Squat defects and rolling contact fatigue clusters - Numerical investigations of rail and wheel deterioration mechanisms
Doktorsavhandling, 2018

Squat defects, a type of localised rolling contact fatigue damage appearing on rail surfaces with rail break as an ultimate consequence, have concerned infrastructure managers for the last couple of decades. In recent years similar types of defects—so-called studs—that are visually resembling squats, have started to appear. In contrast to conventional rolling contact fatigue of rails, these defects are associated with a thin surface layer of brittle material—a "white etching layer". The wheel counterpart of squats/studs are called "rolling contact fatigue clusters". Despite significant research efforts, the exact initiation mechanisms of the defects are still unknown and it is difficult to relate the occurrence of squats/studs and rolling contact fatigue clusters to specific operational scenarios.

The current work aims to deepen the understanding of squat/stud and rolling contact fatigue cluster initiation by comparing and ranking predicted damage from various potential causes of initiation under different operational scenarios. Special emphasis is put on local surface irregularities. These are studied using dynamic vehicle–track interaction simulations to evaluate the impact of e.g. irregularity size, vehicle velocity, wheel–rail friction conditions and position relative to a sleeper. It is seen that surface irregularities might cause substantial fatigue impact. Rolling contact fatigue initiation connected to operational scenarios of specific interest are studied  more in detail by mapping dynamic contact stresses from simulations of vehicle–track interaction to finite element models for subsequent stress analyses and ranking of operational scenarios via ratchetting response and low cycle fatigue impact. Among the results, it is seen that larger irregularities and higher wheel–rail friction promote higher fatigue impact. In order to study the influence of irregularity geometry when macroscopic cracks are present, dynamic contact stresses are mapped onto finite element models of a cracked rail head. The severity is assessed using an equivalent stress intensity factor, which is seen to increase with the size of the irregularity. This effect holds also for clusters of irregularities. It is furthermore seen that even a shallow irregularity can make a substantial impact.

The influence of white etching layers is investigated by simulating thermally induced phase transformations occurring in spots on rail and wheel surfaces, subjected to subsequent mechanical loading. The influence of axle load and wheel–rail friction is investigated with respect to fatigue impact. It is seen that the axle load has a rather low influence whereas an increased frictional loading increases the fatigue impact considerably.

rail surface irregularities

rolling contact fatigue clusters

dynamic vehicle–track interaction

rolling contact fatigue



fracture mechanics

VDL, Chalmers Tvärgata 4C
Opponent: Dr. Richard Stock, LINMAG GmbH, Vancouver, Canada


Robin Andersson

Chalmers, Industri- och materialvetenskap, Material- och beräkningsmekanik

The influence of rail surface irregularities on contact forces and local stresses

Vehicle System Dynamics,; Vol. 53(2015)p. 68-87

Artikel i vetenskaplig tidskrift

Evaluation of stress intensity factors under multiaxial and compressive conditions using low order displacement or stress field fitting

Engineering Fracture Mechanics,; Vol. 189(2018)p. 204-220

Artikel i vetenskaplig tidskrift

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.









Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4391



VDL, Chalmers Tvärgata 4C

Opponent: Dr. Richard Stock, LINMAG GmbH, Vancouver, Canada

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