1 Mechanism of Fatigue in the Absence of Defects and
Inclusions ................................................... 1
1.1 What is a Fatigue Limit? ................................ 1
1.1.1 Steels ........................................... 1
1.1.2 Nonferrous Metals ................................ 4
1.2 Relationship between Stalie Strength and Fatigue
Strength ................................................ 5
1.3 References .............................................. 8
2 Stress Concentration ........................................ 11
2.1 Stress Concentrations at Holes and Notches ............. 11
2.2 Stress Concentration at a Crack ........................ 15
2.2.1 'area' as a New Geometrical Parameter ........... 16
2.2.2 Effective 'area' for Particular Cases ........... 17
2.2.3 Cracks at Stress Concentrations ................. 21
2.2.4 Interaction between Two Cracks .................. 21
2.2.5 Interaction between a Crack and a Free
Surface ......................................... 22
2.3 References ............................................. 24
3 Notch Effect and Size Effect ................................ 25
3.1 Notch Effect ........................................... 25
3.1.1 Effect of Stress Distribution at Notch Roots .... 25
3.1.2 Non-Propagating Cracks at Notch Roots ........... 28
3.2 Size Effect ............................................ 31
3.3 References ............................................. 32
4 Effect of Size and Geometry of Small Defects on the
Fatigue Limit ............................................... 35
4.1 Introduction ........................................... 35
4.2 Influence of Extremely Shallow Notches or Extremely
Short Cracks ........................................... 35
4.3 Fatigue Tests on Specimens Containing Small
Artificial Defects ..................................... 37
4.3.1 Effect of Small Artificial Holes Having the
Diameter d Equal to the Depth h ................. 37
4.3.2 Effect of Small Artificial Holes Having
Different Diameters and Depths .................. 42
4.4 Critical Stress for Fatigue Crack Initiation from
a Small Crack .......................................... 47
4.5 References ............................................. 54
5 Effect of Hardness Hv on Fatigue Limits of Materials
Containing Defects, and Fatigue Limit Prediction
Equations ................................................... 57
5.1 Relationship between ΔKth and the Geometrieal
Parameter, √area ....................................... 57
5.2 Material Parameter Hv which Controls Fatigue Limits .... 60
5.3 Application of the Prediction Equations ................ 62
5.4 Limits of Applicability of the Prediction Equations:
Eqs. 5.4 and 5.5 ....................................... 66
5.5 The Importance of the Finding that Specimens with an
Identical Value of √area for Small Holes or Small
Cracks Have Identical Fatigue Limits: When the Values
of √area for a Small Hole and a Small Crack are
Identical, are the Fatigue Limits for Specimens
Containing these Two Defect Types Really Identical? .... 66
5.6 References ............................................. 71
6 Effects of Nonmetallic Inclusions on Fatigue Strength ....... 75
6.1 Review of Existing Studies and Current Problems ........ 75
6.1.1 Correlation of Material Cleanliness and
Inclusion Rating with Fatigue Strength .......... 75
6.1.2 Size and Location of Inclusions and Fatigue
Strength ........................................ 77
6.1.3 Mechanical Properties of Microstructure and
Fatigue Strength ................................ 78
6.1.4 Influence of Nonmetallic Inclusions Related to
the Direction and Mode of Loading ............... 81
6.1.5 Inclusion Problem Factors ....................... 82
6.2 Similarity of Effects of Nonmetallic Inclusions and
Small Defects and a Unifying Interpretation ............ 85
6.3 Quantitative Evaluation of Effects of Nonmetallic
Inclusions: Strength Prediction Equations and their
Application ............................................ 88
6.4 Causes of Fatigue Strength Scatter for High Strength
Steels and Scatter Band Prediction ..................... 94
6.5 Effect of Mean Stress .................................. 99
6.5.1 Quantitative Evaluation of the Mean Stress
Effect on Fatigue of Materials Containing
Small Defects .................................. 100
6.5.2 Effects of Both Nonmetallic Inclusions and
Mean Stress in Hard Steels ..................... 104
6.5.3 Prediction of the Lower Bound of Scatter and
its Application ................................ 108
6.6 Estimation of Maximum Inclusion Size √areamaх by
Microscopic Examination of a Microstructure ........... 110
6.6.1 Measurement of √areamax for Largest
Inclusions by Optical Microscopy ............... 112
6.6.2 True and Apparent Maximum Sizes of
Inclusions ..................................... 114
6.6.3 Two-dimensional (2D) Prediction Method for
Largest Inclusion Size and Evaluation by
Numerical Simulation ........................... 118
6.7 References ............................................ 122
7 Bearing Steels ............................................. 129
7.1 Influence of Steel Processing ......................... 130
7.2 Inclusions at Fatigue Fracture Origins ................ 130
7.3 Cleanliness and Fatigue Properties .................... 133
7.3.1 Total Oxygen (O) Content ....................... 136
7.3.2 Ti Content ..................................... 136
7.3.3 Ca Content ..................................... 136
7.3.4 Sulphur (S) Content ............................ 137
7.4 Fatigue Strength of Super Clean Bearing Steels and
the Role of Nonmetallic Inclusions .................... 139
7.5 Tessellated Stresses Associated with Inclusions:
Thermal Residual Stresses around Inclusions ........... 142
7.6 What Happens to the Fatigue Limit of Bearing Steels
without Nonmetallic Inclusions? — Fatigue Strength
of Electron Beam Remelted Super Clean Bearing Steel ... 148
7.6.1 Material and Experimental Procedure ............ 148
7.6.2 Inclusion Rating Based on the Statistics of
Extremes ....................................... 152
7.6.3 Fatigue Test Results ........................... 153
7.6.4 The True Character of Small Inhomogeneities
at Fracture Origins ............................ 154
7.7 References ............................................ 159
8 Spring Steels .............................................. 163
8.1 Spring Steels (SUP12) for Automotive Components ....... 163
8.2 Explicit Analysis of Nonmetallic Inclusions, Shot
Pcening, Deearburised Layers, Surface Roughness, and
Corrosion Pits in Automobile Suspension Spring
Steels ................................................ 168
8.2.1 Materials and Experimental Procedure ........... 169
8.2.2 Interaction of Factors Influencing Fatigue
Strength ....................................... 172
8.2.2.1 Effect of Shot Peening ................ 173
8.2.2.2 Effects of Nonmetallic Inclusions
and Corrosion Pits .................... 178
8.2.2.3 Prediction of Scatter in Fatigue
Strength using the Statistics of
Extreme ............................... 180
8.3 References ............................................ 182
9 Tool Steels: Effect of Carbides ............................ 185
9.1 Low Temperature Forging and Microstructure ............ 185
9.2 Static Strength and Fatigue Strength .................. 187
9.3 Relationship Between Carbide Size and Fatigue
Strength .............................................. 190
9.4 References ............................................ 192
10 Effects of Shape and Size of Artificially Introduced
Alumina Particles on 1,5Ni-Cr-Mo (En24) Steel .............. 193
10.1 Artificially Introduced Alumina Particles with
Controlled Sizes and Shapes, Speci mens, and Test
Stress ................................................ 193
10.2 Rotating Bending Fatigue Tests without Shot Peening ... 195
10.3 Rotating Bending Fatigue Tests on Shot-Peened
Specimens ............................................. 199
10.4 Tension Compression Fatigue Tests ..................... 202
10.5 References ............................................ 203
11 Nodular Cast Iron .......................................... 205
11.1 Introduction .......................................... 205
11.2 Fatigue Strength Prediction of Nodular Cast Irons by
Considering Graphite Nodules to be Equivalent to
Small Defects ......................................... 206
11.3 References ............................................ 215
12 Influence of Si-Phase on Fatigue Properties of Aluminium
Alloys ..................................................... 217
12.1 Materials, Specimens and Experimental Procedure ....... 217
12.2 Fatigue Mechanism ..................................... 217
12.2.1 Continuously Cast Material ..................... 220
12.2.2 Extruded Material .............................. 221
12.2.3 Fatigue Behaviour of Specimens Containing an
Artificial Hole ................................ 225
12.3 Mechanisms of Ultralong Fatigue Life .................. 227
12.4 Low-Cycle Fatigue ..................................... 231
12.4.1 Fatigue Mechanism .............................. 231
12.4.2 Continuously Cast Material ..................... 232
12.4.3 Extruded Material .............................. 232
12.4.4 Comparison with High-Cycle Fatigue ............. 232
12.4.5 Cyclic Property Characterisation ............... 235
12.5 Summary ............................................... 238
12.6 References ............................................ 239
13 Ti Alloys .................................................. 241
13.1 References ............................................ 244
14 Torsional Fatigue .......................................... 247
14.1 Introduction .......................................... 247
14.2 Effect of Small Artificial Defects on Torsional
Fatigue Strength ...................................... 248
14.2.1 Ratio of Torsional Fatigue Strength to
Bending Fatigue Strength ....................... 248
14.2.2 The Slate of Non-Propagating Cracks at the
Torsional Fatigue Limit ........................ 253
14.2.3 Torsional Fatigue of High Carbon Cr Bearing
Steel ......................................... 256
14.3 Effects of Small Cracks ............................... 258
14.3.1 Material and Test Procedures ................... 261
14.3.2 Fatigue Test Results ........................... 262
14.3.3 Crack Initiation and Propagation from
Precracks ...................................... 263
14.3.4 Fracture Mechanics Evaluation of the Effect
of Small Cracks on Torsional Fatigue ........... 266
14.3.5 Prediction of Torsional Fatigue Limit by the
√area Parameter Model .......................... 268
14.4 References ............................................ 270
15 The Mechanism of Fatigue Failure of Steels in the
Ultralong Life Regime of N > 107 Cycles .................... 273
15.1 Mechanism of Elimination of Conventional Fatigue
Limit: Influence of Hydrogen Trapped by Inclusions .... 273
15.1.1 Method of Data Analysis ........................ 274
15.1.2 Material, Specimens and Experimental Method .... 275
15.1.3 Distribution of Residual Stress and Hardness ... 276
15.1.4 Fracture Origins ............................... 277
15.1.5 S-N Curves ..................................... 277
15.1.6 Details of Fracture Surface Morphology and
Influence of Hydrogen .......................... 279
15.2 Fractographic Investigation ........................... 291
15.2.1 Measurement of Surface Roughness ............... 292
15.2.2 The Outer Border of a Fish Eye ................. 292
15.2.3 Crack Growth Rate and Fatigue Life ............. 298
15.3 Current Conclusions ................................... 299
15.4 References ............................................ 302
16 Effect of Surface Roughness on Fatigue Strength ............ 305
16.1 Introduction .......................................... 305
16.2 Material and Experimental Procedure ................... 306
16.2.1 Material ....................................... 306
16.2.2 Introduction of Artificial Surface Roughness
and of a Single Notch .......................... 306
16.2.3 Measurement of Hardness and Surface
Roughness ...................................... 308
16.3 Results and Discussion ................................ 312
16.3.1 Results of Fatigue Tests ....................... 312
16.3.2 Quantitative Evaluation by the √area
Parameter Model ................................ 312
16.3.2.1 Geometrical Parameter to Evaluate
the Effect of Surface Roughness on
Fatigue Strength ...................... 312
16.3.2.2 Evaluation of Equivalent Defect Size
for Roughness √areaR .................. 315
16.4 Guidance for Fatigue Design Engineers ................. 319
16.5 References ............................................ 319
Appendix A. Instructions for a New Method of Inclusion
Rating and Correlations with the Fatigue Limit ............. 321
A1 Background of Extreme Value Theory and Data Analysis ... 323
A2 Simple Procedure for Extreme Value Inclusion Rating .... 325
A3 Prediction of the Maximum Inclusion .................... 329
A4 Prediction of √areamax of Inclusions Expected to be
Contained in a Volume .................................. 331
A5 Method for Estimating the Prediction Volume
(or Control Volume) .................................... 333
A6 Prediction of the Lower Limit (Lower Bound) of the
Fatigue Strength ....................................... 337
A7 The Comparison of Predicted Lower Bound of the
Scatter in Fatigue Strength of a Medium Carbon Steel
with Rotating Bending Fatigue Test Results ............. 339
A8 Optimisation of Extreme Value Inclusion Rating
(EVIR) ................................................. 345
A9 Recent Developments in Statistical Analysis and its
Perspectives ........................................... 347
A10 References ............................................. 349
Appendix B Database of Statistics of Extreme Values of
Inclusion Size √areamax ............................ 351
Appendix C Probability Sheets of Statistics of Extremes ...... 357
Index ......................................................... 359
|
