Einfluss von Eigenspannungen auf die Ermüdungsschädigungsentwicklung des martensitischen Federstahls 54SiCr6 im HCF-Bereich

This work contributes to the elucidation of the influence of residual stresses on the fatigue damage evolution of the martensitic spring steel 54SiCr6 in the high cycle fatigue range. The results of the fatigue tests performed show that the fatigue damage evolution starts with the formation of parallel slip bands, whose orientation follow the slip planes with the highest Schmid factors. As fatigue progresses, crack networks form on the sample surface and their crack density rises with increasing number of cycles and the level of the applied stress amplitude. In this process, higher crack densities can be observed under cyclic torsional loading compared to uniaxial cyclic loading. Initiation of fatal cracks occurs exclusively intergranularly along prior austenite grain boundaries with subsequent intergranular and/or transgranular short crack propagation. The short crack propagation represents the dominating phase of the fatigue life and shows an oscillating crack propagation rate, which is due to a barrier effect of prior austenite grain boundaries and grain boundary triple points. In the case of cyclic torsional loading, the comparatively higher crack density leads to a frequently occurring crack coalescence, which is also reflected in the oscillation of the crack propagation rate.
Generally, residual stresses lead to a higher fatigue life and crack initiation at the sample surface is inhibited, resulting in lower crack densities. Furthermore, under uniaxial cyclic loading, the initiation of the fatal crack shifts to the sample interior, where the crack propagation rate is reduced due to vacuum conditions. The cyclic torsion tests on shot peened samples provide evidence that the transition from shear stress controlled short crack propagation to normal stress controlled long crack propagation is impeded due to compressive residual stresses, which probably leads to the higher fatigue life in the case of shot peened samples.