表层强度及其对疲劳的影响

光滑试件疲劳时因表面处于平面应力状态其屈服强度比整体材料低，首先发生滑移，因而造成疲劳损伤，达到一定疲劳周次后积累的损伤导致断裂，因而表层屈服强度和疲劳极限之间存在对应关系。缺口试件引入残余压应力，由于应力集中效应，缺口根部的残余应力集中系数可能高于理论应力集中系数，因而其疲劳强度将超过最高的光滑试件疲劳强度。考虑残余应力对裂纹扩展的作用时应计及其衰减的作用。

Surface Strength and its Effect on Fatigue

Fatigue damage of smooth sample is associated with lower yield strength of the surface layer than that of the bulk material.Accumulation of surface damage leads to failure in a number of loading cycles .Thus the surface yield strength is a factor governing the fatigue strength of smooth sample .Once compressive residual stress is induced in the notched area,the residual stress concentration factor can be higher than the theoretical stress concentration,therefore the notched fatigue limit can be higher than the highest smooth fatigue limit.The effect of residual stress on fatigue crack growth needs to account for its relaxation.