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Corrosion Fatigue Short Crack Growth Behaviour in a High Strength Steel

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Corrosion Fatigue Short Crack Growth Behaviour in a High Strength Steel Corrosion Fatigue Short Crack Growth Behaviour in a High Strength Steel Ghulam Murtaza BSc. Eng. (Hons.) Thesis submitted to the University of Sheffield for the Degree of Doctor of Philosophy in the Faculty of Engineering Department of Mechanical and Process Engineering University of Sheffield March 1992 Preface This thesis is based on the findings of research carried out in the Department of Mechanical and Process Engineering at the University of Sheffield. The content of this thesis is original except where specific reference is made to other work. No part of this thesis has been submitted to any other university for a degree. 1 Acknowledgements I would like to express my hearty gratitude to my supervisors Dr. R Akid and Professor K J Miller for their invaluable guidance, help and encouragement during my time within the department. My thanks are also due to the Head of the De­ partment of Mechanical and Process Engineering for the use of the departmental facilities. I am also thankful to Dr. E R de Los Rios and Dr. M W Brown for their sincere advice and help. I am especially grateful to the Ministry of Science and Technology, Government of Pakistan for a research award and to the Pandrol International UK for providing material and other useful information. I would like to thank members of the technical staff for their help, notably Mr. J Smith for workshop work, Mr. J Goodliffe for maintaining the electronic and computer systems for expeimental facilities, Mr. D Halford for his photographic skills and Mr. K S Morris for tracing the figures. Finally I shall also be indebted to my all family members for their continued encouragement and support throughout my studies. 11 Contents Preface i Acknowledgements ii Contents iii Summary be Nomenclature xi 1 Introduction 1 2 Literature Review- - Air Fatigue 3 2.1 Introduction .. 3 2.2 Historic Survey 3 2.3 Fatigue Mechanisms 5 2.4 Crack Initiation ... 7 2.5 Crack Propagation • 11 2.5.1 Stage I Crack Propagation. ............ 12 111 2.5.2 Stage 11 Crack Propagation . 12 2.5.3 Stage HI Crack Propagation. 13 2.6 Fracture Mechanics . 13 2.6.1 Introduction 13 2.6.2 Linear Elastic Fracture Mechanics 14 2.6.3 Elastic Plastic Fracture Mechanics 17 2.7 Short Crack Behaviour . 20 2.8 Short Fatigue Crack Growth Models 26 3 Literature Review - Environment-Assisted Failure 31 3.1 Introduction ............. 31 3.1.1 Stress Corrosion Cracking. 32 3.1.2 Hydrogen Embrittlement 33 3.1.3 Corrosion Fatigue 35 3.2 Corrosion Fatigue. 35 3.2.1 Introduction 35 3.2.2 Early Studies 37 3.2.3 Failure Mechanisms 37 3.2.4 Crack Initiation .. 38 3.2.5 Crack Propagation . 46 3.3 Variables Affecting Corrosion Fatigue 48 3.3.1 Effect of Loading Variables .. 48 3.3.2 Effect of Environmental Variables 52 iv 3.3.3 Effect of Metallurgical Variables .. 54 3.4 Preventive Measures for Corrosion Fatigue . 55 3.5 Electrochemical Studies . 58 3.6 Environment-Assisted Fatigue Crack Growth Models. 62 3.6.1 Introduction . 62 3.6.2 Long Cracks 64 3.6.3 Short Cracks . 67 4 Experimental Work 72 4.1 Introduction.. 72 4.2 Material...... 73 4.2.1 Chemical Composition. 73 4.2.2 Heat Treatment . 73 4.2.3 Microstructure. 74 4.2.4 Mechanical Properties . 74 4.3 Test Facilities • • • . • . " . 75 4.3.1 Test Specimen . , . .. 75 4.3.2 Test Machine . .. 75 4.3.3 Specimen Grips. • • • . 78 4.3.4 Transducer Cams and Mountings . " . 78 4.3.5 Environment Circulation System • • . .•. 78 4.4 Specimen Preparation . 79 4.4.1 Surface Finishing . .. " . • •. 79 v 4.4.2 Etching . , . 80 4.4.3 Reference Marking . · . 80 4.5 Crack Growth Monitoring 81 4.5.1 Plastic Replication Technique . 81 4.5.2 Direct Observation ... ..... 82 4.5.3 Image Analysis System ... 83 4.6 Fatigue Tests . · .... 84 4.6.1 Test Procedure 84 4.7 Electrochemical Tests 86 4.7.1 Introduction . .... 86 4.7.2 Test Equipment. .. 86 4.7.3 Test Procedure . · .... .. , 87 4.8 Evaluation of Hydrogen Embrittlement . 88 :> Results 90 5.1 Introduction... • • • « • · . 90 5.2 Microstructure . " . 91 5.3 Cyclic Stress-Strain Behaviour . 91 5.4 S-N Curves . • . • . 93 5.5 Crack Initiation and Growth .- . 95 5.5.1 Air Fatigue Tests . • . 95 5.5.2 Corrosion Fatigue Tests . 96 5.5.3 Intermittent Air Fatigue/Corrosion Fatigue Tests . 99 vi 5.6 Electrochemical Tests ..•.. · .. 100 5.7 Evaluation of Hydrogen Effects .. 101 5.7.1 Hydrogen Pre-charged Fatigue Tests ...... 101 5.7.2 Corrosion Fatigue Tests Under an Applied Cathodic Potential 101 5.7.3 Pre-charged Tensile Tests . ...... 102 6 Crack Growth Modelling and Analysis 103 6.1 Introduction. .. .. · .• 103 6.2 Air Fatigue Modelling • .• 104 6.2.1 Short Crack Growth Equation • •• 104 6.2.2 Long Crack Growth Equation . ... 106 6.2.3 Fatigue Lifetime Calculations . .... 107 6.3 Corrosion Fatigue Modelling. .. 108 6.3.1 A Modified Hobson's Model .. · .. 108 6.3.2 A Superposition Model · . · ..... 110 7 Discussion 114 7.1 The Fatigue Limit . , . " . , . · . .114 7.2 Crack Initiation Behaviour. ..•. 115 7.2.1 Air Fatigue . • .. · . · .. 115 7.2.2 Pit Development . · . " . • .• 116 7.2.3 Corrosion Fatigue Crack Ini tiation . · . · •. 118 7.3 Crack Growth Behaviour. ... · .. 118 7.3.1 Air Fatigue . • . , . 119 vii 7.3.2 Corrosion Fatigue •....•........... · . 120 7.3.3 Intermittent Air Fatigue/Corrosion Fatigue ... · . 123 7.4 Stage I - Stage 11 Transition •.... · ....... 125 7.5 Threshold Crack Growth. • • . · .... " . 127 7.6 Electrochemical Studies .••.... · . 128 7.7 Fractography . · . 129 7.7.1 Air Fatigue • • • it • · · · · . · · . 129 7.7.2 Corrosion Fatigue . · · . · . 130 7.7.3 Intermittent Air Fatigue/Corrosion Fatigue · · . • .. 130 7.7.4 Normal and Pre-charged Tensile Tests . · · · .. · · . 131 7.8 Effect of Hydrogen . · . " . · · . · . 131 7.9 Crack Growth Models . · . · . · . 132 8 Conclusions and Future Work 135 8.1 Conclusions. " . " " " " " " " " " " " " . " " . " " " " " " " " . " • • 135 8.2 F\J.ture Work """..""".."""""..,,",,.,,.,,""""""" 137 Bibliography 139 Tables Figures Appendices viii Summary A frequent cause of the premature failure of structural components is Corrosion Fa- tigue Cracking. Historically corrosion fatigue studies have shown that this failure process depends strongly on the interactions between loading mode, metallurgical texture and electrochemical parameters. This has become a serious problem for concerns such as the nuclear, automobile, oil, gas, aerospace and marine industries. This research study was carried out using a quenched and tempered silico- manganese spring steel (DS 250A53). Smooth hour-glass shaped fatigue specimens were tested under fully reversed torsional loading in both laboratory air and acr- ated O.6M NaCI solution environments. Crack growth behaviour in both air and corrosion fatigue tests was monitored using a plastic replication technique. Inter- mittent air fatigue/corrosion fatigue tests were also conducted at sub-fatigue limit stress levels in an attempt to determine an environment-assisted critical (thresh- old) crack length necessary to cause subsequent air fatigue failure and therefore elucidate the mechanisms operative during the first stages of crack development and growth. To assist in this electrochemical experiments were performed to de- termine the corrosion characteristics for this metal-environment system. In air fatigue tests cracks initiated at non-metallic inclusions due to a strain incompatibility between the inclusion and the matrix. Air fatigue modelling was based on the observation that crack growth rate decreases as cracks approach microstructural barriers. In the present study it is suggested that the 4th prior austenite grain boundary was the major barrier to a growing crack. This regime of crack growth is described as the 8hort crack regime and may be represented by • If lX the following equation; After overcoming the major microstructural barrier crack growth rate increases with an increase in its length. This regime of crack growth is represented by long crack regime and may be quantified by the expression; Corrosion fatigue crack initiation was associated with chemical pitting of these inclusions. Failure at stresses close to the 'in-air' fatigue limit was due to the coalescence of a small number of cracks. While at low stresses growth of individual cracks led to failure. Corrosion fatigue crack growth modelling suggested that a chemical driving force arising from chemical reactions was present in addition to the mechanical driving force of the applied stress. The presence of this additional force enabled a crack to continue its propagation at low stresses which would otherwise arrest under air fatigue conditions. Corrosion fatigue crack growth rate was calculated using the following superposition model. where (:~)" and (:~)e represent air fatigue crack growth rate and environmental crack growth rate respectively. ,/ x Nomenclature A, B, G.o, G,o, Gp, (}, f3: constants ao: initial crack length a: surface crack length aov: average crack length a ,: failure crack growth ar : surface area ath: threshold crack length b: Burgers vector d: microstructural dimension dm : dominant deceleration barrier length Dt : threshold crack growth rate ;;;': crack growth rate ( ;;;. )cJ: corrosion fatigue crack growth rate (1!i )e: environmental crack growth rate E: Young's modulus f: cyclic frequency F: Faraday's constant G: shear modulus K: stress intensity factor M: molecular weight N: number of loading cycles Ncl: number of cracks contributing to corrosion fatigue failure xi / NJ: number of cycles to failure Ni: number of cracks initiated in corrosion fatigue N,: number of cycles for long crack regime Np: number of cycles for pit development N.: number of cycles for short crack regime Nt : calculated total number of cycles R: stress ratio T: torque t: time Y: geometry factor z: valence charge on cation T: shear stress AT: shear stress range 0': stress AO': stress range 0'1/: yield stress O'CJI: cyclic yield stress UT: tensile strength TJ/: fatigue limit shear stress AK: stress intensity factor range AKth: threshold stress intensity factor range AKr: mode I stress intensity factor range ro: minimum radius of cross-section "" .
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