Effect of Slip Ratio on the Rolling Contact Fatigue Performance of Wheel Steel Used in EMU

Rolling contact fatigue (RCF) is a significant factor affecting service safety and life of trains,and the slip ratio between wheels and rails greatly affects RCF damage. In order to analyze the influence of slip ratio on the RCF performance of wheel steel utilised in electric multiple unit (EMU), this study conducted a rolling/sliding test for wheel steel under the water lubrication condition at different slip ratios. Then, the traction coefficient, wear and RCF behaviors of the experimental specimens were analyzed. The theory of the shake down map was used to analyze the RCF cracks initiation behavior. Finite element (FE) simulation was conducted to investigate the RCF cracks propagation behavior based on the fluid cavity technology and the maximum circumferential stress criterion. The experimental results revealed that, with an increase in the slip ratio, both the traction coefficient and the wear loss of wheel specimens increased, the degree of oxidation in the contact area intensified, and the plastic deformation of the surface layer of the wheel specimens increased, while the RCF life of the wheel specimens initially decreased and then increased. The analysis based on theory of the shake down map suggested a positive correlation between the traction coefficient μ and the surface RCF damage index FI_surf. With an increase in the slip ratio, the traction coefficient increased, resulting in an increase in the the surface RCF damage index FI_surf, there by facilitating the initiation of fatigue cracks and reducing the fatigue life of wheel specimens. The FE simulation analysis found that the maximum equivalent stress intensity factor, K_θ,max, increased as the slip ratio increased, which promoted the propagation of fatigue cracks and eventually reduced the fatigue life of wheel specimens. In addition, the increase in the wear inhibited the initiation and propagation of the RCF cracks, resulting in the increase of the fatigue life of wheel specimens. Consequently, the competition between RCF and wear resulted in a pattern: in cases of the slip ratio increased from 0.2% to 0.6%, the low wear rate occurred and the damage of the wheel specimens was dominated by the RCF, so the fatigue life of the wheel specimens decreased; in cases of the slip ratio further increased from 0.6% to 1.0%, the wear rate increased and the damage of the wheel specimens was governed by the wear, leading to an increase in the fatigue life of the wheel specimens. Moreover, the experimental observations found that fatigue cracks in the wheel specimens propagated in three directions, and the FE simulation results showed that the maximum equivalent stress intensity factor K_θ,max had three peaks in a contact cycle, meaning that fatigue cracks also had three propagation paths. The propagation directions agreed with the experimental observations, indicating that fatigue crack propagation path under the rolling/sliding condition could be predicted via the fluid-cavity technique and the maximum circumferential stress criterion.