Corrosion 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 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 Cracking. 32

3.1.2 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.