tailieunhanh - High Cycle Fatigue: A Mechanics of Materials Perspective part 47

High Cycle Fatigue: A Mechanics of Materials Perspective part 47. 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. | 446 Applications Intermediate improvements in fatigue performance were observed on Ti-6-7 AC. Marked changes in the slope of the S-N curves were observed in Ti-6-7 WQ owing to pronounced lifetime improvements in the HCF regime while finite life was only slightly improved. These results serve to illustrate the complex nature of the effect of surface treatments on fatigue behavior particularly in the HCF regime and the dependence of such behavior on the characteristics of the specific material such as strain hardening softening. The ability of a surface treatment and the resulting residual stresses that are produced to slow or stop a fatigue crack is probably the most important aspect in fatigue life improvement particularly in the HCF regime where complete crack arrest defines the condition for determining an endurance limit. Guagliano and Vergani 59 address the problem of predicting the conditions for non-propagation of cracks in shot-peened specimens. They adopt the premise that fatigue alleviation due to shot peening is mainly due to the ability of the residual stresses to stop crack propagation and not in preventing fatigue crack initiation. Their approach is based on fracture mechanics and the existence of a threshold for crack propagation AKth. With the aid of an FEM model which considers both the applied loading and the compressive residual stress field and taking into account crack closure and contact between crack faces they are able to explain the presence of non-propagating cracks with typical depths from to mm in a low alloy steel. They note that the depth at which the cracks have stopped is entirely dependent on the residual compressive stress field induced by shot peening. Such a finding can be used as motivation for the use of other surface treatments that provide greater depths of residual compressive stresses. The control of the initiation and early phase of crack propagation of surface cracks is considered to be paramount for prolonging .