From fractographic observations of specimens that have failed due to rolling contact fatigue, it has been concluded that the first stage of damage is the formation of mode 11 fatigue cracks parallel to the contact surface due to the cyclic shear stress component of the contact stress. Although these initial subsurface cracks, in both metals and ceramics, are produced in a direction parallel to the cyclic shear stress, cracks eventually grow in a direction close to the plane of the maximum tensile stress if we apply a simple mode I1 loading to them. The difference between crack growth in simple mode I1 loading and crack growth due to rolling contact fatigue is, we suppose, whether or not there is a superimposed compressive stress. Based on this hypothesis, we developed an apparatus to obtain the intrinsic characteristics of mode I1 fatigue crack growth, and developed a simplified model of subsurface crack growth due to rolling contact fatigue.Some results in terms of da/dN versus AKII relations have been obtained using this apparatus on specimens of steel and aluminum alloys. Fractographs of the mode I1 fatigue fracture surfaces of the various materials are also provided.

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