Acknowledgments .............................................. III
Abstract ....................................................... IV
Contents ........................................................ V
Nomenclature .................................................. VII
1 Introduction ................................................. 1
1.1 Magnesium - History and Overview ........................ 1
1.2 Lightweight Potential of Magnesium Alloys ............... 2
1.3 Production and Application of Magnesium Sheet Metals .... 4
1.4 Motivation and Research Objective ....................... 6
2 Background and Literature Review ............................. 8
2.1 Crystal Structure of Magnesium .......................... 8
2.2 Deformation Mechanisms of Magnesium Alloys .............. 9
2.2.1 Dislocation Slip ................................. 9
2.2.2 Mechanical Twinning ............................. 10
2.3 Magnesium Alloy Systems ................................ 13
2.4 Mechanical Behavior of Magnesium Alloys ................ 17
2.4.1 Quasi-Static Mechanical Properties .............. 17
2.4.2 Pseudoelasticity ................................ 20
2.4.3 Cyclic Deformation Behavior ..................... 22
2.5 Fundamentals of Fatigue Modeling ....................... 25
2.5.1 Local Strain Approach ........................... 28
2.5.2 Energy Based Approach ........................... 36
2.6 Fatigue of Magnesium Alloys - Overview ................. 37
2.6.1 Uniaxial Tension-Compression Tests .............. 37
2.6.2 Modeling of the Stress-Strain Behavior .......... 41
3 Materials and Experimental Procedure ........................ 42
3.1 Investigated Magnesium Alloys .......................... 42
3.2 Microstructural Investigations ......................... 42
3.3 Specimens for Mechanical Tests ......................... 44
3.4 Quasi-Static Tests ..................................... 45
3.5 Cyclic Tests ч ......................................... 46
3.5.1 Test Procedure .................................. 46
3.5.2 Data Recording and Evaluation ................... 48
3.5.3 Definition of the Fatigue Life .................. 49
3.6 Overview of all Performed Mechanical Tests ............. 50
4 Microstructure and Texture .................................. 53
4.1 Microstructure Before Loading ......................... 53
4.1.1 Sheet Metal Surface ............................. 53
4.1.2 Different Spatial Directions .................... 54
4.2 Crystal Orientation Before Loading .................... 55
5 Experimental Investigation of the Quasi-Static Behavior ..... 59
5.1 General Observations ................................... 59
5.2 Stiffness and Microyielding ............................ 62
5.3 Influence of the Strain Rate ........................... 63
5.4 Twin Bands in Compressive Tests ........................ 64
5.5 Serrated Flow .......................................... 67
6 Experimental Investigation of the Cyclic Deformation
Behavior .................................................... 69
6.1 Constant Amplitude Tests ............................... 70
6.1.1 Completely Reversed Stress- and Strain-
Controlled Tests ................................ 70
6.1.2 Influence of the Anisotropy ..................... 73
6.1.3 Twin Bands at Cyclic Strain-Controlled Tests .... 74
6.1.4 Strain-Controlled Tests with Different Strain
Ratios .......................................... 77
6.1.5 Stress-Controlled Tests with Different Stress
Ratios .......................................... 79
6.1.6 Cyclic Hardening Behavior ....................... 83
6.1.7 Evolution of Fatigue Parameters ................. 87
6.2 Variable Amplitude Tests ............................... 90
6.2.1 Load-Time Functions with Single Inner
Hysteresis Loops ................................ 90
6.2.2 Cyclic Stress Relaxation ........................ 92
6.2.3 Load-Time Functions with Multiple Inner
Hysteresis Loops ................................ 94
7 Uniaxial Phenomenological Stress-Strain Models .............. 96
7.1 Correlation of the Masing Model ........................ 96
7.2 Phenomenological Stress-Strain Model for Wrought
Magnesium Alloys ....................................... 97
7.2.1 Mathematical Formulations ....................... 97
7.2.2 Description of the Simulation Code ............. 102
7.2.3 Model Verification ............................. 107
8 Fatigue Life Evaluation Methods ............................ 112
8.1 General Fatigue Behavior .............................. 112
8.2 Fractographic Findings ................................ 114
8.3 Manson-Coffin-Basquin Approach and Smith-Watson-
Topper Model .......................................... 116
8.3.1 Strain-Life Curves of AM50, AZ31B, and ME21 .... 116
8.3.2 Stress- and Strain-Based Mean Stress
Correction Methods ............................. 118
8.4 Energy Based Fatigue Models ........................... 120
8.5 Fatigue Life at Variable Amplitude Tests .............. 125
9 Summary and Outlook ........................................ 129
References ................................................. 133
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