Loads from trains, corresponding to the weight of some ten automobiles, are transferred to the rail via the wheels through a contact patch the size of a thumb nail. This results in severe wheel–rail contact pressures. Together with frictional forces due to traction, braking and curve negotiation, this may lead to rail and wheel damage in different forms.
This work focusses on isolated damage on rail and wheel surfaces. These damage types are known as squats when they appear on rails and Rolling Contact Fatigue (RCF) clusters when they occur on wheels. They consist of crack networks extending below the contact surface and the damage might lead to rail breaks or severe wheel damage. Both the phenomena may cause derailments. To prevent and mitigate squats and RCF clusters, it is of importance to understand the underlying mechanisms behind their formation.
Although the exact root causes are still unknown, several potential damage triggers have been suggested in the literature. These include the influence of local surface irregularities of different forms. The current work evaluates and quantifies influence of important damage triggers. It is therefore valuable in a rational prioritisation of mitigation actions.
One straight-forward approach to achieve such an evaluation would be to perform full scale experiments where trains are operated under strict conditions and the resulting damage is documented. However, high costs and difficulties in keeping parameters, such as wheel–rail friction, at fixed values make such an approach unrealistic. An alternative to overcome these obstacles is to run the experiments on a computer, i.e. perform simulations. This is done in the current thesis. Suitable computer models are developed. Numerical simulations are then performed and conclusions are drawn on, e.g., how the size of a surface irregularity affects the risk of cracks to form.
The thesis consists of an extended summary and six appended papers (A–F). Paper A evaluates rough but fast predictions of the damage caused by surface irregularities. A large number of different irregularity sizes and running conditions are considered thanks to the fast evaluations. Paper B investigates how more detailed, and thus more time consuming, analyses can be performed. This procedure is then employed in Paper C for some relevant cases. Paper D considers how cracks can be incorporated into the analysis. This procedure is employed in Paper F, where cracks in the vicinity of surface irregularities are studied. Paper E investigates the influence of thermal damage, which can be caused by wheel–rail sliding. The risk of crack formation in the vicinity of thermally damaged rail and wheel material under different operational conditions is investigated.
Finally, the thesis summarises the most important findings. These conclusions could serve as a useful guide in the future struggle against squats and RCF clusters.