The term damage is used here to describe a change either in the structure of a rail or of the transverse or longitudinal profile that reduces rail life. The changes of greatest interest here occur at or very close to the point of contact between wheel and rail.
A primary distinction is made here between:
- damage that is continuous or quasi-continuous,
- damage that is periodic or quasi-periodic, or
- isolated damage.
Periodic damage with a wavelength of greater than about 10m (waves) is usually a result of vehicle dynamic behaviour. Damage with a wavelength of less than 3m is usually a form of corrugation.
Three broad and fundamental categories of damage are considered here:
- mechanical deformation
- thermal damage
These are not exclusive: there can be overlap between damage mechanisms and more than one damage mechanism may operate at a particular point on the rail.
Wear usually occurs relatively slowly; mechanical deformation more quickly (although still requiring many cycles of loading), whereas thermal damage typically occurs from a single cycle of loading.
The term spalling is used here to refer simply to detachment of a small particle of material from the rail surface by any cause.
Effects of maintenance are also considered separately to include changes to the running surface of the rail that arise from maintenance of the rail itself.