Predicting rolling contact fatigue crack initiation in highly deformed rail steel

Licentiatavhandling, 2024

Rolling Contact Fatigue crack initiation is often associated with large accumulated plastic deformations in the surface layer of rails and wheels. These deformations are known to affect the mechanical and fatigue behavior of the material. Even so, many fatigue crack initiation predictions in literature do not account for the influence of the deformed near-surface material. This thesis focuses on developing modeling methodologies that account for the long-term accumulation of plastic deformations in fatigue crack initiation predictions.

The first part of the thesis deals with evaluating and improving fatigue crack initiation criteria for severely deformed R260 pearlitic steel. This enhancement is addressed by proposing modified criteria. Three groups of experiments form the basis for the evaluations: Axial-torsion tests with large shear strain increments (predeformation), uniaxial or proportional multiaxial low cycle fatigue tests after different amounts of predeformation, and uniaxial high cycle fatigue experiments. To assess the performance of both existing and proposed crack initiation criteria, a cross-validation procedure is used. The proposed criterion, which accounts for the influence of accumulated plastic strains on fatigue crack initiation, improves the fit to the experimental data. Although there is a tendency to overfitting, this can be mitigated by considering more experiments.

In the second part of the thesis, fatigue crack initiation in a railhead subjected to realistic traffic loading is investigated, focusing on the influence of the large accumulated plastic deformations near the surface. The adopted anisotropic material model is calibrated against experiments with railway-like loading at different material states, corresponding to different depths in the railhead. The identified material parameters are then used to consider spatially varying properties in the railhead. This variation is governed by the accumulated shear strain distribution, obtained from measurements in field samples. By using the stresses and strains from finite element simulations of wheel over-rollings, the previously developed crack initiation criterion is applied. The results highlight the importance of considering the deformed near-surface material in a railhead when predicting fatigue crack initiation, as it is shown to reduce fatigue damage growth during a traffic load sequence.