which can lead to corrosion. Eddy current indications with bobbin-type probes from defects in this region can be difficult to interpret because of excessive signal distortion from tube expansion. Sensitivity may be improved by employing a spring loaded surface probe as discussed in next section.

8.2.6 Testing Tubes with Internal Surface Probes During in-service inspection of tubes, situations arise where conventional circumferential probes (both differential and absolute) prove inadequate. The case of circumferential cracks was treated in Section 7.5. Surface probe designs have also been found to yield improved test results in the case of defects at non-magnetic baffle plates and at heat exchanger tubesheets.

Surface probes have several advantages over bobbin-type probes. They can be made much smaller than tube diameter and hence sample a smaller volume of tube periphery, this provides inherently greater sensitivity to small defects. Spring loading of a surface probe against the tube wall eliminates much of the fill-factor (lift-off) distortion caused by tube expansion in tubesheets. The main drawback to -148-

surface probe tube testing is that a number of scans have to be made for complete circumferential coverage. Conventional probes sample the entire tube in a single scan.

TUBESHEET END 0c INCONEL 600 ROLLED JOINT TUBE WALL

TUBESHEET

CONVENTIONAL SURCACE PROBE PROBE

Fig. 8.15; Comparison of Eddy Current Test Results in Heat Exchanger Tubesheet Region with Conventional and Surface Probes (Dn -12.5 mm, t - 1.2 mm, fgp -200 kHz) Figure 8.15 illustrates surface probe testing at the tubesheet region of a power plant steam generator. It compares signals, from what is believed to be OD corrosion damage at the end of the rolled joint, obtained with conventional and surface probes. The reason for the characteristic A'B'C1 surface probe signal is as follows. As the probe is with- drawn from the tube (direction of arrow) it encounters the start of the expanded area. Failure of the probe to follow this contour exactly results in an increasing lift-off signal, A'B', superimposed on the impedance change, A'C, due to the presence of the tubesheet. Both defect signals were obtained from the same tube, note the considerable improvement in sensitivity obtained with the surface probe. This tube was in fact leaking. -149-

50?, OD ECCENTRIC GROOVE BAP CALIBRATION

BAFFLE (MAXIMUM RAP)

Fig. 8.16: Internal Surface Probe Testing for Fretting Wear under a Non-Magnetic Baffle Plate. (Compare with Fig. 8.13 Results) A second example-of improved sensitivity with an internal surface probe involves fretting wear under non-magnetic baffle plates. Figure 8.16 shows results. Compare with Figure 8.13(a) which shows test results for the same defect obtained with an internal circumferential probe. With no gap, the 50% groove was barely detectable with a conventional probe,while Figure 8.16 shows this defect is easily detected with a surface probe.

8.3 ANOMALOUS EDDY CURRENT SIGNALS

Some eddy current signals can be mistaken for defect indications; these are called false or anomalous signals. They arise because of the high sensitivity of eddy currents to many variables and demonstrate the need for thorough analysis before concluding that every eddy current signal represents a defect. The following examples illustrate more common ones which have been encountered in practice. 8.3.1 Ferromagnetic Inclusions and Deposits

Materials with relative magnetic permeability greater than 1.0 affect eddy current response drastically. Skin depth and probe Inductance are both affected by permeability; permeability values of 50 to several hundred are typical. -150-

Before citing specific examples consider the general approach to identifying signals from magnetic materials. Such signals can be distinguished from real defects by reducing test frequency to move the operating point near the top of the impedance diagram. Figure 8.17 illustrates the procedure where 1, 2 and 3 represent ferromagnetic material on the inside, in the tube wall and on the outside respectively. It may be difficult to achieve a sufficiently high operating point with some instruments and probes when testing low resistivity, large diameter tubes. However, if a low enough frequency is achieved, real defect indications will fall nearly parallel to fill-factor whereas high permeability indications are nearly perpendicular to fill-factor. At 240 kHz (fgg) ln Figure 8.17, 1 and 2 could easily have been mistaken for ID defects. There is no confusion at 10 kHz since it is known that all dofect indications must fall between fill-factor and an 0D defect signal. The following two examples demonstrate the procedure to discriminate false defect (ferromagnetic) indications.

12.7mm

ABSOLUTE PROBE INCONEL BOO TU3E

©'

1.0.

DECREASING FILL FACTOR

i.D. D.D. ©

Fig. 8.17: Coil Impedance Display at Two Test Frequencies -151-

Ferromagnetic inclusions are occasionally encountered during eddy current testing of non-magnetic materials. These arise from chips or filings from steel tooling and handling equipment which are embedded during manufacture. The surface of nominally non-magnetic stainless steels and nickel-base alloys can also become magnetic as a result of cold working or through alloy depletion from oxidation or corrosion.

O.D. DEFECT 1.0. DEFECT

250 kHz FERROMAGNETIC INCLUSION FERROMAGNETIC O.D. INCLUSION I.D.

50 kHz INCLUSION

INCLUSION

10 kHz

Fig. 8.18; Defect and Magnetic Inclusion Signals Obtained from a New Inconel 600 Tube (Dn = 13 mm, t = 1.1 mm) with an Absolute External Coil. fqo =250 kHz

Though one might consider a magnetic inclusion a defect, there are several reasons why it is important to identify the origin of an indication. Even very small, perhaps insignificant, magnetic inclusions can yield sizeable eddy current signals because of the extreme sensitivity to magnetic permeability. A second reason to determine defect origin is so measures can be taken to minimize further damage; magnetic inclusions are nearly always manufacturing defects. Figure 8.18 shows the signal from a magnetic inclusion in new Incon- el.600 tubing at various test frequencies. These results were obtained with an external encircling probe; this explains the reversal in appearance of ID and 0D defects from previous examples. The magnetic inclusion yields a signal whose angular separation from the fill-factor direction increases as test frequency is reduced. The response of real defects is just opposite. -152-

O.D. DEFECT I.D. DEFECT

INTERNAL MAGNETITE 250 kHz MAGNETITE I.D.

MAGNETITE 50 kHz

MAGNETITE O.D. I.D.

10 kHz Fig. 8.19: Defect and Magnetite Signals from an Inconel 600 Tube (Dp - 13 mm, t - 1.1 mm) Obtained with an Absolute Internal Probe. fon - 250 kHz)

Figure 8.19 shows eddy current response to magnetite (Fe^Q^) deposits inside an Inconel 600 tube at various test frequencies. As in the previous example, the existence of ferromagnetic material is verified by lowering test frequency; magnetite signals rotate clockwise whereas defect signals rotate counter-clockwise. One could easily mistake the magnetite signals for real defects at 250 kHz and 50 kHz. Reducing test frequency can also be used to verify the presence of magnetite on the outside of a tube. This approach has been used to measure the height of sludge deposits (containing magnetite) above tubesheets during in-service inspection of vertical heat exchangers.

Figure 8.20 shows the eddy current signals from a Monel 400 steam generator tube with external wall thinning near a tube support. The tube was inspected with an absolute saturation probe and the signals recorded with wall thinning giving a vertically upward signal. At 50 kHz the vertical component of the complex signal is from wall thinning and the horizontal signal is primarily from magnetic deposit. At 200 kHz (2 fgo) the vertical component is again from wall thinning but the horizontal signal is primarily from an increase in tube magnetic permeability because of incomplete magnetic saturation under the carbon steel tube support. At 400 kHz eddy currents just barely penetrate through the wall. In this case the signal is primarily from tube magnetic permeability variations. -153-

0.0. GROOVE

O.D. /DENT DENT

BAFFLE f, = 400 kHz BflFFLE v^. BftFFLE PLflTE PLATE ^S"*!«S5^LflTE BAFFLE f2 = 2OOkHz PLATE f ,* = 50 kHz MAGNET ITEv CALIBRATION TUBE SIGNALS \rmTj

MfiGNETITE

f = 200 kHz f3 = 400 kHz f, =50 kHz 2

ACTUAL DEFECT SIGNAL

Fig. 8.20: Eddy Current Signals from Monel 400 Tube at Baffle Plate Location. (fqf) " 100 kHz)

8.3.2 Conducting Deposits

The most probable conducting deposit which may be encountered during in-service tube testing is copper. Copper taken into solution in one part of a cooling circuit, from brass tubes for example, can re-deposit at another location at the expense of a less noble metal such as iron. An example is shown in Figure 8.21 which is a copper-alloy tube from an air conditione' ' i»- exchanger. Copper deposits occur near tube supports, me. a thickness was 0.05 mm. Even such a thin deposit yields s. _.rge eddy current signal since copper is a good conductor. Figure 8.21 shows response from both absolute and differential internal probes. The absolute probe ga/e eddy current signals with no +Y component, clearly indicating the non-defect nature of the anomaly. -154-

The differential probe signal is not nearly as clear and illustrates another limitation of differential probes. Com- parison of the deposit indication with calibration defects could easily lead one to conclude the presence of an OD defect; particularly if the eddy current results were compressed on X and Y channel recordings as is often the case during in-service inspection. With a differential probe, one has to observe defect sense (arrows) to distinguish between deposit signals and those from real defects. fj—f Copper Deposits

CALIBRATION DEFECT SIGNALS

ABSOLUTE DIFFERENTIAL

DEPOSIT SIGNALS

Fig. 8.21; Eddy Current Indications from Copper Deposits on a Copper Alloy Tube (Dp = 19 mm, t = 1.1 mm, fgp = 57 kHz) figure 8.22 shows simulated copper deposit signals at differ- test frequencies. There is a noticeable change in phase e with increasing deposit thickness as well as test fre- icy• At frequencies above fgg there exists a possibility ^sits c.uld be mistaken for ID defects, even with an solute probe. The procedure for in-service inspection of nuclear power plant boilers specified by ASME(_1_1) leads to test frequencies- between fgg and 2fgg. This appears to be a weakness in the code which may lead to revision if copper deposits prove more common as boilers age. Inspection of Figure 8.22 reveals clearer discrimination between copper and defects is achieved at fgQ /2 than at fgg . Optimum test frequency for copper coated tubes appears to be the frequency -155-

which just leaves copper signals below the horizontal fill-factor direction.

A - '05 OD ECCENTRIC GROOVE B - 10", ID CONCENTRIC GROOVE C - Q.U mr THICK COPPER GROUND TUBE D - 0.05 nn THICK COPPER AROUND TUBE

Fig. 8.22: Eddy Current Signals Obtained with an Internal Circumferential Probe from Simulated Copper Deposits on Tubes

8.4 MULTIFREQUENCY EDDY CURRENT TESTING 8.4.1 Background

Successful in-service Eddy Current inspection relies on eddy current probes that can sense defects and an analysis of eddy current signals. Both aspects are equally important. While scanning each tube, eddy current signals are obtained from baffle plates, magnetite deposits, dents, tubesheets, tube expansion, etc. and maybe defects. One must,therefore, discriminate between defects and insignificant signals and even more important, estimate defect severity when it occurs together with other signal sources. It would be much easier if the data could be processed to contain only defect signals; Multifrequency ET can do this.

In multifrequency testing, two or more sinusoidal signals of different frequencies are fed simultaneously to a single eddy current probe. Gain and phase of the output signal from each frequency can be separately controlled. -156-

THROUGH BAFFLE WALL HOLE PL*TE ID GROOVE 0 D GROOVE 3 MAGNETITE '- m 1 \ J DENT J 15.5 mm

CALIBRATION TUBE R3

100% . o .D. 1 D. ^ >v I I.D 100% DEFECTS ^V^j1 3NETI » ^^ (s^^MAI E1 « \ C ° DENT \ \BAFFLE \ \ PLATE BAFFLE \ PLATE \ \ MAGNETITE \ ft =20 kHz f2 =100 kHz f, =500 kHz

(a) (b) (c)

Fig. 8.23: Internal Probe Response to Various Test Parameters. f^. = 130 kHz •

(c)

Fig. 8.24: Eddy Current Signal at Baffle Plate Position in Tube of Figure 8.11. fgQ = 130 kHz. -157-

These signals can then be combined to eliminate unwanted signals and leave only the defect signal. This method is only effective if a defect signal differs characteristically from unwanted signals and if signals are vectorially additive. The first condition makes detection of internal defects, in the presence of internal variations, impossible. The second requirement makes the method ineffective for detection of fretting wear under non-ferromagnetic baffle plates (Section 8.2.4). As a consequence of combining signals from three different frequencies, defect signal amplitude decreases and instrument noise increases.

Eddy current penetration and phase lag are a function of frequency; increasing test frequency reduces penetration and increases phase lag. Since an eddy current signal is a function of current density and phase lag, it is possible to change the response to various signal sources by changing test frequency. If one simulates a heat exchanger tube with defects, deposits, dents and support plates, one obtains the following results: (a) at high frequencies only internal defects and dents are detectable, Figure 8.23(c). (b) at intermediate frequencies, all features are detectable and there is phase discrimination between internal and external defect signals (because of phase lag across the wall) and other signals, Figure 8.23(b). (c) at low frequencies, baffle plates and magnetite deposits yield predominant signals with little phase separation between internal and external defect signals, Figure 8.23(a). With this background in mind, one can decide which combination of frequencies should be used to eliminate extraneous (unwanted) signals. The following two examples illustrate these effects. For the dented tube example described in Section 8.2.3 (Figure 8.11), the extraneous signals making up the composite signal at f = 100 kHz can be determined by re-inspecting the tube at higher and lower test frequencies. If the signals from the actual defect in Figure 8.24 are compared with the corresponding calibration signals in Figure 8.23, one can see at 500 kHz the signal is primarily from a dent while that at 20 kHz contains a large baffle plate signal component. -158-

8.4.2 Multlfrequency Testing of Dented Tubes

With single frequency eddy current inspection, tube supports and dents tend to mask signals from tube damage under tube supports. This makes detection and estimation of severity difficult and time-consuming. In the remaining section we show how multifrequency simplifies the inspection of the dented tube described previously.

Figure 8.25 illustrates the tube stripping sequence; one or more signals are removed by each mixing of two frequencies. By proper manipulation of the signals from the two lower frequencies, baffle plate and magnetite deposit signals can be eliminated. However, the resultant eddy current signal is still distorted by the 'denting' signal. Again, by combining this resultant signal with the signal from a higher test frequency, the dent signal can also be eliminated. The tube now looks bare. If a defect existed under the baffle plate, it would be very easy to detect, the resultant signal contains only information from the OD corrosion. This process of unwanted signal elimination is like solving three simultaneous equations with three unknowns and solving for the parameter X^ = defect.

h '3 20 kHz 100 kHz 500 kHz

c, = f2 - f,

c2 = c, . f3

'* \

Fig. 8.25; Tube Stripping Sequence by Mult ifrequency -159-

As shown in Figure 8.23, the signal at each baffle plate is a composite signal comprising a baffle plate, magnetite deposit (or baffle plate corrosion products), dent and defect signal. Figure 8.26 illustrates elimination of baffle plate and magnetite signals. The probe is moved back-and-forth under the baffle plate and the signal is monitored on the storage scope in the chopping mode, where both frequency signals are displayed simultaneously.

MAGNETITE

BAFFLE PUTE

BAFFLE PLATE

fI UlTH VERTICAL AXIS I; WITH PHASE COMPRESSED BY 0.74 ROTATION OF 19°

•N RESIDUAL BAFFLE PLATE SIGNAL

RESIDUAL MAGNETITE SIGNAL

Fig. 8.26; Suppression of Baffle Plate and Magnetite Signals

The £2 signal is first rotated to match the fj signal orientation. Then f^ amplitude is changed to match, as nearly as possible, the fj signal size. In this case, this method by itself doesn't work. However, by decreasing the vertical component of the fj baffle plate signal, one obtains a good match. On subtracting the signal, through an electronic mixer (C^), the signals from the baffle plate and the magnetite deposit both nearly disappear. A small residual signal remains due to different approach signals at the two test frequencies, indicated in Figure 8.26 by the two open circles. Although the baffle plate signals -160-

are Identical, the two points do not coincide; the baffle plate Is sensed earlier at the lower test frequency. This residual signal is insignificant for this application though it can become quite serious when testing for small cracks under non-ferromagnetic baffle plates.

C2=Ci-f3 /

RESIDUAL DENT SIGNAL

Fig. 8.27; Suppression of Dent Signal

Figure 8.27 illustrates how one can eliminate the 'denting' signal from the resultant (Cj = ^2~^1^ signal. This is achieved by first matching the phase and amplitude of the Cj and £3 'dent' signals and then using a second mixing module (C2) for subtraction.

Figure 8.28 traces the above sequence for two defective tubes, and shows the eddy current signal becoming simpler to analyze with each step. On comparing defective tube signals with those from a calibration tube, one observes the ^2 defect signal is distorted by the baffle plate, dent and/or magnetite deposit. The C^ signal is only distorted from the dent signal,and C2 is a clear signal indicating 0D pits approximately 50% deep. Even an inexperienced inspector could analyze these results. -161-

I00S 0 D 0 0 10. CALIBRATION TUBE

DENT

DE BAFFLE NT MflGr^T|TE V PLATE c, =1, • 1, f, -- 100 kH2

OEFECTIVE TUBE NO. 2

Fig. 8.28: Multifrequency Eddy Current Signals from Defective Tube

When using multi-frequency to eliminate "ID noise", such as signals from cyclic internal diameter variations ("pilger noise or die chatter"), dents and probe wobble, the signal amplitude from internal defects is drastically reduced. However, signal amplitude from external defects is not altered significantly. Multifrequency is more effective for external defect detection than for detection of internal defects in tubes. -162-

8.5 SUMMARY Defect signal amplitude is a function of its axial and circumferential extent as well as depth. Defect signal phase is primarily a function of depth. For general purpose volumetric inspection of heat exchanger tubes, a suitable test frequency is

2 f90 = 3 p/t , kHz (7.4)

where p is electrical resistivity and t is wall thickness.

Inspection at f^Q allows defect depth to be estimated on the basis of signal phase. To discriminate between defects and ferromagnetic deposits a lower test frequency should be used; normally 10 or 20% of f9Q. Signal response from most significant service induced defects is usually comparable in amplitude to that from a 1.6 mm diameter through hole. Stress corrosion cracking, general corrosion and fretting wear give large signals whereas pitting corrosion and fatigue cracks give small signals.

Testing for fretting wear under non-ferromagnetic support plates is difficult and unreliable with bobbin type probes, because defect and support plate signals are not vectorially additive. A surface type probe should be used. Multifrequency equipment can be used to eliminate unwanted components from complex signals such as support plates and probe wobble. This greatly simplifies signal analysis. -163-

CHAPTER 9 - METALLURGICAL PROPERTIES AND TESTING FERROMAGNETIC MATERIALS

9 .1 INTRODUCTION

One can find numerous references in NDT publications dealing with eddy current measurement of material properties,such as chemical composition, hardness, strength, corrosion damage, degree of cold work and extent of both carburization and decarburization. In fact, none of these properties and material conditions are measured directly. Eddy current testing is sensitive to material properties through their effect on resistivity and magnetic permeability. As such, eddy currents only provide indirect measurement of material properties and care must be taken to insure that some unforeseen material variation does not lead to false conclusions. Two precautions will help avoid false test results:

(a) a sound basic understanding of ET as outlined in previous chapters ( b) use of suitable standards for any particular test; the condition of such standards should be verified by independent methods, e.g., hardness tests, tensile tests .

A complete treatment of materials property evaluation by eddy current testing is beyond the scope of this manual. The basics are covered and a few examples presented.

9.2 ELECTRICAL CONDUCTIVITY

9.2.1 Factors Affecting Resistivity

All materials possess intrinsic resistance to electron flow (current) which is termed resistivity ( P, microhm-centimetres). The resistance of a conductor is given by

R = pJl/A ohms

where A is length (cm) and A is cross-sectional area (cm^). Resistivity values for various materials are listed in Table 9.1

Conductivity (cr, siemens/metre) * is the ease with which electrons can move through a material. It is the reciprocal

*Conversion: a = 108/p, S/m or mho/t -164-

TABLE 9.1 ELECTRICAL RESISTIVITY OF COMMON CONDUCTORS AT 20°C

MATERIAL RESISTIVITY CONDUCTIVITY CONDUCTIVITY (yn.cm) (siemens/m) (% IACS)

Silver 1.6 6.14xl07 105 Copper 1.7 5.81 100 Gold 2.4 4.10 70 Aluminum 2.8 3.55 61 7O75-T6 (Al Alloy) 5.3 1.89 32 Zinc 5.9 1.70 29 Magnesium 4.6 2.17 37 Admiralty Brass 7.0 1.43 24 Iron 9.7 1.03 18 Phosphor Bronze 16 0.63 11 Lead 20.6 0.49 8.4 70 Cu-30 Ni 37.4 0.27 4.5 Mo nel 48.2 0.21 3.6 Zirconium 50 0.20 3.4 Ti tanium 54.8 0.18 3.1 304 SS 70 0.14 2.5 Zircaloy-2 72 0.14 2.4 Inconel 600 98 0.10 1.7 Hastelloy X 115 0.087 1.5 Was paloy 123 0.081 1.4 T1-6A1-4V 172 0.058 1.0

of resistivity. In eddy current testing, conductivity is frequently given as a percentage of the International Annealed Copper Standard (% IACS). In this system conductivity of pure, annealed copper at 20°C is set to 100% and conductivity of other materials is given as a percentage of copper. Conductivity of a material can be calculated from its resistivity,

% IACS - 172/p

Increasing temperature normally increases resistivity (decreases conductivity) as shown in Figure 9.1. Over a limited temperature range the variation is usually linear according to the relation

p = PQ(1 + aAT)

where P is resistivity at temperature T (°C), P is o -1 resistivity at a reference temperature To, a ("C is thermal coefficient of resistivity and AT is the temperature difference (T-To). For common metals and alloys values of a range from less than 0.001 to over 0.01, 0.004 is fairly typical. -165-

Alloying normally Increases resistivity. Figure 9.2 shows even small alloy additions to aluminum can increase resistivity appreciably. The conductivity of binary Cu-Ni

60 /

/TITANIUM 50 / a» 0.04 metre s ) I u 40 — 1 y a « 0.004 | 30 i ~ 1 ,/

20 'IT Y COPPER 10 ~ / aft* 0.00 5 - RESISTI V Ul.—i "1 i 1 1 1 1- 200 400 600 800 1000 1200 1400 TEMPERATURE (°K)

Fig. 9.1: Effect of Temperature on the Resistivity of Copper, Platinum and Titanium

MANGANESE

MAGNESIUM

1-0 2.0 3.0 4.0 5.0 6.0 ALLOY CONTENT (wt. %)

Fig. 9.2: Effect of Alloying Elements on the Electrical Resistivity of Aluminum. -166-

alloys is shown in Figure 9.3. The dependence of conductivity on composition provides one basis for eddy current sorting of mixed alloys. Oxygen impurity in zirconium and titanium alloys changes resistivity considerably. Figure 5.19 showed a non-uniform oxygen distribution in a zirconiun-niobium alloy detected by eddy current testing.

TOO -

COPPER/NICKEL ALLOYS

6-3

40 60 100 HEIGHT % COPPER

Fig. 9.3: Variation in Electrical Conductivity of Nickel- Copper Alloys with Composition

Cold work increases resistivity through introduction of lattice defects in metals. At normal temperatures, cold work has a relatively small effect on conductivity (<10Z) and can usually be ignored. The degree of cold work in some austenitic stainless steels can be determined by ET, this is possible because cold work makes them ferromagnetic, not because of a resistivity change. 9.2.2 Material Sorting by Resistivity

This is normally an eddy current surface probe method. Two instrument types are commonly used. Impedance display instruments offer a comparative method as treated in Section 5.8.2; the lift-off curves for unknown materials are compared with those of known standards and the resistivity of the unknown is estimated by interpolation. Meter readout instruments are also available with built-in "lift-off" -167-

compensation which are calibrated directly in % IACS . Both types of instruments require care on the part of the operator to insure meaningful results. Effects which can contribute to erroneous results follow (for more detail see Section 5.8.2):

(a) too low a test frequency can make material thickness appear similar to resistivity changes. (b) sample curvature affects coil coupling and hence its response (edge and other geometry effects have a similar response). (c) too high a test frequency could sense alloy changes at the surface of oxidized or corroded materials. (d) conducting and nonconducting coatings affect test coil impedance. (e) ambient temperature variations result in changes in sample resistivity and test coil resistance. The above potential error sources can largely be overcome through use of suitable standards which duplicate materials to be tested.

I 10 100 1000 TIME AT TEMPERATURE (hi

Fig. 9.4: Variation of Mechanical Properties and Conductivity in 7075-T6 Aluminum Exposed at 2Q5°C -168-

An example of eddy current testing to determine heat treatment state of an aluminum alloy is shown in Figure 9.4. These results are from Pellegrini(JJO) who indicates the technique can be used to judge the fitness of overheated material for further service. A similar approach has been used to assess heat treat condition of titanium alloys. 9.3 MAGNETIC PROPERTIES

For eddy current purposes one can classify materials as ferromagnetic (magnetic) or non-ferromagnetic (nonmagnetic). Diamagnetic and paramagnetic materials can be considered nonmagnetic. Ferromagnetism has its origin in a quantum mechanics effect, the "exchange interaction". It occurs in the elements iron, cobalt, nickel and some of the rare earth inetals. These elements have partially filled d and f electron shells. Alloying with elements which have a higher electron to atom ratio fills these d and f shells and makes the resulting alloys less magnetic, e.g., copper added to nickel (Monel) and chromium added to iron (stainless steel).

The main feature separating magnetic from nonmagnetic materials is magnetic permeability, y , which is a measure of a material's intrinsic ability to conduct magnetic flux. It is defined as ths induced magnetic flux density, B, divided by external magnetic field intensity (magnetizing force), H,

y = B/H

For air and nonmagnetic materials y is a constant,

V = 4TT x 10 webers/ampere-met re when B is in teslas* (T) or webers/metre^ and H is in ampere/metre (A/in) .

Simplification results if one uses relative permeability, which is defined as

(dimensionless)

Relative permeability has the same value in all magnetic systems of units. For magnetic materials Mr can be very large, whereas for nonmagnetic materials Mr = 1.0.

•Conversion: 1 tesla = 10^ gauss; 1 A/m=0.012566 oersted. -169-

9.3.1 Magnetic Hysteresis When a material Is magnetized in a coil, the magnetic field intensity, H, is proportional to coil current. If alternating current is applied to a magnetizing coil a B-H loop results as shown in Figure 9.5. As H increases from zero for the first time, 6 increases along the DC curve, path No. 1. When H decreases, B also decreases but along path No. 2. The difference between paths 1 and 2 is termed hysteresis. When H has fallen to zero a residual flux density remains in the material, Br, called retentivity or residual flux density. On decreasing H further (reverse or negative current) flux density decreases to zero at Hc which is the coercive magnetic Intensity or coercive force. Decreasing H still more drives the curve to point Sj. Additional AC cycles will retrace the loop. At point S2 the material is saturated, from S2 to S3 the B-H curve is lin^.ir with slope Vo . Flux density at saturation depends on material; carbon steel saturates at about B = 2 tesla (20 kilogauss) whereas Monel 400 saturates at about 0.3 tesla (3 k ilogauss).

Fig. 9.5: Hysteresis (or B-H) Loop -170-

9.3.2 Magnetic Permeability For eddy current inspection of ferromagnetic materials several kinds of permeability play an important role. Normal permeability, Mr, is a measure of a material's ability to conduct magnetic flux; it is an important factor when determining the ease with which a magnetic material can be saturated.

Another permeability of concern in ET is relative incremental or recoil permeability, It is defined as

AB/AH

where AB is the change in flux density which accompanies a change in magnetizing force, AH, created for example by an eddy current coil's alternating current. An incremental AH can be superimposed at any point on a DC magnetization curve as illustrated in Figure 9.6.

0.8 -

S 0- 6 -

0.4 -

0. 2 -

I0D 200 300 400 500

MAGNETIZING FORCE (A / m )

Fig. 9.6; DC Magnetization Curve and Recoil Permeability for Iron -171-

At H-0 we have the relative initial permeability, V-^ . In a magnetic material without a biasing DC magnetic field, the normal permeability is equal to the incremental permeability,

^r " Ui = MA In eddy current testing, test coil inductance and depth of penetration are influenced by incremental permeability not normal permeability. However, throughout this report it is assumed that the eddy current test is performed without DC bias and with a low magnetizing force (low alternating coil current). In this case, yr = V& , and for simplification purposes yr is used in the skin depth and inductance equations and impedance diagrams; yr is used throughout the manual to denote incremental permeability (y,) unless otherwise stated.

When an increasing DC magnetizing field is applied, a nonlinear B-H relationship results as shown in Figure 9.7. The incremental permeability continuously decreases until saturation is achieved. At saturation y^ =1.0. The normal permeability,instead, first increases to a maximum value and then decreases gradually, see Figure 9.7; at saturation it can still be very large.

i i i 1 1 1 1

Q.3 3 Re 60

-—-" ' 0. 2 - -

0. 1 - - / / I 1 1 i i 1 I 1

60 - - ~ B / H

SO UJ 40 ^\ - X 1 \^ JJ ~- o. \ —• ~ CU 20 = a B / a H -

••« ———

JJ i : i i 1 1 1 1

H x 10" (4 In )

Fig. 9.7; Magnetization Curve, Incremental Permeability and Normal Permeability for a 3Re6O Tube Sample -172-

9.3.3 Factors Affecting Magnetic Permeability Ferromagnetic materials do not have unique magnetization curves but depend strongly on factors such as thermal processing history, - mechanical processing history, chemical composition, internal stresses, - temperature (if close to Curie temperature). The following examples illustrate the effect of some of these variables•

Figure 9.8 shows B-H curves, at room temperature, for three supposedly identical Monel 400 tube samples. The differences are attributed to variations in nickel/copper content within the normal alloy specification range. Figure 9.9 shows variation of magnetic permeability with cold work in Type 300 series stainless steels(^). In these "nonmagnetic" austenitic steels a ferromagnetic martensite phase forms during cold working increasing the magnetic permeability. In contrast, most normally ferromagnetic materials exhibit a decrease in permeability as a result of cold work. The 300 series stainless steels can also become ferromagnetic as a result of welding, a magnetic delta ferrite phase forms during solidification.

MONEL 100 TUBES FROM (UNTICOKE G.S.

I / ' ' 1 L 1 1 / 5 10 i. — — 25 30

UBE 1-250

D.G - - I 1 1 1 _/ — "

- / ,,-""" TUBE A-25 1 •"TUBE »-252

10 12 14 IB 18 20

H OERSTEDS Fig. 9.8; Magnetization Curves for Various Monel 400 Samples" -173-

- AUSTENITIC STAINLESS STEEL

1. 0

40 60 80 100

% COLD DORK

Fig. 9.9: Variation of Relative Permeability with Cold Reduction for Various Austenitic Stainless Steels(2.)

I. 5

6 MPa NO STRESS

_ 1.0

0. 5 ~

I 25 50 75 100 MAGNETIZING FORCE (A I mI

Fig. 9.10: Effect of Elastic Strain on the Magnetization of Iron -174-

Figure 9.10 shows changes in B-H curves for iron with internal stress. Note that these stress levels are purely elastic, well below the yield strength. The changes in B-H (and permeability) are due to magnetostriction. The above examples illustrate the inherent variability of B-H and hence permeability of ferromagnetic materials. Incremental permeability affects an eddy current coil's inductance as well as depth of eddy current penetration into a material. The large variations in permeability shown above make conventional eddy current testing for defects in magnetic materials very difficult if not impossible.

The best solution to eddy current testing of a magnetic material for defects is to bring it to a condition where HA. =1.0. A few slightly magnetic materials can be heated above their Curie temperature to make them nonmagnetic. Monel 400 heated to between 50° and 70°C has been tested in this manner. Most materials have too high a Curie temperature to be tested by this approach. The only other way to decrease y^ to unity is by magnetic saturation. This topic is treated in a subsequent section.

9 .4 TESTING MAGNETIC MATERIALS

9.4.1 Simplified Impedance Diagrams A qualitative understanding of the effect of permeability on coil impedance can also be obtained by the equivalent circuit and its associated semicircular impedance diagram treatment of Section 3.5. Coil inductance is a function of magnetic flux through it; flux increases in the presence of a magnetic material. For a cylinder surrounded by an encircling coil, coil inductance is proportional to both the cylinder's permeability and its cross-sectional area,

a Lp V rD o

the where L_ is primary coil (probe) inductance, Ur = VIA. is cylinder's incremental permeability and DQ its diameter. An increase in permeability or diameter will increase coil inductance. By a similar treatment to that presented in Chapter 3, one can generate the Impedance diagrams of Figure 9.11. Figure 9.11(a) is obtained by plotting the encircling coil impedance normalized to the inductive reactance in air. It illustrates the effect of permeability and cylinder diameter. As permeability or cylinder diameter increases (with constant coil diameter) oil impedance increases drastically. (This explains the good response to ferromagnetic inclusions and deposits discussed in Sections 6.5.1 and 8.3.1). There is no phase separation and hence no discrimination between variations in permeability and cylinder diameter. However, there is about 90° phase separation and hence excellent discrimination between variations in permeability and resistivity. -175-

1.0 1.0

0.5 0.5

RL/wLp (b) CYLINDER (c) PLATE

Fig. 9.11; Simplified Impedance Diagrams for Ferromagnetic Cylinders and Plates

Figure 9.11(b) is obtained by plotting the encircling coil impedance normalized to its inductive reactance with the ferromagnetic cylinder inside the coil. This figure indicates the effect of permeability and cylinder diameter on operation point location. An increase in both permeability and cylinder diameter moves the operating point DOWN the impedance curve (for constant fill factor). Surface probe inductance also depends on test sample permeability (L. is proportional to yr ). An increase in permeability moves the operating point UP the impedance locus as shown in Figure 9.life) • However, unlike curves for a cylinder where the semicircle increases drastically in size, the curve for a surface probe increases only a small amount as previously shown in Figure 5.10. This results from much less efficient coupling with surface probes as compared to encircling coils. A surface probe with a ferrite core (or cup) coil permits better magnetic coupling (decreased magnetic reluctance) and hence yields a larger impedance diagram than a similar air core coil. An additional observation can be made from Figure 9.11(c); magnetic permeability has the same effect as electrical resistivity and hence these two parameters cannot be separated when usiag a surface probe. -176-

70 329 STAINLESS STEEL

10 kHz

100 kHz

I 10 20 30

NORMALIZED RESISTANCE

Fig. 9.12: Experimental Normalized Impedance Diagrams for Three Type 329 Stainless Steel Samples Tested with a Long Encircling Coil

9.4.2 Impedance Diagrams

Figure 9.12 shows experimental Impedance curves for three different Type 329 stainless steel samples tested with long encircling coils. These curves differ markedly from a semicircle at the lower section of the impedance diagram, where the curve approaches the Y-axis at 45° rather than 90° These curves are nearly identical in shape to that presented in Figure 7.6 for a nonmagnetic cylinder. But, while the nonmagnetic curve intersects the reactance axis (Y-axis) at 1.0, the Figure 9.12 curves intersect this axis at their respective Vr values. Magnetic saturation of these samples would reduce them to a common curve intersecting the axis at 1.0. This figure is another example of typical permeability variations which may be encountered in supposedly "identical" samples. -177-

INCREASING PROBE DIAMETER

INCREASING FREQUENCY

INCREASING PERMEABILITY

INCREASING RESISTIVITY

NORMALIZED RESISTANCE

Fig. 9.13; Impedance Diagram for Ferromagnetic Material Showing Effect of Material and Test Parameters

Figure 9.13 shows an actual surface probe impedance diagram for magnetic material. The shape differs appreciably from a semicircle. Most test variables have a similar effect on the impedance diagram as for surface probes on nonmagnetic material (Section 5.5). To measure magnetic permeability in the presence of lift-off noise, probe diameter and test frequency should be chosen to operate in region A. Eddy current inspection of magnetic materials for defects is difficult or impossible because of random permeability variation as discussed in Section 9.3.3. In addition there are skin depth limitations. Without saturation, the initial permeability can range from 50 to over 500. Since depth of penetration is inversely proportional to the square root of permeability and test frequency,

6 cc l//fin to obtain equal penetration requires a reduction in frequency by the same factor of 50 to over 500. Unfortunately, lowering frequency moves the operating point to Region B in Figure 9.13 where there is poor signal separation between lift-off, permeability and rosistivity as well as •&'aced sensitivity to defects. -178-

Before leaving Figure 9.13 consider the characteristic 2 parameter, r' u)yra (Section 5.6). Figure 9.13 shows the parameter is not generally valid for ferromagnetic materials. It indicates an increase in Pr should move the operating point down the impedance curve like increasing frequency or probe diameter. In practice exactly the opposite occurs. The characteristic parameter should only be used for finding operating point of surface probes on nonmagnetic materials.

9.4.3 Material Sorting by Magnetic Permeability

Detailed treatment of this topic is beyond the scope of this manual. This section is essentially a warning, Many properties of magnetic materials affect permeability as discussed in Section 9.3.3. Eddy current testing has been used to sort mixed alloys as well as measurement of hardness, decarburization, carburization, degree of cold work, strength, ductility,etc. A standard, ASTM E566-76, offers broad guidelines on this eddy current application.

Meaningful results with such testing requires at least the f ollowing: understanding of the variables affecting a material's electrical and magnetic properties a sound knowledge of eddy current testing adequate standard samples verified by destructive examination or other independent methods. 9.4.4 Testing for Defects in Magnetic Materials

Previous sections explained why saturation is required to suppress effects of usually harmless permeability variations which could be mistaken for, or obscure, defect signals. We only consider testing of cylindrical materials; similar techniques can, at least in theory, be applied to surface probe testing.

Manufacturing inspection of rods, wires and tubes is accomplished fairly simply by external, water cooled magnetizing coils through which the material is passed. ASTM standard E309 covers such testing. In-service inspection again presents the most difficult situation due to access and space limitations. -179-

SUPPORT O.D. PLATE FLAT PITS DEFECT HOLE CALIBRATION 7 TUBE

EDDY CURRENT TEST WITHOUT SATURATION SLIGHT BEND IN TUBE

EDDY CURRENT TEST WITH MAGNETIC SATURATION (10 X ABOVE GAIN)

Fig. 9.14: Eddy Current Signals from a High Magnetic Permeability Monel 400 Tube. Test Frequency - 50 kHz -180-

Figure 9.14 compares Y-channel eddy current signals from a Monel 400 tube at fgQ without and with magnetic saturation. Saturation results in good defect detection. Permeability variation due to cold work and internal stresses at a slight bend in the tube are completely suppressed by saturation. This tube was saturated by superimposing the AC eddy current signal on DC magnetization power. Saturation of Monel 400 is also achieved by incorporating permanent magnets in the probe(jJ) .

Saturation with DC magnetization is limited by coil heating. Heat dissipation is proportional to current squared and coil wire resistance (Power =I^R). To increase magnetization (H is proportional to I) pulse saturation is used. The saturation current (DC) is switched on-and-off at regular intervals thereby reducing the heating effect. The test current (AC) is superimposed on the saturation current and the eddy current signal is sampled only at maximum saturation. One commercial instrument, operating on this principle, is currently available. Testing speed is a function of pulse rate, in general it is much slower than conventional testing.

If magnetic saturation at defects is not complete, an eddy current test becomes a test for permeability, not eddy current testing as described in previous chapters. This can be understood from Figure 9.15 which illustrates the change in eddy current signals from calibration defects in a magnetic stainless steel tube as degree of saturation is increased. The eddy current signals were obtained with an absolute bobbin type probe. Since defect signal amplitude decreases as saturation is approached, instrument gain was doubled for the 20 and 40 ampere saturation results. Magnetization was achieved with an external, water cooled coil; 10 amperes produced about 2.8 x 10^ A/m or 350 oersteds. Figure 9.15 shows one has to be saturated well past the knee in the magnetization curve (over 20 amperes) before eddy current defect signals appear normal, like those from nonmagnetic materials.

The reason for the characteristic eddy current signals from partially saturated tubing is more clearly apparent in the eddy current impedance display of Figure 9.16 which includes impedance response as magnetization level increases. This figure shows, at partial saturation (less than 10 amperes), defect signals consist nearly entirely of increasing and decreasing permeability. The initial increasing permeability signal component is attributed to less saturation on either side of machined calibration defects while the decreasing permeability component is due to more intense saturation in the reduced tube-wall region at defects.

Similar results are obtained with internal saturation using DC magnetization or permanent magnets. A single rare-earth permanent magnet was found to be equivalent to about 5 amperes (175 oersteds) of an external magnetizing current for this tube size and material. -181-

EXTERNAL MAGNETIZING COIL THROUGH HOLE \

INTERNAL ABSOLUTE PROBE

A §

A BC D

A PROBE WOBBLE 8 THROUGH HOLE C O.D. GROOVE D I.D. GROOVE

MAGNETIZING CURRENT I A)

Fig. 9.15; Eddy Current Signals from E-Brite 26-1 Tube With Increasing Saturation, (fgp » 100 kHz at Complete Saturation)

Eddy current testing at partial saturation may seem attractive since defect sensitivity is very high, it may in fact develop into a useful NDT technique. However, there are drawbacks; y^ is greater than one and is variable. This means eddy current penetration is not defined and conventional phase analysis is impossible. Testing tubes for defects at magnetic lupports could be a very questionable procedure since large permeability signals would be encountered which could be mistaken for or obscure defects. Even the best available saturation methods still encounter problems in detecting defects at steel baffle plates in some Monel A00 tubes which are only slightly magnetic.

Eddy current testing at partial saturation only measures permeability in a thin surface layer adjacent to the test coil. This classifies the technique with NDT methods such as magnetic particle inspection and leakage flux testing. Leakage flux testing responds to the distortion of magnetic flux at defects in a magnetized material using pickup coils or Hall effect sensors. Partial saturation ET with surface probes has an advantage over encircling (or internal) probes in the ability to separate permeability from lift-off variations (Figure 3.13). -182-

EXTERNAL MAGNETIZING COIL THROUGH HOLE \

INTERNAL ABSOLUTE PROBE

BALANCE POINT IN^AIR

WILL-

INCREASING FLUX DENSITY , (DECREASING PERMEABILITY)

MAGNETIZING / CURRENT (AMPS)\ \ \ \ 10 A - PROBE WOBBLE B - THROUGH HOLE C - 0. D. GROOVE D - I. D. GROOVE B C D

Fig. 9.16: Eddy Current Signals from E-Brite 26-1 Tube with Increasing Saturation, fgp = 100 kHz -183-

An example of the dangers of ET ferromagnetic materials at partial saturation is illustrated in Figure 9.17. It shows eddy current signals from calibration defects in a 3Re60 heat exchanger tube tested with a differential probe. (3Re6O requires a flux density of about 0.6T for complete saturation). Calibration defects yield signals which change in phase with increasing depth leading to the conclusion one may have a viable test technique. However, elastic deflection of the tube at a support plate gives change of permeability signals nearly identical to serious (50% and 75%) defects. Thia is due to magnetostriction: changes in magnetic propertiea due to elastic stress such as shown in Figure 9.10.

PARTIAL SATURATION PRDBE 1 \ \ 50% 75% HOLE

5 mm 1 mm 3mm 2mm 0

Fig. 9.17: Eddy Current Signals from 3Re6O Tube With Partial Saturation for Various Levels of Elastic Stress. Test Frequency 230 kHz. -184-

The problem of Figure 9.17 was overcome with a multimagnet probe similar to that developed for Monel 400 tubing (8). This eliminated the false defect signals at tube supports and made these heat exchangers inspectable by conventional ET techniques. It was fortunate these particular heat exchangers had nonmagnetic, Type 304 stainless steel, support plates. This permits tube saturation in the vicinity of supports. If the supports had been magnetic they would have provided a low reluctance alternative path to the saturation field leaving the tube only partially saturated. Nonmagnetic support materials improve inspectability of ferromagnetic tubes even though fretting wear may be difficult to detect with a conventional bobbin-type probe as discussed in Section 8.2.4.

9.5 SUMMARY Eddy current testing can be used to measure electrical resistivity and magnetic permeability. This parameter, in some cases, can be correlated to a material's chemical composition, hardness, heat treatment, etc. and therefore provide an indirect measurement of material properties. Material sorting by electrical resistivity can be done with general purpose eddy current instruments or with special instruments with meter output calibrated in % IACS. Care must be taken to obtain reliable results. Material sorting by magnetic permeability is not simple. It requires a sound knowledge of magnetic properties and eddy current testing. Most of the commercial equipment make use of hysteresis distortion and the method is empirical. It is more reliable to use general purpose eddy current equipment to roughly measure magnetic permeability and then correlate to material property.

Testing ferromagnetic materials for surface defects is possible but often unreliable. If material can be magnetically saturated, it appears as non-ferromagnetic material to the eddy currents. Testing at partial saturation results in good sensitivity to defects and to ferromagnetic anomalies but can result in false indications. It is possible to magnetically saturate some ferromagnetic tube alloys in unsupported tube sections, but nearly impossible under ferromagnetic baffle plates.

Magnetic permeability affects the following: - depth of penetration - probe inductance - operating point on impedance diagram - characteristic defect signal is no longer dependent on phase lag - drastically decreases signal-to-noise ratio. -185-

9.6 WORKED EXAMPLES 9.6.1 PROBLEM: Convert resistivity of 5.5 microhm-centimetres to conductivity in % IACS. SOLUTION: X IACS - 172/p

- 172/5.5 ' 31.3% 9.6.2 PROBLEM: Pure annealed iron under a magnetizing force, H, of 40 A/m results in a magnetic flux density, B, of 0.028T. Determine magnetic permeability and relative permeability in (a) the tesla, ampere/metre system of units and (b) the gauss, oersted system.

SOLUTION:

(a) y = B/H = 0.028/40 = 7.0 x 10~4 henry/m

7 Pr = U/VQ = 7,0 x 10~ /4TT x 10" = 557 (dimensionless)

(b) B = 0.028 x 104 = 280 gauss

H = 40 x 0.012566 = 0.503 oersted

y = B/H = 280/0.503 = 557 gauss/oersted

= 557/1.0 = 557 (dimensionless) -186-

9.6.3 PROBLEM: Calculate standard eddy current depth of penetration in carbon steel at a test frequency of 10 kHz (a) without saturation and (b) with complete saturation. P .= 15 microhm-centimetres, p. = 300

SOLUTION: yA = yr i

(a) From Equation 2.13(a)

6 = 50 /pTfy",

15 50 10 x 300

= 0.11 mm (0.004")

(b) 1.0 at saturation

/ 15 50 I104 x 1.0

= 1.94 mm (0.077") -187-

CHAPTER 10 - DEFINITIONS, REFERENCES AND INDEX

10.1 DEFINITIONS

This section lists the most common terms covered in the manual* For each term, the symbol, the SI units and the section where the topic is covered is given, followed by the definition.

ABSOLUTE PROBE - See Sections 5.2 and 7.2. - A probe having a single sensing coil.

ALTERNATING CURRENT - IA C , amperes; see Chapters 2 and 3. - A current flow changing in amplitude and direction with time.

ANOMALY - See Sections 6.5 and 8.3. - An unexpected, unclassified eddy current signal. - A false defect indication.

BRIDGE - See Section 4.2.1. - Electrical circuit incorporating four impedance arms. CALIBRATION STANDARD - A test standard used to estimate defect size and set—up instrument.

CAPACITIVE REACTANCE - Xc, ohms; see Section 3.2. - The opposition to changes in alternating voltage.

CHARACTERISTIC PARAMETER -"r2wa)J , dimensionless , see Section 5.6. - It allows test coil operating point to be specified in terms of a single quantity rather than four independent variables.

CHARACTERISTIC OR LIMIT FREQUENCY - fe, hertz, see Section 7.3.3.

CHARACTERISTIC FREQUENCY RATIO -f/fg - dimensionless, see Section 7.3.3. - It allows the test coil operating point to be specified in terms of a single quantity rather than four independent variables.

CIRCUMFERENTIAL COIL - see encircling and internal probes.

CONDUCTIVITY - CT(sigma), siemens/m; see Sections 2.4 and 9.2. - Measure of the ability of a material to conduct current (alternating or direct current).

CONDUCTOR - Material capable of carrying electrical current. -188-

COUPLING - The coil's magnetic field couples to the test sample. - The change in probe impedance is directly proportional to probe-sample coupling.

CURRENT -I, amperes, see Section 3.3 - Flow of electrons.

DEPTH OF PENETRATION (STANDARD) - S (delta), millimetres; see Section 2.4. - The depth at which the eddy current density has decreased to 1/e or 36.8% of the surface dens ity• - Also referred to as skin depth.

DEFECT - A flaw or discontinuity that reduces a material's integrity or load carrying capacity - may involve a loss of material.

DIFFERENTIAL PROBE - see Sections 5.2 and 7.2. - A probe having two sensing coils located side-by- side.

DIRECT CURRENT - I nc . amperes; see faction 3.3. - A current flow i:hat is constant in amplitude and direction with time.

DISCONTINUITY - A defect.

EDDY CURRENTS - see Chapter 2 and Sections 5.2.2 and 7.2.3. - A closed loop alternating current flow induced in a conductor by a varying magnetic field.

EDDY CURRENT METHOD - An electromagnetic NDT method based on the process of inducing electrical currents into a conductive material and observing the interaction between the currents and the material. In France it is known as the 'Foucault currents' method.

EDGE EFFECT - see Section 5.8.2. Signal obtained when a surface probe approaches the sample's edge.

EFFECTIVE DEPTH OF PENETRATION - see Section 2.4. - Depth at which eddy current density drops off to 5% of the surface density. -189-

END EFFECT - see Section 5.8.2. - Signal obtained when an Internal or encircling probe approaches the end of a tube or rod (similar to edge effect). ENCIRCLING PROBE (Coll)-see Section 7.2. - Also referred to as a feed-through coil. - A probe which completely surrounds test material; can be absolute or differential. FEED-THROUGH COIL - see encircling probe.

FERRITE - Ferromagnetic oxide material. - Used for cores in high frequency transformers. FLAW - A defect . FERROMAGNETIC - see Section 9.3. - A material with a relative magnetic permeability greater than 1.0

FILL-FACTOR - n (eta), Jimensionless; see Section 7.3. - It is a measure of coupling between the coil and test object. - Fraction of the test coil area filled by the test specimen. FOUCAULT CURRENTS METHOD - In France the Eddy Current Method is known as the 'Foucault currents' method.

FREQUENCY - f, hertz, see Section 2.4. - Number of cycles of alternating current per second. FREQUENCY (ANGULAR) -w(omega), radians/second; see Section 3.2. Angular velocity, where to = 2 irf. HYSTERESIS - See Section 9.3. - Magnetization curve.

a IACS - -rACS , %> see Section 9.2. - International Annealed Copper Standard. - Conductivity as a percentage of pure copper. INDUCTANCE - L, henries, see Section 3.2. - Ratio of the total magnetic flux-linkage in a coil to the current flowing through the coil. -190-

IMPEDANCE - Z, ohms, see Section 3.2. - The total opposition in an electrical circuit to flow of alternating current. - Represents the combination of those electrical properties that affect the flow of current through the circuit. IMPEDANCE METHOD - Eddy current method which monitors the change in probe impedance; both phase and amplitude.

INDUCTIVE REACTANCE - XL, ohms, see Section 3.2. - The opposition to a change in alternating current flow. INDUCTOR - A coil.

INTERNAL PROBE (COIL) - see Chapters 7 &?•*. 8. - A probe for testing tubes (or holes) from the inside. The coil(s) is circumferentially wound on a bobbin.

LIFT-OFF - L.O., mm, see Sections 5.5 and 5.8.4. - Distance between the coil of a surface probe and sample. - It is a measure of coupling between probe and s ample. MAGNETIC FLUX - , webers, see Section 9.3. MAGNETIZING FORCE - H, amperes/metre, see Section 9.3, Magnetic field intensity.. MAGNETIC FLUX DENSITY - B, tesla, see Section 9.3.

MODULATION ANALYSIS - An instrumentation method which separates desirable from undesirable frequency signals from the modulating envelope of the carrier frequency signal. - Test sample must move at constant speed.

NOISE - Any undesired signal that obscures the signal of interest. It might be electrical noise or a signal from specimen dimensional or property variations. -191-

NULL BALANCE - see Section 4.2.1.

OHM'S LAW - Electromotive force across a circuit is equal to the current flowing through the circuit multiplied by the total impedance of the circuit.

OPERATING POINT - see Sections3.5, 5.6 and 7.3.3. - Point on the impedance diagram that specifies the normalized inductive reactance and resistance of a coil.

OSCILLATOR - The electronic unit in an eddy current instrument that generates alternating probe excitation current.

PARAMETER - A material property or instrument variable. PERFORMANCE STANDARD - Also referred to as Reference Standard. A test standard used to qualify and calibrate a test system for a particular test. PERMEABILITY (MAGNETIC) - y(mu), henry/metre; see Sections 2.A and Section 9.3. or yr, dimensionless, relative magnetic permeability. - Ratio between flux density, B, and magnetizing force, H. Permeability describes the intrinsic willingness of a material to conduct magnetic flux lines.

PHASE LAG - 3(beta), radians or degrees; see Section 2.4. - A lag in phase (or time) between the sinusoidal currents flowing at the surface and those below the surface. PHASOR - see Section 3.3. - A vector describing sinusoidal signals; it has both amplitude and phase. PRIMARY FIELD - The magnetic field surrounding the coil due to the current flowing through it.

PROBE - Eddy current transducer.

REFERENCE COIL - Coil which enables bridge balancing in absolute probes. Its impedance is close to test coil impedance but does not couple to test material. -192-

RESONANCE - See Sections4.3, 5.9 and 7.2.5. A circuit having an inductor and capacitor connected in series or parallel. When inductive reactance equals capacitive reactance the circuit is tuned or in resonance.

RESISTANCE - R, ohms; see Section 3.2. - The opposition to the flow of electrical current. - Applies to DC and AC. RESISTIVITY - p , microhm-centimetre; see Sections 2.4 and 9.2 - Reciprocal of conductivity (p=l/o). SATURATION (MAGNETIC) - A condition where incremental magnetic permeability of a ferromagnetic material becomes 1.0. SECONDARY FIELD - The magnetic field produced by induced eddy currents. SEND-RECEIVE - See Sections3.4, A.5 and 5.4. The variations in the test object which affect current flow within the test object can be detected by observing their effect upon the voltage developed across a secondary receive coil. SIGNAL - A change in eddy current instrument output voltage; it has amplitude and phase.

SIGNAL-TO-NOISE RATIO - Ratio between defect signal amplitude and that from non-releyant indications. Minimum acceptable ratio is 3:1. SKIN DEPTH - See depth of penetration. SKIN EFFECT - See Section 2.4. - A phenomenon where induced eddy currents are restricted to the surface of a test sample. Increasing test frequency reduces penetration.

SURFACE PROBE - See Chapters5 and 6. - A probe for testing surfaces, which has a finite coverage. The coil is usually pancake in shape.

TEST COIL - Coil coupled to test material. It senses geometric, electric and magnetic changes in test material. -193-

VOLTAGE - V, volts, see Section 3.3. - Electric potential or driving force for current. - Output signal from an eddy current instrument. VOLTMETER - The instrument used to measure voltage.

VECTOR - see Section 3.3. - A quantity having amplitude (magnitude) and direction. Normally represented as a line whose length represents the quantity's magnitude and the angular position the phase (relative to some reference). -194-

10.2 REFERENCES

1. H.S. Jackson, "Introduction to Electric Circuits", 2nd edition, Prentice-hall, Inc., Englewood Cliffs, New Jersey (1965) .

2. C.V. Dodd, "The Use of Computer-Modelling for Eddy Current Testing", Research Techniques in NDT, Vol. Ill, edited by R.S. Sharpe, Academic Press Ltd., London, p. 429-479 (1977).

3. H.L. Libby, "Introduction to Electromagnetic Nondestructive Test Methods", Wiley-Interscience, New York (1971) .

4. "Nondestructive Testing Handbook", Vol. II, edited by R.C. McMaster, Ronald Press, New York, p. 36.1-42.74 (1963) .

5. "Eddy Current Testing, Classroom Training Handbook", General Dynamics/Convair Division, San Diego, California (1979). CT-6-5 Second Edition.

6. W.J. McGonnagle, "Nondestructive Testing", 2nd edition, Gordon and Breach, New York, p. 346-390 (1961).

7. F.R. Bareham, "Choice of Frequency for Eddy Current Tube Testing", British J. Applied Physics, VL, 218-222 (1960).

8. V.S. Cecco, "Design and Specifications of a High Saturation Absolute Eddy Current Probe with Internal Reference", Materials Evaluation, 2Z» 51-58 (1979).

9. J. Stanley, "Electrical and Magnetic Properties of Metals", American Society for Metals, Metals Park, Ohio (1963).

10. H.V. Pellegrini, "Assessing Heat Damage in Aluminum Alloys with an Eddy Current Testing Technique", Metals Progress, 117, 60-63 (1980).

11. ASME Boiler and Pressure Vessel Code, Section V, Article 8, Appendix 1, "Eddy Current Examination Method for Installed Non-Ferromagnetic Steam Generator Heat Exchanger Tubing" (1978).

12. "Nondestructive Inspection and Quality Control", Metals Handbook, Vol. 11, 8th edition, American Society for Metals, Metals Park, Ohio, p. 75-92 (1976).

13. R. Hochschild, "Electromagnetic Methods of Testing Metals", Progress in Nondestructive Testing, Vol. 1, MacMillan Co., New York, p. 59-109 (1959). -195-

10.3 INDEX

Absolute Probe - 56,105-109 Alternating Current - 8,16,21-23 Anomaly - 98 Bridge - 34-37 Bridge Balance - 34-37 Calibration Standard - 101-103,125 Capacitive Reactance - 20 Characteristic or Limit Frequency - 120-125,128 Characteristic Frequency Ratio - 120-125,130 Characteristic Parameter - 55,74-7 6,87,88,120 Circumferential Coil - 105,109,125 Conductivity - 11,163-166 Coupling - 25,29,55,107,113 Current - 5-10,21-23 Defect - 55,65,66,78,89-97,101,131-147,178 Depth of Penetration (Standard) - 13,17,18,79 Differential Probe - 57-58,105-109 Direct Current - 21,22 Discontinuity - 188 Eddy Currents - 6-18,59,60,109,110,132 Eddy Current Method (Testing) - 1,19,55,89,98,131,164 Edge Effect - 81 Effective Depth of Penetration - 14 Encircling Probe (Coil) - 105,113,116,120,151 End Effect - 189 Excitation Coil - 6,11,45,67 Faraday's Law - 9,17,49,60,69,116 Faraday, M. - 2 Farad - 20 Feed-Through Coil - 189 Ferrite - 41,189 Ferromagnetic - 10,30,98,168 Fill-Factor - 29,113-115,150 Flaw - 189 Forster - 3,120 Foucault Currents Method - 189 Frequency - 5,8,13,17,72,120,123,124,129,130 Frequency (Angular) - 8 Frequency Response - 53 Hall Detector - 6,33,46,181 Henry - 19 Hysteresis (B-H curve) - 169-172 IACS - 163-166 Impedance - 8,9,20,25,32 Impedance Diagrams - 25-31 Impedance Method - 24,33 Inductance - 19,61,62,63,110,111 Inductive Reactance - 20,27,69,176 Inductor - 19 -196-

Internal Probe (Coil) - 106 Lenz's Law - 9,23 Lift-off - 43-47,84 Limit Frequency - 120-125 Magnetic Field - 6-7 Magnetic Flux - 7-10 Magnetic Flux Density - 7,168,169 Magnetic Permeability - 11,13,71,72,94,98,99,149,150,168-174 Magnetic Saturation - 168-171,178-184 Magnetizing Force - 168,171 Modulation Analysis - 50 Noise - 34,37,41,50,87,161,190 Non-ferromagnetic - 10,98,151 Null Balance (Bridge Balance) - 34-36 Oersted - 6,8 Ohm's Law - 8,17,61,117 Operating Point - 27-29,31,76,98,99,120-122,133,150 Oscillator - 5,33,34,43 Parameter - 65,191 Performance Standard - 191 Permeability (Magnetic) - 11,13,71,72,94,98,99,149,150,168-174 Phase - 76,78 Phase Lag - 2,14-17,78,93 Phasor - 21 Primary Circuit - 8,25 Primary Field - 191 Probe - 55-60,105-110 Receive Coil - 6,24,67,81 Reference Coil - 36,56,57,106 Resistance - 19,28-31,131-13 3 Resistivity - 13,17,71,72,80,100,163-168 Resonance - 38,39,85,86,112 Saturation (Magnetic) - 171,178-184 Secondary Field - 10,191 Secondary Voltage - 78 Send-Receive - 6,24,33,45-48,81 Sensing Coil - 6,24 Signal - 192 Signal-to-Noise Ratio - 63,192 Similarity Condition (Law) - 75,122 Sinusoidal - 5,12 Skin Depth - 13,14,17,125 Skin Effect - 11 Speed of Response - 53 Standard Depth of Penetration - 13,14,17,79 Surface Probe - 55-59 Test Coil - ->6-57 Vector - 23 Voltage - 8,9,21,33 Voltmete r - 6. ISSN 0067 - 0367 ISSN 0067 - 0367

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