Squat defects and rolling contact fatigue clusters - Numerical investigations of rail and wheel deterioration mechanisms
Sammanfattning: 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.
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