Keywords: rail surface defect, squats and studs, ultrasonic mapping, field monitoring, asset management, field experimental technique, multi-stage growth.

Rail surface defects (such as rail squats, corrugation, rolling contact fatigue damages, wear, etc.) are one of the top priorities in infrastructure maintenance management. Typically rail squats are classified as the growth of any cracks that have grown longitudinally through the subsurface. In addition, some of the cracks can branch off from initial longitudinal cracks with a depression of rail surface, and then transversely propagate to the bottom of rails. The rail surface defects are commonly referred to as 'squats' when they were initiated from damage layer 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. For over 60 years, these rail surface defects have been often observed in railway tracks operated for either light passenger or heavy freight traffics and with a variety of train speeds: low, medium or high speed trains all over the world. The exception exists at some locations such as sharp curves where large wear takes place under severe friction between wheel flange and rail gauge face. The rail surface defects become a much-more significant issue when the crack grows and sometimes flakes off the rail, resulting in severe rail surface irregularities. It is often noticed that the rail surface defects induce wheel/rail impacts and large amplitude vibrations of track structure, resulting in poor ride quality. The rail squats/studs have occasionally turned into broken rails in many countries such as Australia, Germany, France, Great Britain and Japan. Many research investigations from the fracture mechanical and material scientific points of view are being carried out in order to evaluate the root cause, early warning signs and preventive solutions. Some patterns of squat/stud development related to both of curve and tangent track geometries have been observed, and squat growth has also been monitored for individual squats using ultrasonic plotting techniques. This paper investigates squat/stud distribution and its growth in the fields. The squat/stud growth has been examined and monitored using the ultrasonic measurement device on a reference grid applied to the rail surface. Three dimensional contours of rail squat cracks can then be formulated by the measurement records. It has been found that crack propagation is non-linear to repeated train loads when the crack is getting larger. In this study, the crack propagation of medium-scale squats/studs can be observed to be almost linear to repeated train loads called as accumulated passing tonnages up to a certain degree of propagated crack length.

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