On the transition behavior between rolling contact fatigue and thermal fatigue of freight wheels under various contact parameters/modes during tread braking process

During tread braking, railway wheels are subjected to severe thermo-mechanically coupled loading, which promotes the formation of a tribologically transformed structure layer (TTSL) in the near-surface region and may trigger thermal fatigue (TF) cracking in addition to rolling contact fatigue (RCF) cracking. In this study, a blockwheel-rail coupled contact test apparatus was employed to systematically investigate the effects of contact parameters such as brake block normal force, wheel speed, and wheel-rail normal force on wheel fatigue damage under tread braking conditions. The propagation paths of TF cracks under different contact modes were further comparatively analyzed. The results demonstrate that wheel fatigue damage is governed by the combined effects of the wheel-rail friction coefficient and the braking-induced thermal input, rather than by any single contact parameter. Based on the evolution of TTSL coverage under different conditions, wheel fatigue damage can be classified into three distinct regimes: RCF-dominated, RCF-TF competitive, and TF-dominated. Furthermore, TF crack propagation exhibits significant contact-mode dependence, growing nearly vertically under the blockwheel-rail contact mode, but following shallow inclined paths under the wheel-rail contact mode.