The role of local plasticity during very high cycle fatigue crack initiation in high-strength steels

Cracks leading to failure in very high cycle fatigue (VHCF) of high-strength steels often initiate at subsurface inclusions with stress intensity factors (SIF) below the classical threshold SIF Kth for long cracks, especially for negative stress ratios. A characteristic fine granular area (FGA) can then be observed at the fracture surface in the vicinity of those crack-initiating inclusions. According to various researchers, the FGA formation might be responsible for the late initiation of a propagable long crack. Microstructural investigations reveal strong local grain refinement along the fracture surface inside the FGA. The origin of the FGA and its influence on the crack initiation in VHCF is discussed controversially in the literature. The grain refinement might be the result of local plasticity around inclusions in VHCF prior to crack initiation or propagation and thereby causal for the late failure. If this is true, there should be a correlation between the FGA dimension and the local plastic zone size. This hypothesis is investigated in detail in this study. For this purpose, the spatial geometry of FGA is systematically analyzed for different fatigued specimens of the high-strength steel 100Cr6. The results are compared to plastic zone size models and the influence of local plasticity on VHCF is discussed. Based on the results, the VHCF failure mechanism proposed by Grad et al. (2012) [1] is extended and improved.