Two unique types of surface defects resulting from contact fatigue have been observed in rail used in high-speed lines in Japan (speed greater than 150 km/h). One of these results from the dark spots which randomly appear on the running surface and develop into transverse defects, and the other is the head check which arises at the gage corner.

An experimental simulation of these defects was attempted through rolling contact fatigue tests. The resulting defects were similar to those found on the running surface. For their initiation, a slip component of load was necessary, in addition to the normal load. Examinations of these defects indicated that they initiated on the contact surface, as did those found in rail, and not at the subsurface where the shear stresses are maximum due to Hertzian contact.

To investigate the effects of microstructure on the initiation and propagation behavior of these defects, coarse pearlitic, fine pearlitic, and tempered martensitic structures were examined. The fatigue limit of each structure showed a good correlation with its tensile strength. In the rolling contact fatigue tests, the fine pearlitic structure exhibited the longest lifetime. However, the tempered martensite had the lowest crack growth rate. Therefore, it is concluded that, to optimize the durability of rail steels, the effect of microstructure should be considered in addition to the tensile properties.