Effect of mean stress and stress concentration factor on fatigue limits of middle and high strength steels

Ultrasonic fatigue tests were performed on notched specimens of medium strength steel, 2.25Cr-1Mo, and the bolt specimens of high strength steel, SNB7, under mean stress application, which was enabled by our original loading device in house. Stress concentration factor, α, and nominal stress of the notched specimens were obtained by means of static and dynamic analyses of finite element method. For the notched specimen of 2.25Cr-1Mo steel, stress amplitude, σa, at fatigue limit hardy reduced by the application of mean stress, σ_m, since maximum stress, σ_max, exceeded the yield stress, σ_ys, by a small amount of tensile σ_m application and the shakedown behavior took place. While for the bolt specimen of SNB7, σ a at fatigue limit reduced by tensile σ_m application, however, it tended to be constant when the σ_max exceeded the σ_ys to result in the shakedown behavior. It has been widely recognized that αis necessary to be considered for σ a but not for σ_m in order to predict the effect of stress concentration on the fatigue limit. However, αshould be considered for both σ_a and σ_m depending on the certain combinations of the material strength and the stress concentration condition.