Studs and squats: The evolving story

Abstract

Several railways suffer from a defect that has been christened a “stud”, which appears superficially similar to a squat but is very different in character. Although both initiation and propagation of studs are poorly understood, at least a couple of railways have already benefitted from exploiting the less malevolent nature of studs, particularly the fact that these do not themselves initiate transverse defects.This paper is based primarily on field work undertaken in NSW to reveal some of the characteristics of stud defects, in particular in contrast to rolling contact fatigue (RCF) of which squats are a classical example. This work has been supplemented by a very much smaller sample of field work undertaken in Switzerland and metallurgical analysis of studs that was undertaken at Sheffield University of samples from Swiss Railways. Studs are often associated with sites where there is high traction, such as the exit from stations. In NSW they commonly initiate at about 10–20° to the vertical towards gauge on the high rail in curves, then grow into the rail at an angle of about 20° to the rail surface. Studs can at first develop very quickly e.g. to a depth of 2.2mm in 6MGT. The stud fans out across the rail from the initial surface crack, developing across the rail at a substantially constant depth of 3–6mm. If the stud is left, it may rise to the surface at the opposite side of the railhead, giving rise to an ugly spall with a fracture surface typical of a conventional fatigue crack. There is no evidence whatsoever that studs become transverse defects, nor should this occur with a crack that develops across rather than along the rail as there is an absence of flexure to drive the crack. Studs have grown in rails in which there is no significant plastic work, and accordingly also no significant depth of compressive residual stress. These characteristics are not those of a classical squat.Although some transverse defects (TDs) have been associated with studs, both gauge corner cracking (GCC) and studs have coincided in all of the cases examined. In these cases the TD has clearly developed in the conventional and well understood manner from the GCC. The stud has given rise to a dynamic load that accelerates growth of the TD. But if the GCC had not existed, the TD would not have developed.The paper provides some guidance on treatment and maintenance of studs as well as updating a hypothesis that still rests substantially on circumstantial evidence. It is strongly recommended that railways move away from referring to these defects as “squats”, particularly when it is clear (as is increasingly the case) that the defect described is not a squat.