Fracture mechanisms in rails with mechanically and thermomechanically-induced white etching layers under three-point bending

Abstract

White etching layers (WELs) on rail surfaces play a crucial role in dictating rail life and guaranteeing safety during transport. It has been reported that the presences of WELs correlate with wheel-rail tribological properties and rail defects, including squats and studs. It is impossible to directly measure fracture mechanisms in WELs when rails are in service because they are subject to numerous wheel passage cycles, and WELs are routinely spalled due to dynamic impact throughout their lifetime. Consequently, samples containing mechanically and thermomechanically-induced WELs, as well as WEL-free samples were sectioned from ex-service rails and subjected to three-point bending to investigate crack initiation in WELs and compare the subsequent propagation of cracks into the base material leading to fracture. The results show that WELs are subject to intergranular brittle fracture and promote ductile crack growth before macroscopic river-like cleavage failure. In contrast, the WEL-free rail fails via a mixed-mode of quasi-cleavage fracture. Mechanically-induced WELs are more detrimental to rail life compared to their thermomechanically-induced counterparts on account of the former’s high hardness and ultrafine microstructure resulting in more severe and numerous cracks. Moreover, the heavily deformed pearlite at mechanically-induced WEL-base material interfaces contributes to the lower fracture resistance and enhanced crack penetration from WEL to rail.