Very high cycle fatigue behavior under constant and variable amplitude loading

Components and structures are often exposed a very high number of cycles. The investigations in the field of very high cycle fatigue (VHCF) are mainly focused on experiments without mean stresses and with constant amplitude loading. Therefore, within the scope of this paper, investigations with constant and variable amplitudes with different mean stresses will be presented. For studying variable amplitude loadings in the VHCF regime systematic two-step block loading experiments have been performed, in which the maximum load amplitudes of the high block and the number of cycles of the low block with amplitudes below the fatigue strength have been varied. Moreover, the standardized load-time-histories Felix/28 and WISPER have been used. The influence of different reconstructions as well as the amount of the amplitudes beneath the fatigue strength of the investigated high strength steel on the initiation site, the S-N curve and the lifetime prediction has been investigated. Due to the variable amplitude loadings arrest marks are produced within the fish-eye surrounding the inclusion. The sizes and the area, where arrest marks are observable, as well as the spacings between the arrest marks are influenced by the different load sequences. By counting and measuring the arrest marks an average crack growth rate for the crack propagation within the fish eye can be calculated. Moreover, the short crack growth curve is used for calculating lifetimes using fracture mechanical approaches.