Mechanisms of high-strength structural materials fatigue failure in ultra-wide life region

For attractive technical applications such as cars and trains, the durability sought for some components is between 108 and 1010 cycles. However, only few experimental results beyond 107 cycles are available. The S–N curve in the ultra-wide life region must be determined in order to guarantee the real fatigue strength and safe life of components. This paper presents a study of the fatigue behaviour of the bearing steel 100Cr6 and austempered ductile iron between 103 and 1010 cycles (in the ultra-wide life region). These tests were carried out using the following testing techniques: rotating bending, tension-compression, high-frequency tension-compression loading. Based on the experimental results and the fractographic analysis of fracture surfaces, the initiation of surface and subsurface fatigue cracks and the influence of the type of loading on the shape of the S–N curves are discussed.