tailieunhanh - High Cycle Fatigue: A Mechanics of Materials Perspective part 56

High Cycle Fatigue: A Mechanics of Materials Perspective part 56. 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. | 536 Appendix E comparisons provide some useful information concerning the capabilities and accuracy of the Fs approach. In all cases the predicted threshold stress levels exceeded the experimental values. However the differences between the actual and modeled notch dimensions may have contributed to the discrepancies to some degree. For example in notch 1 the actual notch depth was approximately 10 greater than the modeled value. This would increase the effective stress concentration factor thereby reducing the nominal threshold stress level. The differences in notch dimensions were less severe in the other cases. Nevertheless it is apparent the Fs method tends to underestimate the notch strength reduction in components containing very small sharp notches with severe stress gradients. However the accuracy of the method improves in cases of more blunt notches 1 . FRACTURE MECHANICS WORST CASE NOTCH APPROACH The fracture mechanics approach employed to predict the nominal HCF threshold stress Ao-jh assumes that microcracks can initiate relatively early in the life of a notched component due to a variety of reasons such as the damage imparted by an initial FOD impact the highly concentrated HCF stresses associated with either an FOD notch or edge-of-contact zone or the intermittent occurrence of low cycle fatigue LCF . Moreover since the stress gradients at sharp notches and edge-of-contact zones are steep and die out at relatively short distances from the notch surface it is necessary to consider the unique behavior of small fatigue cracks when performing a fracture mechanics analysis. The method used termed the Worst Case Notch WCN model enables the boundaries between crack initiation crack growth followed by arrest and crack growth to failure to be delineated. In this model true crack initiation is assumed to actually occur as predicted by the classical S-N approach when the applied stress range is equal to the endurance limit divided by the elastic stress .