Influence of mean stress and variable amplitude loading on the fatigue behaviour of a high-strength steel in VHCF regime

Components and structures are often exposed a very high number of cycles. Within the scope of this paper the effect of the mean stress in terms of the stress ratio (R-ratio) on the fatigue properties of a high-strength steel (34CrNiMo6) in the VHCF regime was experimentally investigated under axial loading. For both analyzed R-ratios the cracks nucleate predominately at non-metallic inclusions in the interior of the specimen and a fish-eye is formed, but ODAs were not visible on the fracture surface. Thus, continuous S-N curves are observable, where the slopes as well as the fatigue strengths are dependent on the R-ratio. Moreover, the number of cycles to failure is influenced by the R-ratio. In order to investigate the influence of variable amplitudes on the lifetime fatigue tests were performed using the standard helicopter load spectrum FELIX. Due to variable amplitude loading arrest marks are created surrounding the non-metallic inclusion, which are used to calculate the fatigue crack growth rate. The S-N curves of the VA tests are shifted to higher lifetimes depending on the reconstruction sequence. The results are used for the evaluation of different approaches calculating short crack growth threshold ranges and the fatigue lifetime.