Keywords: wheel-rail contact, friction, adhesion, tangential stress, white etching layer, friction induced martensite, rolling contact fatigue.

A rolling train wheel causes a moving, three-dimensional stress field along the rail surface. This field is, in a convective reference system, ideally constant in time but may vary due to changes in geometry, friction, material properties or loading configuration of the wheel-rail system. To this external stress field corresponds, at a fixed material point, a transient strain field inside the rail. This material response is, for sufficient axle loading, non-elastic in the upper layer. As a result, a 3D strain history accumulates in the surface layer as a function of born tonnage. Experience shows that this process is accompanied by the growth of interface layers, on top of the rail parent material, of an extreme hardness as compared to the parent material. These layers themselves may show rather different behaviour and characteristics, with respect to grain size, boundary configuration, demarcation with respect to the parent material, thickness and also deformation texture of the underlying base material. The literature knows two forms of interface layers: white etching layers (WEL) and friction induced martensite (FIM). The distinction into a purely mechanical and a purely thermal origin may however be inadequate to describe all features that are found. The near-surface material constitutive behaviour (in comparison to the parent material) and its wear properties are important because there is a clear relationship with the external manifestation of rail defects such as squats and also corrugation.

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