Numerical investigations of rolling contact fatigue crack growth in a rail head

Doktorsavhandling, 2024

Rolling Contact Fatigue (RCF) cracks in railway rails are critical due to their impact on safety and maintenance costs. A rail break can be the ultimate consequence of progressive crack growth. Understanding influential mechanisms involved in RCF initiation and propagation is essential for developing effective inspection, maintenance, and replacement strategies. Despite extensive research on understanding mechanisms, and predicting RCF crack growth, there are still open questions, particularly regarding the direction and rate of RCF crack growth. This is due to the complex non-proportional and compressive loading from wheel‒rail contact in combination with rail bending and thermal loads.

This thesis aims to develop and employ a numerical framework for predicting directions and rates for RCF crack growth in railway rails under operational loading. A numerical procedure for simulating a propagating crack in 2D is developed. The finite element model features an inclined surface-breaking crack 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 criterion. The influence of crack face friction and crack growth rate predictions are added to the numerical framework. It is observed that frictional cracks tend to go deeper into the rail under a pure contact load and under a combination of bending and contact loads, while friction has a moderate influence on crack paths under combined tensile thermal and contact loads. Furthermore, friction reduces crack growth rates.

The numerical framework for a frictionless stationary crack is extended to the 3D case of a semi-circular gauge corner crack embedded in a 60E1 rail. Results are evaluated at points along the crack front. The influence of crack size and inclination, magnitude and position of the contact load, wheel‒rail tractive forces, rail bending with different track support conditions, and varying thermal loads is studied. Rail bending and tensile thermal loads are found to promote downward growth compared to pure contact load cases. The location of the contact load significantly influences predicted growth rates while the influence of a tractive force decreases (rapidly) with depth. Rail lives computed from the integration of predicted growth rates are comparable to growth rates found in field investigations.