Surface roughness and plastic flow in rail wheel contact

Abstract

Material in railway rails is loaded repeatedly by the passage of the wheel. The maximum contact pressure which the material can carry elastically in the steady state is known as the ‘shakedown limit’. With an operating contact pressure below the shakedown limit the rail would be expected to remain elastic with a very long life. However, examination of rail cross-sections shows severe plastic deformation in a sub-surface layer of a few tens of microns thickness; the contact patch size is in tens of millimetres. This raises two questions: firstly, why should plastic flow occur if the shakedown limit is not exceeded; and secondly, why should plastic flow be confined to a thin sub-surface layer? It is hypothesized that asperity contacts are responsible for the observed plastic flow. This hypothesis was investigated in experiments on a twin-disc machine and was found to be correct. Numerical analysis showed that roughness causes the contact pressure to deviate from the assumed Hertzian (smooth) to one which is spiky.