A Possible Mechanism for Rail Dark Spot Defects

Rail dark spot defect, also termed squat failure or shelling, which is a kind of rolling contact fatigue failure and occurs frequently on running surfaces of railway rails carrying high speed traffic, is one of the most dangerous rail failures. The dark spot crack is characterized by a principal crack propagating in the direction of traffic and a second crack growing in the direction opposite to traffic. By using a newly developed two-disk machine, the authors have succeeded in reproducing very similar dark spot cracks to those which appear in actual rails. It is found that the dark spot defects are caused by frequent repetitions of dry and wet runnings, and that the traction force plays an important role for the occurrence of the cracks. The principal crack may occur from a tiny pit formed a posteriori on the contacting surface and after that, the second crack is formed by cracks branched from the extended principal crack. It has also been proved experimentally that water is capable of entering the tip of the crack. Furthermore, a possible mechanism for the dark spot cracking has been proposed on the basis of the fracture mechanics approach.