Experimental investigation on rolling contact fatigue and thermal fatigue in freight wheels prefabricated with tribologically transformed structure layer

The tribologically transformed structure layer (TTSL) induced by tread braking markedly modifies the mechanical response of freight wheel treads, intensifying rolling contact fatigue (RCF) and even promoting thermal fatigue (TF) cracking. In this study, a block-wheel-rail contact apparatus was employed to prefabricate a TTSL on the wheel specimen surface. A series of thermo-mechanically coupled and decoupled tests were conducted to systematically evaluate the effects of contact mode, water medium, and TTSL on wheel RCF and TF damage. The results show that water plays fundamentally different roles in wheel fatigue damage under different contact modes. In the wheel-rail contact mode, water acts as a hydraulic medium and markedly accelerates RCF crack propagation. In the block-wheel-rail contact mode, water mainly serves as a cooling medium, promoting the initiation of TF cracks. The TTSL exerts a dual influence on RCF damage, increasing the susceptibility to local brittle fracture while retarding the establishment of the hydraulic-wedge effect. RCF cracks within the TTSL exhibit a two-stage propagation behavior, from TTSL interfacial extension to shear-dominated inclined growth. In addition, TF crack initiation requires critical thermo-mechanical loading conditions and is strongly promoted by the microstructural degradation associated with TTSL formation.