A combined rolling contact fatigue damage mechanism for EMU train axle box bearings

The fatigue fracture of bearings is a major threat to the service safety of EMU trains. This study systematically investigates the evolution of spalling in axle box bearing outer rings through detailed characterization and in-depth analysis, proposing a combined rolling contact mechanism. Two distinct regions are identified in the spalls, respectively corresponding to subsurface- and surface-induced rolling contact fatigue (RCF) damages. Crack propagation and branching behaviors are thoroughly revealed in different spalling regions. According to this study, spalling begins with subsurface cracks originating at the depth consistent with the peak maximum shear stress under Hertzian contact, forming concentric ellipses with deep pits. As spalling progresses, surface-initiated cracks from spall-edge stress concentration can lead to further, shallow material removal that broadens with increasing over-rolling cycles. Electron backscatter diffraction and nanoindentation show that local microstructural refinement and hardening occur along with crack formation. A critical threshold stress intensity factor that dominates RCF crack growth is found, which is crucial for RCF modelling and life prediction. The study also identifies two crack-carbide interaction modes, cutting-through and bypassing, with the transition between them determined by a critical carbide diameter of 1 µm. These findings provide crucial insight into the RCF mechanism and offer practical guidance for long-life railway bearing steels.