Squat initiation mechanism model in a rail-wheel contact

• Squat initiation on a transverse plane section across the rail is investigated. • Parameters governing the squat initiation mechanism from a mechanical point of view proposed. • Impact of the contact patch and its variations on the squat-like crack development assessed. • The effect of transversal forces and changed frictional properties on squat initiation quantified. Observations in a railway track frequently show the start of a squat-form crack right next to the wheel-rail running band on the rail head surface. This article investigates the potential of a head check-like surface defect near the running band to initiate a squat formation. Cross sectioning of the rail samples having squats in the initial stage of development suggested the crack initiation in a transverse plane (perpendicular to the rail traffic direction), implying that broadening to a typical squat shape in the longitudinal direction (rail traffic direction) occurs afterwards. The influence of possible factors that might trigger the squat formation particularly on the transverse plane across the rail is elaborated in the numerical study. The tendency of one defect to grow rather than another is analysed by configurational force theory in order to determine both the crack driving force value and the crack growth direction. The effect of the contact patch variations and changed frictional properties at the crack faces on the squat-like crack development are studied in detail. The numerical results show that analysed cracks are able to grow under particular wheel loading in the directions that are in accordance with observed squat growth. These results suggest lateral forces as one of the main suspected determinants favouring the squat initiation.