Studs: a squat-type defect in rails

Abstract

In the mid-2000s, a new rail defect that was initially classified as a squat became increasingly common on London Underground's track. By 2006, there were about 600 of these and they had become the Underground's single most common rail defect. This defect occurred almost exclusively on lines carrying relatively new rolling stock. The work reported here was undertaken initially to characterize this defect, advise as to whether it was indeed a squat, and propose a hypothesis that explained its mechanism of formation. This paper includes observations and track measurements made in the field and initial results of a metallurgical analysis. The hypothesis for formation of the defects is presented, and both similarities and differences are discussed between these defects and the classical 'squat'. The defect on London Underground appears to be the same as that described by Marich and his colleagues in Australia and by Li and his colleagues in the Netherlands. It is evidently not a conventional rolling contact fatigue defect. In order to avoid confusion arising from simple misuse of an established term, it is proposed that these defects be given a different name, for which 'stud' is proposed. Evidence to date is that the 'stud' is a significantly more benign defect than the 'squat'.

Dynamic Wheel–Rail Interaction Over Rail Squat Defects

“…For instance, a certain level of noise excited by rail squats was observed by residents near the Woolloomooloo viaduct in Sydney, Australia [1]. Reportedly, the cost of rail replacement due to rail squats and studs has become a major part of whole-life-cycle track maintenance costs in European countries [2]. The rail squats and studs are typically classified as the growth of any cracks that have grown longitudinally and laterally through the subsurface.…”

The potential for suppressing rail defect growth through tailoring rail thermo-mechanical properties

“…Evidence from defects cut open for examination is that plastic damage is almost completely absent in stud defects [4], so thermal input in the generation of the WEL has been examined. Severe thermal input has three main consequences, which last over different durations: (i) temporary thermal expansion of the steel, (ii) permanent metallurgical transformation of the steel to WEL, and (iii) permanent locked-in stress produced by the change of material volume associated with the metallurgical transformation.…”

3D characterization of rolling contact fatigue crack networks

“…The initiation of squats is thought to originate due to local plastic deformation of the surface from rolling contact fatigue (RCF) loading, including dynamic wheel-rail contact forces [1]. When local thermal damage on the rail surface is believed to affect the initiation mechanism, the defect is rather referred to as a stud [1,7]. In this work we have not determined the cause of initiation due to limited background information, and have chosen to use the name 'squat' for the defects studied.…”

“…In order to develop a proper maintenance system, it is important to firstly understand the nature, causes, and consequences of different defect types. In recent years, squats and squat-like defects have been the subject of several investigations [4][5][6][7] and are proving to be an important cause of failure and thus a cost-driver for maintenance worldwide. Squats are a particular type of defect that generally manifest themselves as planar crack networks parallel to the surface within a shallow depth from the rail head; however, in some cases they may develop into cracks perpendicular to the surface, causing transverse rail fracture.…”