A B S T R A C T Surface crack growth of silicone nitride ceramic bearings under rolling contact fatigue has been investigated from the viewpoints of contact stresses (ring crack model) and fluid pressure (wedge effect model). The mechanisms of these two models have been investigated independently; however, it was impossible to separate the effects of contact stresses and fluid pressure on surface crack growth. In this paper the effects of contact stresses (ring crack model) on surface crack growth are investigated. In the ring crack model the crack growth is caused by contact stresses around the circumference of the contact circle. The growth of surface cracks located inside and outside the contact track was observed in order to obtain data from which we could reexamine the ring crack model. The outside cracks under rolling contact fatigue were propagated by contact stresses alone and also the inside cracks grew as slowly as the outside cracks. We concluded that the cracks are propagated by the single effect of contact stresses. Preliminary observations of surface crack growth showed that the cracks were unaffected by wear and residual stresses. III = normalized stress intensity factors in mode I, mode II and mode III K I 1 , K II 1 and K III 1 = stress intensity factors caused by tensile stress on unit area when calculating stress intensity factors K I 2 , K II 2 and K III 2 = stress intensity factors caused by shear stress (τ xy ) on unit area when calculating stress intensity factors K I 3 , K II 3 and K III 3 = stress intensity factors caused by shear stress (τ yz ) on unit area when calculating stress intensity factors K Ic = fracture toughness K * II = normalized stress intensity factor range in mode II K * III = normalized stress intensity factor range in mode III O = centre of contact circle O = centre of initial crack O = position of O in Y axis R a = surface roughness Correspondence: K. Kida.
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