TAILIEUCHUNG - High Cycle Fatigue: A Mechanics of Materials Perspective part 42

High Cycle Fatigue: A Mechanics of Materials Perspective part 42. 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. | 396 Applications CTm MPa Figure . Haigh diagram for smooth and notched specimen data compared with fracture mechanics-based computations of allowable stresses based on initial flaw size aj. . Application to LCF-HCF Looking at the same general problem of relating a Haigh diagram to a threshold plot Nicholas and Zuiker 7 evaluated behavior under combined LCF-HCF loading. Using another set of data and addressing the problem from yet another perspective an analysis was conducted to create a Haigh diagram corresponding to an initial crack size based on actual threshold data. Details of the analysis for general LCF-HCF loading conditions are presented in Appendix I. The specific problem addressed the initiation of a crack from LCF loading and the propensity for that crack to grow when subjected to HCF loading. No load history or interaction effects were assumed in this study. The analysis considered the initiation phase using the threshold stress intensity AKth and was handled with a Haigh diagram that was treated as representing a damaged material having an equivalent crack depth of mm. This crack depth was chosen to correspond to that used on experimental data in the literature where this depth was taken to be the definition of initiation 8 . The Haigh diagram for initiation to a crack of depth of mm in Ti-6Al-4V obtained from data of Guedou and Rongvaux 8 and others was represented as a straight line that went from o-alt 240 MPa at o-mean 0 to CTalt 0 at o-mean 1200 MPa the ultimate strength of the material. For essentially the same alloy and heat treatment threshold data were also taken from Hawkyard et al. 9 which show that AKth decreases linearly from 5MPaVm at R 0 to at R and remains constant at that value for R . As a second assumption the straight-line decrease of AKth was continued beyond R by Nicholas and Zuiker for comparison purposes. To compare initiation criteria from a Haigh diagram to those based on threshold .

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