1 The Effect of Temperature and Strain-Rate on the Deformation and Fracture of Mild Steel Charpy Specimens The,,,;.is Submitted in Suppliction for the Degree of Doctor of Thilosophy by SLoicor. Rodney Uilshaw D.Sc. A.12.3.M. (London) 1961. Deprtmeilt of -.ysicalietallurrsy- Imperial College University of London December 1964 2 ABSTRACT High nitrogen mild steel Charpy specimens were deformed at room temperature in three-point bending; the distribution of plastic deformation revealed by Fry's etch was measured at different applied loads for both substantially plane stress and plane strain conditions. Specimens were deformed to fracture at striker velocities of 0.05, 50 and 30,000 cm/min within the temperature range - 196°C to + 100°C. These studies have revealed the existence of ; a) a transition from ductile tearing at the notch root, to internal cleavage, b) at a lower temperature a bimodal distribution of fracture loads, indicating a transition in the mode of cleavage fracture and c) a decrease in the fracture load associated with the onset of twinning. 3 The relationship between these transition temperatures and the strain-rate may be expressed by an Arrhenius eauation with different apparent activation energies, which are not comparable with the activation energies for yielding. From this it is concluded that the effect of temperature and strain-rate on the cleavage stress is not entirely due to their influence on the yield stress. The implication of this when predicting notch impact transitions from tensile data is disc-p.ssed and a method of predicting the existence of a Crack arrest temperature is postulated. ERRATA page line 12 3 degress should read degrees 17 16 slip 11 II ship 27 8. maitrix II 1/ matrix 2 11 49 6PGY - 565 Kg/m pGY = 565 Kg 8 P = 505 40 Kg/mm2 11 PGY = 505 ±40 K GY 51 10 505 ± 40 Kg/mm2 11 II 505 ± 40 Kg 55 13 depositied fl II deposited 63 7 low 11 11 four 69 11 due to plastic zone 11 11 due to the plastic zone 70 6 alip 11 11 slip 9 connot 11 cannot 80 19 fired 11 fixed 81 11 Taly-Surface It Taly-Surf 107 6 at It Il and airx 1 + 2 eic/ez 110 3 Equation 3 II 0; 2 + . 131 11 derivate 11 derivative 161 5 odes 11 modes 199 10 w-isume that condition 11 II assume that the condition 202 12 Marjoine 11 Manjoine 211 16 Llider 11 'Alders 212 19 above If II below -1 22 t= 10s II 11 105 sec 222 11 mode It model 223 5 region which if region in which 229 19 effect 11 11 effects Reference Alexander J. M. and Komoly T. J. 1962. J. Mech. Phys. Solids 10 265. 14 CONTENTS Page Abstract 2 Contents 4 Chapter 1. Introduction and Previous Work 1.1. Introduction 10 Review of Previous Work 1.2. Classical Theories of Fracture 15 1.2.1. Energy Criterion 16 1.2.2. Critical Displacement Criterion 19 1.2.3. Stress Criterion 20 1.3. Micromechanisms of Cleavage Fracture 21 1.3.1. Slip Initiated Mechanisms 21 1.3.2. Deformation Twinning and Crack Initiation 25 1.3.3. The Ductile-Brittle Transition 27 1.4- Deformation and Fracture of Mild Steel at Low Temperatures 29 1.4.1. The Ef_rect of Strain—Rate 32 1.5. The Effects of a Notch 33 5 Page Part I Chapter 2. The Deformation of Charpy Specimens before General Yield 39 2.1.1. Summary 40 2.1.2. Choice of Material 40 2.1.3. Specimens 44 2.1.4. The Deformation Jig 44 2.1.5. The Load Deflection Curve 47 2.2.1. The Distribution of Plastic Strain below General Yield 50 2.2.2. The Schlieren Technique 51 2.2.3. Fry's Etch Technique 55 2.2.4. Plastic Zone Size 57 Discussion 2.3.1. Plane-Stress Deformation 59 2.3.2. Plane-Strain. Deformation 62 2.3.3. Effect of Specimen Width 64 2.3.4. Elastic Stress-Concentration Factor 68 2.3.5. Distribution of Stress below the Notch 72 - 6 Page Chapter 3. The Deformation of Charpy Specimens beyond General Yield 79 3.1.1. Summary 79 3.1.2. Experimental 79 3.1.3. Measurement of Root Strain 81 3.1.4. Calibration 87 3.1.5. Strain-Measurement 92 Discussion 97 3.2.1. Deformation Sequence 98 3.2.2. Strain at the Notch Root 101 3.2.3. Crack Opening Dislocation (C.O.D.) 1014 3.2.4. Strain-Rate 107 3.2.5. Biaxiality 109 7 Page Part II Chapter 4. The Effect of Temperature on the Fracture of Notched Bars 4.1.1. Summary 112 4.1.2. Experimental 112 4.2.1. Initiation Transition 119 4.2.2. Results 120 4.2.3. Metallographic Examination 133 4.3.1. Cementite Cracks 137 4.4.1. Brittle Fracture above General Yield 148 4.5.1. The Bimodal Transition 151 4.5.2. The Probability of Microcrack Formation 154 4.5.3. The Stability of Microcracks 155 4.5.L. Double Notched Charpy Tests 161 4.6.1. The Twinning Transition 162 - 8 Page Chapter 5. The Effect of Strain-Rate on the Fracture of Notched Bars 5.1.1. Summary 172 5.1.2. Instrumented Charpy Impact Tests 173 5.1.3, Intermediate and Slow Bend Tests 177 Discussion 5.2.1. The Effect of Strain-Rate on the Load-Deflection Curve 182 5.2.2. Instrument Impact Tests 185 5.2.3. Slow Bend Tests 188 5.2.4. The Effect of Strain Rate on the Fracture Load Temperature Diagram 188 5.2.5. Measurement of Dynamic Yield Stresses 199 5.2.6. Prediction of Dynamic Transition Temperatures from Tensile Data 200 9 Page Chapter 6, Tensile Tests 205 6.1. Experimental 206 6.2. Impact Tensile Tests 207 6.3. Discussion 210 6.4. Strain-Hardening 214 6.5. Fracture at - 190C 216 Chapter 7. Discussion of Conclusions 7.1. Stress and Strain Distributions around a Notch 219 7.2. Cementite Cracks 221 7.3. Crack Initiation 223 7.4. Crack PPopagation 225 7.5. Adiabatic Heating 227 7.6. The Char:Dy Impact Test 228 7.7. Summary of Conclusions 232 Acknowled7ements 236 Referenco 237 - 10 - CHAPTER 1 Introduction and Literature Survey 1.1. Introduction Mild steel structures have been known to fracture in a catastrophic and unpredictable manner. This phenomenon is generally termed brittle fracture and is recognised by the small amount of plastic deformation accompanying such a fracture. The source of failure is usually a stress concentration in the form of a sharp notch, or a crack, which is the result of metallurgical damage occurring during construction or service. A brittle crack can propagate through steel olates under a nominal stress of about 30 per cent of the yield stress, absorbing a relatively small amount of energy and travelling at a velocity of about one third the velocity of sound in the material. It is well known that conditions of low temperature, high strain-rate and a triaxial state of stress, which prevails beneath a notch, all tend to - 11 — favour the initiation of fracture. A number of large and small scale laboratory tests have been devised to simulate the actual service failures and to assess a material's susceptibility to such failures. (see Tipper 1963). Whilst some of these tests appear to emphasise the initiation of cleavage cracks and others the stopping of a propagating brittle crack, almost all of them involve the introduction of a notch and the observation of the onset of brittle behaviour as the test temperature is lowered. Since each of these tests emphasise different features of the brittle-fracture phenomenon to varying degrees it is not surprising that they evaluate the ability to resist brittle fracture in different ways. (A.A.C.S.S. Report P.9. H.M.S.°. 1960). The most common laboratory test for brittle fracture is the V-notch Charpy impact test. The details of this test are given in B.S. 131 : Part 2 1959. British Standards Inst itut ion. A small rectangular prismatic specimen containing a notch is broken in three point bending under impact loading at various temperatures and the energy absorbed — 12 — during fracture is measured. This energy changes considerably over a transition range of a few tens of degrees, and the fracture appearance changes accordingly from the fibrous appearance associated with the high energy ductile fractures to the shiny crystalline appearance resulting from cleavage on the -c1001 planes. The temperature at which 15 ft lbs of energy is absorbed is defined as a ductility transition temperature Td and is associated with the temperature region in which cleavage cracks initiate the final fracture. At higher temperature, a fracture transition Tf for 50 per cent fibrous appearance satisfies a condition for which brittle fracture may be initiated by a ductile crack, and is also used as a design criterion. Although the use of such data has met with some degree of success, particularly in the design of ships, each type of service requires a new correlation. The inability to correlate data, from various tests on a variety of steels, has made it desirable to pursue this problem on a fundamental basis. - 13 - The most detailed studies of deformation and fracture in notched bars have been performed by Green and Hundy (1956), Cmssard et al (1956) and Knott (1962). The latter author fractured deeply notched specimens under slow four point bend and related the fracture mechanisms to the deformation characteristics of the specimen, at various temperatures.