Initiation and propagation mechanism of fish-scale-like fatigue cracks on a U75V quenched rail

Abstract

After rails are returned to service following rehabilitative grinding, severe rolling contact fatigue damage characterized by fish-scale-like oblique cracks occurs on the inner side of the U75VH rail tread. Metallurgical tests were performed on the damaged surfaces and cross-sections to analyse the initiation and propagation mechanisms of the fish-scale-like cracks. The results indicated that the rapid initiation of the microcracks could be attributed mainly to the generation of a thin white etching layer (WEL) during rail grinding. Microcracks initially propagated along the interface of the WEL and matrix, growing horizontally or downward in a “wavy” form and ultimately manifesting as fish-scale-like oblique cracks on the contact surface. Moreover, the WEL was rapidly removed and became much thinner under severe alternating wheel-rail stresses. The longitudinal profile of the fish-scale-like cracks could be divided into “circuitous cracks” and “downward cracks”. The circuitous cracks propagated at a small angle and tended to propagate to the surface in a wave-like pattern, thus resulting in spalling pits. Conversely, the downward cracks tended to propagate deeper into the matrix. Most branch cracks from the main cracks initiated at the peak of the upper crack face. The main propagation mechanism of the branching cracks and the main crack was transcrystalline fracture.