An investigation of rail squats from several perspectives

A rail squat is a crack growing below the running surface of a rail, leading to a depression in the rail surface, caused by plastic flow above the crack. Squats are a major problem on track in a number of countries, such as France and Australia, and have become more common in recent years. Severe grinding or more commonly replacement of the rail affected is needed to deal with the problem, incurring significant maintenance costs. In Australia, the Co-operative Research Centre for Railway Engineering has funded a project to investigate the factors influencing the formation and growth of rail squats. The problem is being approached differently at three institutions. Metallurgical investigations of squat cracks at the University of Queensland indicate the presence of a thin brittle white etching layer (WEL) on the rail surface, resulting from severe local transient heating of the rail due to wheel slip. This is only about 30 microns thick, but does help initiate cracks. At Central Queensland University vehicle dynamics studies have being conducted to identify failure indices indicative of the formation of rail squats, in situations such as in a transition into a curve. As well, experiments are underway to detect the white etching layer on track using eddy current measurements. Crack initiation can only occur if wear does not remove flaws rapidly enough, so a failure criterion must account for this trade-off. Crack growth of squat cracks is being studied at Monash University and at the University of Queensland. Progress includes the first accurate measurement of the growth of cracks shorter than 1mm in rail steel, enabling identification of a crack growth law for short cracks in rail steel. This law is appropriate in the fatigue regime applying during early growth of a squat crack, a situation where Paris law cannot be used. The effect on crack growth of entrapped fluid in short cracks in rail steel is also being studied experimentally, and is found to be quite significant even with use of a SENT test specimen. Growth of squats on track is being monitored by ultrasonic measurements in several locations in Sydney by Railcorp