Advancing the understanding of short fatigue crack propagation: Leveraging ultrasonic testing device to approach rolling contact fatigue

This paper uses ultrasonic testing devices to approach the rolling contact fatigue (RCF) stress state experienced during rolling on an indented surface, in order to understand the primary cause of failures of rolling element bearings in aeronautics. It relies on testing specimens made of M50-VIM/VAR steel while inducing compressive preload. This leads to a localized multi-axial and non-proportional stress field, induced by an artificial surface defect created via electro-discharge machining (EDM). Observations reveal that the surface crack initiation occurs along the EDM beyond $10^8$ cycles, with no shift observed from surface defects to sub-surface defects, as commonly seen in very high cycle fatigue (VHCF) regime. Our analysis suggests that the stress intensity factor range, $\Delta K$, may govern surface initiation in the VHCF regime, particularly when the formation of fine granular area (FGA) is not feasible. Consequently, under fixed stress conditions, there exists a critical surface defect size below which short crack initiation becomes improbable. These results mirror the behavior usually observed for indentations and thereby connect ultrasonic loading with RCF. Besides, initiations of fatigue butterfly and FGA appear to be associated with VHCF tests, compression, high levels of multi-axial stresses, and the refinement of microstructure at low temperatures. These findings shed light on a potential link between fatigue butterfly and FGAs, attributed to the same underlying cause: cross-slip.