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Civil-Comp Proceedings
ISSN 1759-3433
CCP: 98
Edited by: J. Pombo
Paper 96

Squat Formation on Train Rails: Origination

M.J.M.M. Steenbergen

Railway Engineering Group, Delft University of Technology, the Netherlands

Full Bibliographic Reference for this paper
M.J.M.M. Steenbergen, "Squat Formation on Train Rails: Origination", in J. Pombo, (Editor), "Proceedings of the First International Conference on Railway Technology: Research, Development and Maintenance", Civil-Comp Press, Stirlingshire, UK, Paper 96, 2012. doi:10.4203/ccp.98.96
Keywords: squat, rail failure mechanism, rail crack, white etching layer, shear stress, rolling contact fatigue.

A theory is presented for the origination and physical nature of squat defects on train rails. This theory is formulated departing from and validated against both experimental and field evidence.

Examination of the geometry of the surface-breaking crack pattern, which may be linear or branched (typically V-shaped), shows a consistent position and typical orientation of this pattern in the running band. Characteristics of the pattern are its asymmetry, with the presence of a leading and a trailing branch, and the crack reflection or deviation at the border of the running band. Bending tests of samples taken from the rail surface and containing squats, together with longitudinal cross-sectioning, reveals the internal and essentially three-dimensional crack morphology: an internal pair of crack planes ('wings') at shallow angles corresponding to the pair of surface cracks. These planes are ideally flat and form a 'wedge', spatially expanding in time in the form of 'growth rings' in the subsurface.

Optical microscopy of both transverse and longitudinal cross-sections the rail upper layer shows two metallurgical principles of crack initiation: edge delamination of white etching shelves, embedded in the surface, and transverse fracture of white etching material. The role of white etching material appears therefore as crucial in the early stage. This analysis moreover reveals a strongly anisotropic and essentially three-dimensional texture of the upper layer, which is developing as a function of the load history, under combined shear surface tractions. Transverse tractions are directed towards the rail gauge face, as a result of conicity, and longitudinal tractions are directed opposite to the running direction for driven wheels.

Mechanical interpretation of the three-dimensional crack morphology shows that the leading or single branch of the surface-breaking crack pattern is a shear-induced fatigue crack, following the anisotropic microstructure when growing into the railhead. The trailing crack of a branched squat defect, which is found to run crosswise through the material texture, is explained as the result of a transverse, bilinear brittle failure mechanism of the surface layer. The leading branch of this mechanism is given by the pre-existing leading shear fatigue crack. The failure mechanism is wedge-shaped at the surface. It develops under transverse shear loading towards the rail gauge face, and may develop either within the elliptical actual Hertzian contact patch (typically for 'baby' squats) or the envelope of potential local contact patches (typically for larger squats). As in practice the running band consists of a superposition of many contact patches whose locations vary within a bandwidth in transverse direction, both mechanisms play a role.

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