Investigation of the crack initiation of subsurface rolling contact fatigue in railway wheels

To investigate the crack initiation behaviour of the subsurface rolling contact fatigue (RCF) of railway wheels, a novel experimental procedure was developed to determine the actual shapes and sizes of macroscopic defects in the wheel rim. The experimental results show that the macroscopic defect can be approximated as an ellipse with an axis ratio of 2.5-7. Most of the macroscopic defects in the wheel rim are composed of different oxides, which is in accordance with observations for scrapped wheels subjected to subsurface RCF. The mode II threshold stress intensity factor range Delta K-II,(th) of the wheel steel was measured using a double cantilever specimen. Finally, the initiation behaviour of subsurface RCF crack was evaluated based on a fracture mechanics approach, considering the morphologies of the macroscopic defects. The evaluation results show that the subsurface RCF is unlikely to occur in railway wheels that satisfy the requirements of relevant standards under the straight condition, while it will likely occur under the limited condition. For a given defect size, the initiation of subsurface RCF crack is the least likely when the axis ratio of the macroscopic defect is controlled to be a/b = 2.