Rail squats: understand its causes, severity, and non-destructive evaluation techniques

Generally, rail squats are defined by the growth of any cracks that have grown longitudinally through the rail subsurface and some of the cracks propagating to the bottom of rails transversely have branched from initial longitudinal cracks with a depression of rail surface. The rail defects are commonly referred to as ‘squats’ when they were initiated from damage caused by rolling contact fatigue, and as ‘studs’ when they were associated with white etching layer caused by the transform from pearlitic steel due to friction heat generated by wheel sliding or excessive traction. Such rail surface defect induces wheel/rail impact and large amplitude vibration of track structure and poor ride quality. In Australia, Europe and Japan, rail squats/studs have occasionally turned into broken rails. The root cause and preventive solution to this defect are still under investigation from the fracture mechanical and material scientific point of view. This paper highlights the root causes, dynamic wheel/rail interaction and its application, severity and consequences, and field monitoring of squat/stud distribution and its growth. A non-destructive testing technique to detect and evaluate crack forms in a three dimensional contour is also demonstrated using ultrasonic measurement method.