Very high cycle fatigue of high-strength steels: Crack initiation by FGA formation investigated at artificial defects

Abstract

It is well known, that high-strength steels do not show a classical fatigue limit and failure occurs still after 107 cycles. The reason for this late failure is that the fatigue properties in the long life region are strongly affected by non-metallic inclusions inside the material (Murakami et al. (1989)). After VHCF a characteristic fine granular area, or short FGA, can be observed at the fracture surface. The FGA formation is responsible for the late initiation of a propagable long crack (Grad et al. (2012); Sakai et al. (2002)). In recent years a lot of research was carried out to reveal the mechanism responsible for the crack initiation by FGA formation (Grad et al. (2012); Murakami et al. (1999); Sakai, Kokubu, et al. (2015); Shiozawa et al. (2008)). Multiple different theories exist in literature trying to explain VHCF failure. Because of its occurrence solely below the surface by now only the fracture surface could be observed after the failure occurred. This makes it impossible to gain a full understanding of the mechanisms leading to failure. Consequently all proposed mechanisms remain unprovable theories, up to now.In order to further investigate FGA formation we have simulated the VHCF-failure at artificial surface defects. According to literature the absence of any environment seems to be crucial for VHCF subsurface failure by FGA formation in high-strength steels (Billaudeau et al. (2004)). Thus, fatigue tests were performed in ultra-high vacuum to simulate subsurface condition at surface defects. Thereby the FGA formation can be reproduced at the surface and gets observable quasi in situ. Microstructural investigations were carried out with transmission electron microscopy inside the FGA at artificial defects. These measurements show the comparability of the resulting microstructure at artificial defects with the FGA microstructure after failure at subsurface inclusions. Therefore, it is possible to gain new insights into FGA formation gained with the use of artificial defects. Thereby fatigue test with artificial defect can be interrupted and enable the in situ investigation of pre-stages of FGA formation by microstructural analyses with transmission electron microscopy. With this testing procedure it should be possible to enhance the knowledge of the very high cycle fatigue in high-strength steels and the associated failure mechanisms.