Influence of rolling speed on fatigue crack initiation and growth of wheel steel under dry-wet rolling/sliding contact

In order to examine the effect of rolling speed on rolling contact fatigue (RCF) of wheel steel, dry-wet rolling/sliding tests were carried out at varied rolling speeds. Wear loss, friction coefficient, crack morphology, and crack size distribution of test wheel discs were analyzed. Finite element (FE) analysis was subsequently performed to evaluate the RCF crack growth according to the max ΔKeq criterion. By combining the findings from both experimental and FE analyses, potential influences of rolling speed on RCF crack initiation and growth were postulated. It was discovered that the crack initiation is governed by the ratcheting mechanism under dry condition. As the rolling speed increases, the initiated crack size first increases and then decreases, probably due to the concurrent rising of friction coefficient and yield strength. Furthermore, the predicted crack growth paths using FE model with and without considering the trapped fluid are consistent with test results at lower and higher rolling speeds, respectively. The driving force of crack growth in depth and branching direction decreases with an increase in rolling speed, probably owning to the reducing hydrostatic pressure in the crack cavity induced by the increasing air volume. This ultimately results in smaller crack size and reduced wear loss under wet condition.