Two visually similar forms of surface rail defects appear to have very different initiation mechanisms, and can lead to different outcomes, with only one so far causing potentially dangerous rail breaks. Over the past decade, we have been investigating a new form of rail surface damage, which bears similarities to the form of rolling contact fatigue (RCF) commonly known as 'squats'. Although the appearance is very similar, we believe that the newer defect type, which we have christened a 'stud' or 'squat-type unidentified defect', has an entirely different cause and is not initiated by a RCF mechanism. Our hypothesis to explain the development of studs is based on the features that have been observed to date and the circumstantial evidence regarding their appearance. We believe that studs are initiated by thermal damage of the rail, which results from limited wheelslip, possibly associated with localised areas of poor adhesion. Studs are associated with vehicles having AC traction or thyristor-controlled DC traction, which both have more aggressive wheelslip and traction control than conventional DC-motored vehicles. Studs appear on open track and not in tunnels because wheel/rail friction is more variable than in tunnels, triggering damaging transient responses from the wheelslip and traction control systems. In tunnels, the limiting friction is commonly (but not always) sufficiently high to sustain high traction ratios. The mechanism by which studs propagate is still unclear, but may be a low-cycle fatigue mechanism associated with contact stresses, of a similar form to that which causes shells to propagate initially parallel to the rail surface.
Telling the difference between studs and squats
Railway Gazette International ; 168 , 8 ; 36-38
2012
3 Seiten, 8 Bilder, 1 Tabelle, 11 Quellen
Aufsatz (Zeitschrift)
Englisch
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