Rail squats: progress in understanding the Australian experience

Abstract

Rail squats or studs, being a sub-surface crack in a track below a depression of the rail surface, have been studied in the Australian CRC for Rail Innovation project R3.105, from a number of perspectives. Examination of squats on track by the Rail Corporation of NSW has revealed statistics about their distribution. Examination with optical and scanning electron microscopy at the University of Queensland has shown the frequent presence of a brittle white etching layer (WEL) on the rail surface. This has led to successful adaptation of eddy currents to detect WEL rather than cracks. Tests at the University of Queensland have investigated whether the WEL can form below the normal 720°C needed to form austenite. Examination of crack surfaces shows beach marks indicative of growth in modes II and III. Neutron diffraction testing of Australian rail has shown residual stresses in a railhead with a WEL, similar to those reported elsewhere, but with a broad surface layer in compression, not a narrow running band. Elasto-plastic finite element simulation has shown residual compression transforms into residual shear at a crack tip, which increases that due to plastic deformation from successive wheel passages, tending to encourage the crack to continue in a direction of sub-surface growth. The rate of growth of squats measured on-track in Sydney shows crack growth that corresponds to a power law with a low exponent, implying that the stress intensity at the crack tip is not a strong function of crack length. This is partly due to the localised nature of contact stresses, which allows the crack to grow beyond the region loaded. However, accounting for this effect leads to the prediction of a smaller reduction in growth rate than that observed. It is likely the low exponent reflects a smaller effect of water on crack growth, as the crack enlarges. There is also a redistribution of contact loading that occurs as a crack grows