Initiation and propagation behaviour of fatigue cracks in titanium alloys

226 Pages·2003·18.66 MB·English

Dottorato di Ricerca in Ingegneria dei Materiali – XVI ciclo

IInniittiiaattiioonn aanndd pprrooppaaggaattiioonn bbeehhaavviioouurr ooff
ffaattiigguuee ccrraacckkss iinn ttiittaanniiuumm aallllooyyss

MMaatttteeoo BBeenneeddeettttii

February 2004

Doctoral Thesis

Initiation and propagation behaviour of
fatigue cracks in titanium alloys

Matteo Benedetti

Tutor:
Prof. Ing. Vigilio Fontanari

Doctoral Committee:
Prof. Gian Domenico Sorarù, Università degli Studi di Trento
Prof. Roberto Contro, Politecnico di Milano
Prof. Gianni Nicoletto, Università degli Studi di Parma

Date of dissertation: 11th February 2004
Collana Dipartimento di Ingegneria dei Materiali
e Tecnologie Industriali.

Doctoral Thesis No. 03/2004

ISSN 1724-7454
“Die höchste Aufgabe des Menschen ist zu wissen, was
einer sein muss, um ein Mensch zu sein“
Immanuel Kant

To
Carmen, my wife to-be, and my parents,
Mario and Daniela

CONTENTS

Preface ix
Nomenclature xi

1 Introduction and overview 1

PART ONE: STATE OF THE ART 5

2 Fatigue propagation of long cracks 7
2.1 Characterization of crack growth 8
2.2 Conditions of K-dominance 10
2.3 Microscopic stages of fatigue crack growth 10
2.4 Different regimes of fatigue crack growth 12
2.4.1 Near-threshold fatigue crack growth 12
2.4.2 Intermediate region of crack growth 15
2.4.3 High growth rate regime 16
2.4.4 Mean stress effects 17
2.5 Summary 18

3 Retardation mechanisms in fatigue crack growth 19
3.1 Intrinsic and extrinsic crack growth mechanisms 20
3.2 Fatigue crack closure 21
3.2.1 Plasticity-induced crack closure 22
3.2.2 Roughness-induced crack closure 25
3.3 Fatigue crack deflection 27
3.4 Crack bridging and crack front geometry effects 28
3.5 Summary 31

4 The small crack problem 33
4.1 The similitude concept 33
4.2 Definition of small cracks 34
4.3 Sub-threshold small fatigue crack growth 35
4.4 Crack initiation in commercial alloys 39
4.5 On the origins of the anomalous small crack behaviour 42
4.5.1 Microstructural aspects of small crack growth 45
4.5.2 Effects of physically smallness of fatigue cracks 53
4.5.3 Effects of near-tip plasticity on small crack growth 58
4.5.4 Effects of notch-tip plasticity on small crack growth 63
4.6 Summary 70

5 Titanium alloys 73
5.1 Classification of titanium alloys 74
v
Contents
5.2 Processing and microstructure of α+β alloys 76
5.3 Mechanical properties of α+β titanium alloys 78
5.4 Processing and microstructure of metastable β alloys 82
5.5 Mechanical properties of metastable β titanium alloys 84
5.6 Summary 87

PART TWO: RESEARCH ACTIVITY 89

6 Objectives and outline 91
6.1 Titanium alloys for aircraft applications 91
6.2 Synopsis of the research activity 93

7 Behaviour of small cracks emanating from notches 97
7.1 Introduction 97
7.2 Material and experimental procedure 100
7.2.1 Material and specimen 100
7.2.2 Fatigue testing 102
7.2.3 Crack detection 103
7.3 Results and discussion 104
7.3.1 Stress field analysis 104
7.3.2 Fatigue crack initiation 106
7.3.3 Fatigue crack growth 108
7.3.4 Microstructurally long cracks 114
7.4 Summary 116

8 The microstructural influence on small crack behaviour 119
8.1 Introduction 119
8.2 Material and experimental procedure 121
8.2.1 Material characterization 121
8.2.2 Fatigue testing and crack detection 123
8.3 Results and discussion 124
8.3.1 Stress field analysis 124
8.3.2 Fatigue crack initiation and early propagation stage 126
8.3.3 Fatigue crack growth 130
8.3.4 Linear elastic fracture mechanics approach to crack growth 135
8.3.5 The effect of crack closure on small fatigue cracks 137
8.4 Summary 141

9 The long crack growth behaviour 143
9.1 Introduction 143
9.2 Material and experimental procedure 145
9.2.1 Material and microstructures 145
9.2.2 Mechanical testing 146
9.2.3 Estimation of crack bridging and crack closure 147
9.3 Long crack growth behaviour 148
9.4 Comparison between long and small crack growth behaviour 153
9.5 Role of crack front geometry on crack growth resistance 154
9.5.1 Background 154
9.5.2 Experimental procedure 155
9.6 Results and discussion 158
9.6.1 Fatigue crack growth curves through microstructural gradients 158
vi
9.6.2 Crack closure effects 160
9.6.3 Fracture toughness 163
9.6.4 Crack front geometry effects 165
9.7 Summary 167

10 Fatigue and fracture of β titanium alloys 169
10.1 Introduction: soft zones along β-grain boundaries 169
10.2 Material and experimental procedure 170
10.3 Results and discussion 176
10.3.1 Tensile properties 176
10.3.2 Smooth-bar stress-life fatigue 177
10.3.3 Fatigue crack growth curves 179
10.3.4 Fracture toughness 182
10.3.5 Crack front geometry effects 185
10.4 Summary 189

11 Concluding remarks 191

References 197
Curriculum Vitae 211
vii