Fatigue of Structures and Materials

527 Pages·2004·31.322 MB·English

by  Jaap Schijve (auth.)

Fatigue of Structures and Materials
Fatigue of Structures
and Materials
by
JAAP SCHIJVE
Professor of Aircraft Materials (Emeritus),
Delft University of Technology,
Faculty of Aerospace Engineering,
Delft, The Netherlands
Previously:
Head of the Structures and Materials Division of the National Aerospace Laboratory NLR,
Amsterdam/North-East polder, The Netherlands
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Preface
This book is primarily a textbook. It is written for engineers, students and teachers, and it
should also be useful for people working on various topics related to fatigue of structures
and materials. The book can be used for graduate and undergraduate courses and for short
courses for people already working in the industry, laboratories, or research institutes.
Furthermore, the book offers various comments which can be useful to research-workers
in order to consider the practical relevance of laboratory investigations and to plan future
research.
An important theme of the book is the understanding of what happens in the material
of a structure in service if the structure is subjected to a spectrum of cyclic loads.
Knowledge of the fatigue mechanism in the material and how it can be affected by a large
variety of practical conditions is essential for dealing with fatigue problems. The designer
of a dynamically loaded structure must“design against fatigue”. This includes not only the
overall concept of the structure with related safety and economic aspects, but also questions
on detail design, joints, production and material surface quality. At the same time, the
designer must try to predict the fatigue performance of the structure. This requires a
knowledge of the various influencing factors, also because predictions on fatigue have their
limitations and shortcomings. Similar considerations arise if fatigue problems occur after
a long period in service when decisions must be made on remedial actions.
Fatigue of materials and structures covers a rather wide scope of different topics. The
purpose of the book is to highlight these topics, to indicate how they can be analyzed, and
how this can contribute to the solution and prevention of in-service structural fatigue
problems. The book is not “cookbook engineering” with a set of rules, data sheets and
equations, which are supposed to reduce the fatigueproblem to a routine procedure. It may
even be dangerous to deal with fatigue properties of a structure in this way. Fatigue
predictions are covered in the present book, but an essential message is that prediction
without understanding the relevant aspects involved is unacceptable. The challenge of
working on fatigue of structures and materials is in the understanding.
Chapter 1 gives a general survey of the fatigue problem of structures with some brief
comments on the significance of the aspects involved. This serves as a kind of a program
for the following chapters, which are grouped in five different parts, see the Contents. A
final chapter has been added as Part 6, dealing with the fiber-metal laminates (Arall and
Glare), fatigue resistant sheet materials developed in Delft. This final chapter should be
considered as a kind of an extra text outside the main scope of the book which is
concentrated on structures of metallic materials.
Each chapter is more or less a self-contained coverage of the subject of the chapter,
which could be helpful for including or omitting certain chapters in a course. At the end of
each chapter the highlights of that chapter are recalled, not a summary of the chapter, but
just the more important points to remember.
The number of references to the literature is limited, and much smallerthan usual for
a monograph. But this book is primarily a textbook, and students need not consult the vast
amount of publications from which our present knowledge has emerged. Research-workers
v
vi Preface
who want to investigate specific problems in detail will find the access to the literature by
various computerized retrieval systems. Literature sources from which results or pictures
have been used in this book are listed at the end of each chapter. The lists are supplemented
by a small number of general references, mainly books and conference proceedings. Most
references are in the English language which now appears to be the most used language for
publications and conferences.
The present book has some predecessors, a course book on Fatigue of Materials (in Dutch),
with co-authors another one on Fatigue of Structures (also in Dutch). A course book (in
English) for students in Delft was written in the eighties, and more recently course notes for
an International Course on Fatigue and Damage Tolerance. Although my professional
background has been mainly in aircraft structures, I always had a keen interest in fatigue of
all kinds of structures, on which I have done quite a bit of work. After working some 40
years on a large variety of fatigue problems time came available to write the present book.
Acknowledgments:
I have been working for about 20 years in the Structures and Materials laboratory of the
National Aerospace Laboratory NLR in Amsterdam and the North-East polder, and for
another 20 years in the Structures and Materials laboratory of the Faculty of Aerospace
Engineering in Delft until my formal retirement, and since then as an emeritus. In both
laboratories, I have been working in a fine cooperation with many people, too many to be
mentioned here. Various people contributed to lively discussions, planning programs,
designing and building test set-ups, carrying out experiments and microscopic examinations,
manufacturing of specimens, evaluation of test results, and all the secretarial work. This
stimulating cooperation has significantly contributed to the work which we have done, and
to the experience and the insights gained and reflected in the present book. For me, it was
a privilege to work on problems in which I was interested, and even more important to be
surrounded by people who appreciated the cooperation with great integrity.
I am also aware how much I learned from international (and national) contacts with
laboratories, research institutes, and universities, both in personal contacts as well as
through publications. The same is true for discussions with people of the industry on design
problems. Moreover, I learned a lot from investigatingfatigue failures in service. Finally,
it was stimulating to work with students, undergraduates and post graduates, having an open
and imaginative mind, and a large variety of questions.
With respect to preparing the manuscript of this book, I want to acknowledge the
unselfish help of Dr. Scott Fawaz, who did his doctor thesis work in Delft. He read all
chapters and suggested various improvements and corrections. The same was done by my
friend and colleague, professor Hans Overbeeke. He also offered useful questions.
Indispensable assistance on microscopy and photographs was offered by Frans Oostrum.
Last but not least, without my love, Janine, this book could never have been written. Her
dedication is unsurpassed.
Jaap Schijve
Delft, March 2001
CONTENTS
Preface v
Frequently used symbols, acronyms and units xiv
Chapter 1
Introduction to Fatigue of Structures and Materials 1
PART 1: INTRODUCTORY CHAPTERS ON FATIGUE
(Chapters 2 - 8)
Chapter 2
Fatigue as a Phenomenon in the Material 7
2.1 Introduction 7
2.2 Different phases of the fatigue life 8
2.3 Crack initiation 9
2.4 Crack growth 11
2.5 The fatigue mechanism in more detail 13
2.5.1 Crystallographic aspects 15
2.5.2 Crack initiation at inclusions 17
2.5.3 Small cracks, crack growth barriers, thresholds 20
2.5.4 Numberofcracknuclei 23
2.5.5 Surfaceeffects 26
2.5.6 Crack growth andstriations 29
2.5.7 Environmental effects 32
2.5.8 Cyclictensionandcyclictorsion 35
2.6 Characteristicfeaturesoffatiguefailures 37
2.7 Main topics ofthepresentchapter 42
References
Chapter 3
Stress Concentrations at Notches 45
3.1 Introduction 45
3.2 Definitionof 46
3.3 Calculations on stressconcentrations 48
3.4 Effectofthe notch geometryon 53
3.5 Someadditional aspectsofstress concentrations 60
3.6 Superposition ofnotches 63
3.7 Methodsforthedeterminationsofstressconcentrations 65
3.8 Maintopicsofthepresentchapter 69
References
vii
viii Contents
Chapter 4
Residual Stresses 71
4.1 Introduction 71
4.2 Different sources of residual stresses 73
4.3 Measurements of residual stresses 78
4.4 Estimation of the residual stress at a notch after a high load 79
4.5 How to remove a residual stresses 81
4.6 Main topics of the present chapter 83
References
Chapter 5
Stress Intensity Factors of Cracks 84
5.1 Introduction 84
5.2 Different types of cracks 86
5.3 Definition of the stress intensity factor 87
5.4 Examples of stress intensity factors 90
5.5 K-factors obtained by superposition 99
5.6 Cracks with curved crack fronts 101
5.7 Crack opening and the state of stress 104
5.8 Crack tip plasticity 106
5.9 Some energy considerations 109
5.10 Determination of stress intensity factors 110
5.11 The similarity concept and the application of the stress intensity factor K 111
5.12 Main topics of the present chapter 113
References
Chapter 6
Fatigue Properties of Materials 115
6.1 Introduction 115
6.2 Description of fatigue properties of unnotched specimens116
6.3 Some general aspects of the fatigue strength of unnotched specimens 122
6.3.1 Relation between and 122
6.3.2 Mean stress effects 124
6.3.3 The size effect for unnotched specimens 126
6.3.4 Type of loading, tension, bending, torsion 128
6.3.5 Combined loading 131
6.4 Low-cycle fatigue 133
6.5 Main topics of the present chapter 138
References
Contents ix
Chapter 7
The Fatigue Strength of Notched Specimens. Analysis and Predictions 141
7.1 Introduction 141
7.2 The fatigue limit of notchedspecimens at 142
7.2.1 The similarity principle and the notch sensitivity 142
7.2.2 The size effect on the fatigue limit of notched specimens 144
7.3 The fatigue limit of notched specimens for 150
7.4 Notch effect under cyclic torsion 156
7.5 Notch effect on the fatigue limit for combined loading cases 158
7.6 Significance of the surface finish 160
7.7 Discussion on predictions of the fatigue limit 162
7.8 The S-N curve of notched specimens 168
7.9 The major topics of the present chapter 172
References
Chapter 8
Fatigue Crack Growth. Analysis and Predictions 174
8.1 Introduction 174
8.2 Description of fatigue crack growth properties 175
8.2.1 Fatigue crack growth described by test results 175
8.2.2 The stress intensity factor and the similarity principle 177
8.2.3 Constant- tests 180
8.3 Fatigue crack growth regions 181
8.4 Crackclosure 187
8.4.1 Plasticity induced crack closure and 187
8.4.2 Plane strain / plane stress 192
8.4.3 Thickness effect on fatigue crack growth 194
8.4.4 Othercrackclosure mechanisms 194
8.5 Crack growth data of different materials 195
8.6 Predictions of fatigue crackgrowth 200
8.6.1 Some basic aspects 200
8.6.2 Crack growth predictions for through cracks 203
8.6.3 Crack growth predictions for part through cracks 207
8.6.4 Additional comments on fatigue crack growth predictions 210
8.7 Major topics of the present chapter 211
References
x Contents
PART 2: LOAD SPECTRA AND FATIGUE UNDER VARIABLE-AMPLITUDE
LOADING (Chapters 9 - 11)
Chapter 9
Load Spectra 215
9.1 Introduction 215
9.2 Different types of loads on a structure in service 216
9.3 Description of load histories 221
9.4 Determination of load spectra 233
9.5 Service-simulation fatigue tests and load spectra 239
9.6 Major topics of the present chapter 243
References
Chapter 10
Fatigue under Variable-Amplitude Loading 245
10.1 Introduction 245
10.2 The Miner rule 246
10.2.1 Effect of load cycles with stress amplitudes below the fatigue limit 248
10.2.2 Effect of notch root plasticity 248
10.2.3 Crack length at failure 250
10.2.4 What is basically wrong with the Miner-rule? 251
10.3 Results of fatigue tests under VA loading 252
10.4 Alternative fatigue life prediction methods for VA loading 257
10.4.1 Non-linear damaging increase, characterized by a single damage parameter 257
10.4.2 Damage calculations and extrapolation of S-N curves below the fatigue imit 258
10.4.3 The relative Miner rule 261
10.4.4 The strain history prediction model 261
10.4.5 Predictions based on service-simulation fatigue tests 264
10.4.6 Predictions based on crack growth from an equivalent initial flaw 268
10.5 Discussion of fatigue life predictions for VA loading 268
10.5.1 Life estimates for a specific component and the Miner rule 269
10.5.2 Considerations on the effect of the design stress level 270
10.5.3 Comparison between different options for design improvements 270
10.5.4 Comparison of different load spectra 270
10.6 Major topics of the present chapter 271
References
Chapter 11
Fatigue Crack Growth under Variable-Amplitude Loading 274
11.1 Introduction 274
Contents xi
11.2 Crack growth under simple VA stress-histories 275
11.3 Crack growth under complex VA stress histories 286
11.4 Fatigue crack growth prediction models for VA loading 292
11.4.1 Non-interaction model for crack growth under VA loading 293
11.4.2 Interaction models for crack growth under VA loading 295
11.5 Evaluation of predictions of fatigue crack growth under for VA loading 302
11.6 Major topics of the present chapter 305
References
PART 3: FATIGUE TESTS AND SCATTER
(Chapters 12 and 13)
Chapter 12
Fatigue and Scatter 310
12.1 Introduction 310
12.2 Sources of scatter 310
12.3 Description of scatter 311
12.4 Practical aspects of scatter 320
12.5 Major topics of the present chapter 327
References
Chapter 13
Fatigue Tests 329
13.1 Introduction 329
13.2 Purposes of fatigue test programs 330
13.3 Specimens 330
13.4 Fatigue test procedures 335
13.5 Some aspects of the evaluation of fatigue test results 338
13.6 Aspects of crack growth measurements 339
13.7 Main topics of this chapter 347
References
PART 4: SPECIAL FATIGUE CONDITIONS
(Chapters 14 - 17)
Chapter 14
Surface Treatments 349
14.1 Introduction 349
14.2 Aspects of surface treatments 349
14.3 Some practical aspects of surface treatments 360
14.4 Topics of the present chapter 361
References