Optimization of Apparent Fatigue Limits of Shot-peened Specimens

Threepoint bending fatigue limits of shotpeened 40Cr steel specimens were determined and the influence of grinding after shot peening was studied. Their change rules and mechanisms are discussed in relation to the surface residual stress fields and the sites of fatigue crack sources. The technology optimization of shot peening is discussed and a criterion of optimization is proposed, which suggests that the depths of surface compressive residual stress fields produced by shot peening should reach 10～15 times the average diameter of austenite grains and it is better to grind some crucial parts after shot peening. It is also proposed a method to calculate the apparent fatigue limits of specimens shotpeened under optimum conditions according to a concept of internal/surface fatigue limit.