Metallurgical and physical understanding of rail squat initiation and propagation

This paper describes the initiation and propagation behaviour of squat-type cracks in rail steel using mechanical and metallurgical analysis simultaneously. Rails with squat defects were removed from the site and inspected visually, ultrasonically, optically and by using electron microscopy. The contact stresses on a rail from a wheel are critically evaluated and rolling contact fatigue crack direction, initiation and propagation have been evaluated using stress theory and validated with metallurgical observations. The hardness of the rail surface was much higher than that of the adjacent rail matrix and this was comparable with that of a white etching layer. Squats were observed near the white etching layer. The squats formed an angle of 20–30° with the horizontal surface. Ultrasonic testing predicted the actual shape of the squat under the surface. Scanning electron microscopic observation revealed differences in the rolling contact behaviour between the direction of train travel and the opposite direction. Based on metallurgical research, a mechanism for squat initiation and propagation has been proposed in this paper.