SER Cover.cdr

323 Pages·2010·3.83 MB·English

University of Alberta
Department of Civil &
Environmental Engineering
Structural Engineering Report No. 285
Reliability-Based Management of
Fatigue Failures
by
G. Josi
and
G.Y. Grondin
February, 2010
University of Alberta

RELIABILITY-BASED MANAGEMENT OF FATIGUE
FAILURES

by

Georg Josi

and

Gilbert Y. Grondin

Structural Engineering Report 285

Department of Civil and Environmental Engineering
University of Alberta
Edmonton, Alberta, CANADA

February, 2010
ABSTRACT
Fatigue assessments have been carried out predominantly with quasi-deterministic
approaches, such as the use of S–N curves. However, both the loading and the
resistance of fatigue prone components are subjected to significant uncertainties.
Consequently, a prediction of the remaining fatigue life based on deterministic load
and resistance models can lead to unreliable results. This work presents a general
reliability-based approach to predict fatigue life of steel components. The approach
incorporates prediction of fatigue crack initiation, modeled with a strain-based
correlation approach, and propagation, modeled using a linear elastic fracture
mechanics approach, and is applicable to new, cracked or repaired structural
components.
Based on the analysis of existing test results and additional crack initiation and
propagation tests on weld metal, the relevant probabilistic fatigue material
properties of grade 350WT steel and a matching weld metal were established. An
experimental program was carried out on welded details tested either in the as-
welded, stress-relieved, conventionally peened, or ultrasonically peened condition.
It was demonstrated that ultrasonic peening is superior to the other investigated post
weld treatment methods. Using finite element analyses, the results of the tests were
deterministically predicted for several different initial conditions, including initial
flaw and crack sizes and locations, as well as different levels of residual stresses. A
model incorporating an initial flaw and accounting for crack closure and the
threshold stress intensity factor range was retained.
A probabilistic analysis using Monte Carlo Simulation was carried out to calibrate
the relevant parameters. A general reliability-based approach, which includes both
the loading and resistance sides of the limit state function was proposed and applied
to three practical examples: prediction of test results from two test programs and the
prediction of the remaining fatigue life of a cracked component as a function of the
safety index. These three applications demonstrated that accurate fatigue life
predictions targeting a predefined safety index are achieved.
ACKNOWLEDGEMENTS
This research was funded by Syncrude Canada Ltd and the Natural Sciences and
Engineering Research Council of Canada through a collaborative research and
development grant.
Financial support to the senior author in the form of scholarships from Cohos Evamy
Intagratedesign (Structural Engineering Scholarship), the CISC Alberta Region (Geoffrey
L. Kulak Scholarship), Alberta Ingenuity, the Faculty of Graduate Studies (Mary Louise
Imrie Award), and the Graduate Student Association is acknowledged with thanks.
The authors would like to thank Dr. Khaled Obaia of Syncrude Canada Ltd for the
continuing assistance with establishing contacts with Syncrude's technical staff who
provided the direction and feedback required to provide the direction for this project.
The experimental work for this project was conducted in the I.F. Morrison Laboratory of
the University of Alberta and the Syncrude Research Laboratory. The support from the
technical staff at both these facilities was invaluable. Their support throughout the
experimental program is acknowledged with thanks.
TABLE OF CONTENTS
1. INTRODUCTION 1
1.1 Motivation 1
1.2 Objectives and Scope 1
1.3 Methodology 2
1.4 Organization of the Thesis 3
2. BACKGROUND 6
2.1 Introduction 6
2.2 Fatigue 6
2.2.1 General 6
2.2.2 Parameters Influencing Fatigue Life 6
2.2.3 Fatigue Design Methods 9
2.2.4 Fracture Mechanics 12
2.2.5 Fatigue Failure Criteria 25
2.3 Fatigue Repair Methods 26
2.3.1 Introduction 26
2.3.2 Mechanical Repairs 27
2.3.3 Welded Repairs 27
2.4 Reliability Concepts and Probabilistic Analyses 30
2.4.1 Introduction 30
2.4.2 Reliability Concepts in Structural Engineering Applications 31
2.4.3 Probabilistic Analyses of the Fatigue Behaviour of Welded
Structures 33
2.5 Summary 36
3. LITERATURE REVIEW OF SELECTED TOPICS 38
3.1 Introduction 38
3.2 Fatigue Material Properties 38
3.2.1 Introduction 38
3.2.2 Crack Initiation 38
3.2.3 Crack Propagation 40
3.2.4 Summary and Conclusions 44
3.3 Initial Flaw Size and Shape 45
3.3.1 Introduction 45
3.3.2 Codes and Recommendations 46
3.3.3 Crack Sizes Reported in Research Projects 48
3.3.4 Summary and Conclusions 54
3.4 Residual Stresses 55
3.4.1 Introduction 55
3.4.2 Repair Weld 56
3.4.3 Fillet Welds and Groove Welds at T-Joints 57
3.4.4 Relaxation of Residual Stresses during Fatigue Loading 60
3.4.5 Summary and Conclusions 61
3.5 Ultrasonic Peening 62
3.5.1 Introduction 62
3.5.2 Peening 62
3.5.3 Peening Intensity – the Almen Method 64
3.5.4 History of Ultrasonic Peening 65
3.5.5 Mechanism of Ultrasonic Peening 66
3.5.6 Effects of Ultrasonic Peening on the Fatigue Behaviour of Welds 67
3.5.7 A Review of Practical Applications of Ultrasonic Peening in
Structural Engineering 70
3.5.8 Summary and Conclusions 77
3.6 Summary 78
4. FATIGUE MATERIAL PROPERTIES 80
4.1 Introduction 80
4.2 Base Metal: Grade 350WT Steel 80
4.2.1 Introduction 80
4.2.2 Material Standard Requirements 80
4.2.3 Cyclic Properties 81
4.2.4 Crack Initiation Properties 81
4.2.5 Crack Propagation Properties 83
4.2.6 Summary 86
4.3 Weld Metal: Matching Grade 350WT 86
4.3.1 Introduction 86
4.3.2 Chemical Composition 87
4.3.3 Welded Plates 88
4.3.4 Tension Coupon Tests 88
4.3.5 CVN Tests 90
4.3.6 Crack Initiation Tests 93
4.3.7 Crack Propagation Tests 105
4.3.8 Summary 114
4.4 Comparison with Properties Reported in the Literature 114
4.4.1 Crack Initiation 114
4.4.2 Crack Propagation 116
4.5 Summary and Conclusions 117
5. EXPERIMENTAL PROGRAM 122
5.1 Introduction 122
5.2 Small Scale Tests 122
5.2.1 Introduction 122
5.2.2 Welded Plates 123
5.2.3 Test Specimens and Test Matrix 123
5.2.4 Preparation of the Coupons 124
5.2.5 Test Set-Up and Instrumentation 128
5.2.6 Results of Tests on Ground Flush Specimens 130
5.2.7 Specimens in the As-Welded Condition 130
5.2.8 Discussion of Small Scale Tests 135
5.3 Large Scale Test Set-Up 136
5.3.1 Introduction 136
5.3.2 Description of Test Specimens and Set-Up 137
5.3.3 Preparation of Test Specimens 140
5.3.4 Test Procedure 143
5.4 Initial Gouging and Welding Procedure 149
5.4.1 Introduction 149
5.4.2 Test Matrix 149
5.4.3 Test Results 150
5.4.4 Examination of Fracture Surfaces 152
5.4.5 Discussion 153
5.5 Improved Gouging and Welding Procedure 153
5.5.1 Introduction 153
5.5.2 Test Matrix 154
5.5.3 Test Results 156
5.5.4 Examination of Fracture Surfaces 159
5.5.5 Discussion 161
5.6 Summary and Discussion 161
6. DETERMINISTIC PREDICTION OF THE SMALL SCALE TEST RESULTS 163
6.1 Introduction 163
6.2 Effect of Residual Stresses during Stable Crack Propagation 164
6.3 General Approaches to Fatigue Life Prediction 166
6.3.1 Introduction 166
6.3.2 Initial Flaw or Crack Shape 167
6.3.3 Initial and Transitional Flaw or Crack Size 168
6.3.4 Small Crack Effect during Crack Propagation 168
6.3.5 Residual Stress Effect during Crack Propagation 169
6.3.6 Fatigue Material Properties 169
6.3.7 Stress Intensity Factors 169
6.3.8 Final Crack Size 171
6.3.9 Summary 172
6.4 Finite Element Analyses 174
6.4.1 Introduction 174
6.4.2 Global Finite Element Model 174
6.4.3 Local Finite Element Models 176
6.4.4 Application of External Load and Residual Stresses 180
6.4.5 Results 181
6.5 Fatigue Life Predictions of Tested Specimens 185
6.6 Discussion of Analytical Results 186
6.7 Summary 187
7. VALIDATION OF FATIGUE PREDICTION MODELS THROUGH
PROBABILISTIC ANALYSIS 189
7.1 Introduction 189
7.2 Reliability Methods 189
7.2.1 Introduction 189
7.2.2 Monte Carlo Simulation (MCS) 190
7.2.3 First Order Reliability Method (FORM) 191
7.2.4 Evaluation and Implementation of MCS and FORM 193
7.3 Choice of Fatigue Life Prediction Model 194
7.4 Probabilistic Modeling of Parameters 195
7.4.1 Introduction 195
7.4.2 Fatigue Initiation and Propagation Properties 196
7.4.3 Initial Flaw Size and Shape 197
7.4.4 Transitional Crack Size 197
7.4.5 Final Crack Size 198
7.4.6 Residual Stresses 199
7.4.7 Strain Amplitudes and Maximum Stresses in Initiation 200
7.4.8 Stresses in Crack Propagation 201
7.4.9 Summary 201
7.5 Probabilistic Fatigue Life Predictions 202
7.6 Calibration of Probabilistic Parameters 204
7.7 Summary 208
8. GENERAL RELIABILITY-BASED APPROACH 210
8.1 Introduction 210
8.2 Target Reliability 210
8.3 Loading 211
8.3.1 Introduction 211
8.3.2 Design Code Approach 212
8.3.3 Loading According to In-Situ Measurements 215
8.3.4 Equivalent Strains and Stresses 217
8.4 Resistance 219
8.4.1 Introduction 219
8.4.2 Fatigue Material Properties 220
8.4.3 Detail with no Imperfections 220
8.4.4 Severity of Imperfections and Residual Stresses 221
8.4.5 Failure Criterion 222
8.5 General Reliability-Based Approach to Predict Fatigue Performance 223
8.6 Sample Applications 223
8.6.1 Introduction 223
8.6.2 Fatigue Repair with Hole-Drilling and Expansion 224
8.6.3 Fatigue Life of Welded Non-Load-Carrying Cruciform Specimens 229
8.6.4 Fatigue Life Prediction of an Excavator Boom 237
8.7 Summary 244
9. SUMMARY, CONCLUSIONS, AND RECOMMENDATIONS 247
9.1 Summary 247
9.2 Conclusions 248
9.3 Recommendations 250
List of References 252
Appendix A Probability Density Functions 270
Appendix B Gouging and Welding Procedure Specification 273
Appendix C Load History of Cracked Reaction Beam 276
Appendix D Crack Closure 279
Appendix E Mesh Refinement Study 284
Appendix F Sample Fatigue Life Calculation 289
Appendix G Deterministic Predictions of Test Results 294
Appendix H Equivalent Strains and Stresses 309