Squat formation and propagation: metallurgical and mechanical points of view

A squat is a rolling contact fatigue crack below the surface of a rail that leads to a depression on the running surface. Squats were thought to initiate by ratcheting, but a thermal origin is now considered likely in many cases. Rails with squat defects in different stages were removed from the site investigated and inspected visually, ultrasonically, optically and by using electron microscopy. Microscopic observations revealed primary, secondary and tertiary cracks in the longitudinal section of rails, and differences in crack surface appearance. Secondary and tertiary cracks initiated from the primary crack at a definite angle, indicative of the shape of a squat. The primary cracks initiate at an angle of 10-15º to the rail surface and turn to 30º from the original direction. The secondary cracks initiated at an angle of 150º from the original direction of the primary crack and further bifurcated in tertiary cracks. A finite element model is created to check how the stress intensity factors (SIFs) vary around a 3D semi-elliptical squat. Traction ratio is considered as well as lateral load and it is shown the superposition of a lateral load ratio of 0.5 and traction ratio of 0.2 can increase the equivalent stress intensity factor by up to 200%. This may explain why squats have a greater chance of appearing on transverse curved rails. This study shows water entrapped between the crack faces can be pressurized and change crack propagation direction.

Media Info

  • Pagination: 10p. ; PDF
  • Monograph Title: Rail - the core of integrated transport: CORE 2012: conference on railway engineering, 7-10 September 2012, Perth, Western Australia

Subject/Index Terms

Filing Info

  • Accession Number: 01532137
  • Record Type: Publication
  • Source Agency: ARRB
  • Files: ATRI
  • Created Date: Jul 29 2014 11:57AM