1.1 Wear and plastic flow
Wear occurs whenever a wheel rolls over a rail and transmits a tangential force. Wear rates are low unless sliding between wheel and rail and/or tangential forces are consistently high. In practice the highest wear rates occur where slip is greatest, in particular when the wheel flange contacts the gauge face. In straight track a rail crown wear rate of 0.05-0.1mm per 10MGT of traffic would not be unusual, whereas wear rates in curves could be an order of magnitude greater than this and gauge face wear rates higher again (in the absence of lubrication).
Plastic flow is bodily deformation, without loss of material, that results from high loading normal and tangential to the wheel/rail contact. Both wear and plastic flow change the shape of a rail.
In straight track the crown of the rail wears to become slightly “flat”, conforming to the shape of the wheels that run over it. Some plastic flow occurs towards the gauge and field corners. The measurement shows a rail that has rolled a little to field, so contact is not along the centre of the rail crown and there is more wear of the gauge shoulder.
The photograph shows a case of abnormally rapid wear and plastic flow. This occurred in only a few days because the rail was transposed i.e. turned, so that the field side of the rail became the gauge. Contact stresses on the gauge corner were extremely high and flow occurred quickly to change the rail shape and reduce stresses to those that could be carried.
In curves that are sufficiently severe for there to be flange contact, the high rail usually wears until it conforms almost exactly to the corresponding flange root area on the wheel. Wear of the gauge face is known as side cutting.
If friction is sufficiently high, the tangential force can pull material down the gauge face of the high rail, causing a lip of plastically flowed material that is detached by wheels. These slivers of material can then be seen lying alongside the rail.
Wear and deformation are best controlled with adequate lubrication, which can be applied to either the rail or the wheel.
The low rail tends to be flattened as a result of wear and plastic flow. Gauge and field corners can be quite abrupt as a result of the flowed material.
In extreme cases, plastic flow gives rise to flaking of the low rail with clear signs of detached material. This usually occurs only where friction is consistently high e.g. in tunnels and very dry, unlubricated areas. The loss of plastically flowed material (as in the photo) would be reflected in a high wear rate. The measurement of a low rail shows an early stage of the condition known as collapsed head. This occurs more readily in curves because tangential loads are higher.