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High Cycle Fatigue: A Mechanics of Materials Perspective part 41

High Cycle Fatigue: A Mechanics of Materials Perspective part 41. 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. | 386 Applications 8.2. FRACTURE MECHANICS CONSIDERATIONS The implications of the shape of the Haigh diagram as described by the modified Jasper formulation for example can be examined in terms of the fracture mechanics parameter related to the threshold for crack growth namely AKth. The FLS which can be defined as the maximum stress under which a crack will not initiate and grow should be related to the threshold for crack propagation using the following logic. At a stress just above the fatigue limit a crack will both initiate and grow to failure in a smooth bar. If it is assumed that crack initiation can be defined as initiation or nucleation to a defined size then the driving force in terms of AK must be equal to or exceed AKth in order for failure to occur. As an illustrative example the locus of stresses in terms of the alternating and maximum stresses corresponding to the modified Jasper equation used to fit the data on Ti-6Al-4V plate material are plotted against mean stress in Figure 8.6. The fit to experimental data is extended into the negative mean stress region in this plot. A line corresponding to R 0 is also shown for reference purposes in this Nicholas-Haigh diagram. At values of the stress ratio above R 0 all stresses are positive throughout the load cycle. One can consider two extreme cases for AKth for a fixed crack size independent of R for crack initiation. First is the case where crack propagation and hence the threshold for crack extension is governed by the total stress range. In this case AKth is proportional to the alternating stress and would be represented by a horizontal line marked AKth constant in Figure 8.6. A second scenario one commonly used in fatigue crack Mean stress MPa Figure 8.6. Locus of maximum and alternating stresses corresponding to modified Jasper equation fit to experimental data. The Nicholas-Haigh diagram that plots both alternating and maximum stress was discussed in Chapter 2. HCF Design Considerations 387 Figure .