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High Cycle Fatigue: A Mechanics of Materials Perspective part 33

High Cycle Fatigue: A Mechanics of Materials Perspective part 33. The nomenclature used in this book may differ somewhat from what is considered standard or common usage. In such instances, this has been noted in a footnote. Additionally, units of measurement are not standard in many cases. While technical publications typically adhere to SI units these days, much of the work published by the engine manufacturers in the United States is presented using English units (pounds, inches, for example), because these are the units used as standard practice in that industry. The graphs and calculations came in those units and no attempt was made to convert. | 306 Effects of Damage on HCF Properties Figure 6.42. Cross-section of a fretting fatigue cracked specimen prior to fracture 47 . Contact surface 112x 20.0 kV 100 m AMRAY Loading direction Figure 6.43. Cross-section of failed fretting fatigue sample showing crack orientation 52 . fatigue experiment. Their criterion for initiation of a crack is related to the direction of maximum shear stress range but propagation is governed by the direction of the maximum tangential stress range around the crack tip. They propose a numerical integration scheme where the crack length is incremented and the orientation continually updated. In the numerical example presented however they find that the fretting fatigue crack growth curve under mixed mode agrees well with the fatigue crack growth curve under mode I except for the initial first few data points. 6.12. A COMBINED STRESS AND K APPROACH An example of an investigation where many of the features needed for an accurate life prediction scheme are incorporated is that of Golden and Grandt 22 . There they combined an initiation model with fracture-mechanics-based crack growth modeling to try to predict experimental data from several fretting-fatigue experiments. The crack growth Fretting Fatigue 307 Figure 6.44. Smooth bar and fretting test data adjusted for stressed surface area using equivalent stress 22 . portion accounted for the three-dimensional nature of the cracks and their subsequent growth accounting for aspect ratios. The initiation modeling accounted for multiaxial and mean stress effects while making use of an equivalent stressed surface area as done in 33 see also Appendix E . From their initiation modeling alone Figure 6.44 shows how the use of an equivalent stress parameter that is adjusted to account for the gradient of stress using the Fs approach can consolidate data from a number of fretting-fatigue tests but does not match smooth bar fatigue data. The reason for this is the assumption that the initiation model