Surface and subsurface rolling contact fatigue characteristic depths and proposal of stress indexes

The rolling contact fatigue is distinguished into subsurface initiated (spalling and case crushing) and surface initiated (pitting and micropitting). A characteristic depth was identified for each of these mechanism. The characteristic depth of the case crushing is the hardening depth, while for the spalling it is the maximum cyclic shear stress depth. The pitting depth is the size of the crack for which the mode I stress intensity factor range, due to the fluid pressurization, is higher than the threshold. This depth can be similar to the micropitting depth, in the order of 10 µm, for heavily loaded small radius contacts. Rolling contact fatigue cyclic shear stress indexes are then defined on the basis of the characteristic depths, and they identify the load intensity of each rolling contact fatigue mechanism. The characteristic depths and the stress index approach can be used to relate specific tests to component design, without any size effect misinterpretation.