Crack growth paths in rolling contact fatigue — Numerical predictions

Sammanfattning: Rolling contact fatigue (RCF) cracks in railway wheels and rails are costly and complex to deal with. Despite the extensive research efforts that have been put into understanding the mechanisms and developing appropriate predictive models for RCF crack growth, there are still a lot of open questions. This is the case regarding the direction and growth rate of RCF crack propagation under multiaxial wheel‒rail contact loading, which also interplays with rail bending and thermal loads during the operational life of a rail. In the first paper of this thesis (Paper A), a numerical procedure is developed to evaluate the effect of different operational loading scenarios on the predicted crack paths in rails. A 2D linear elastic finite element model of a rail part with an inclined surface-breaking crack has been implemented. The rail part is subjected to wheel‒rail contact load, rail bending, and temperature drop as isolated scenarios and in combinations. The effective crack propagation direction is predicted based on an accumulative Vector Crack Tip Displacement (VCTD) criterion that accounts for crack face locking effects through a reversed shear threshold parameter. It has been shown that the crack path for combined thermal and contact loads varies gradually between the pure load cases while the combination of bending and contact loading has an abrupt change in predicted crack paths. Furthermore, the dependency of the results on the reversed shear threshold parameter is investigated. The influence of crack face friction on the crack path is investigated in the second paper (Paper B). The numerical procedure developed in Paper A is utilised, and crack face friction is modelled by a Coulomb friction model.  Qualitative predictions are obtained for varying magnitude of the coefficient of friction, as well as for varying parameters of the crack growth criterion. It is observed that the frictional crack tends to go deeper into the rail under a pure contact load and for a combination of bending and contact loads, while the friction has a moderate influence on the crack path for combined thermal and contact loads. Furthermore, assessment of the ranges of crack face deformation indicates that friction reduces the crack growth rate.