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'infor- mation 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 ferro- magnetic 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