High Cycle Fatigue: A Mechanics of Materials Perspective part 46. 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. | 436 Applications surface treatment may be different. Specifically in HCF cracks may take a long time to initiate but with the aid of compressive residual stresses they may arrest. This may provide a measurable improvement in the endurance limit or the fatigue limit strength at a large cycle count. . Application to notches As noted above the effect of residual compressive stress is to move the mean stress in the negative direction. For a smooth bar this simply shifts the Haigh diagram a plot of alternating stress against mean stress to the left. In most cases for a smooth bar the compressive residual stress near the surface produces a tensile residual stress field in the interior the magnitude of which is dependent on the compressive stress profile and the cross-sectional dimensions. In most cases under tensile loading fatigue failures will occur in the interior rather than at the surface. Under bending however failure can occur at the surface or interior to the surface depending on the geometry of the specimen applied stress gradient and the residual stress distribution profile. If there is a notch in a specimen or component a residual compressive stress field will also reduce the local stresses in the vicinity of the notch. The residual stress field produced by a surface enhancement procedure such as conventional shot peening CSP low plasticity burnishing LPB laser shock processing LSP or other will depend on whether the surface treatment is applied before or after the notch is introduced. The notch depth as well as the depth of the residual compressive stresses must also be considered. For illustrative purposes the assumption will be made in the following that the residual stress field extends beyond the notch depth so that the local notch stress field due to applied loading is reduced uniformly by the residual stresses that are assumed to be uniform over this depth. The net effect will be to shrink rather than just shift the Haigh diagram as illustrated in
