Mode II surface crack growth under rolling contact fatigue and cyclic shear stress in Si3N4

The effects of friction on surface crack growth behavior from pre-indentations of HIP-Si3N4 were investigated under rolling contact fatigue (RCF). Near surface contact stress distributions were calculated by Hanson’s analytical method, and the crack growth behavior was discussed in terms of stress intensity factors under rolling contact fatigue. It was found that friction promoted surface crack growth and the cracks grew in mode by shear stress (ΔK ؒ 3.6 MPam1/2, Kmax ؒ 1.8 MPam1/2, stress ratio R =-1). In order to confirm the effects of shear stress on surface crack growth, crack propagation tests under cyclic shear stress were carried out. We found the features concerning Modecrack growth: 1) The cracks grew by cyclic shear stress in Mode (initial value of ΔK is 3.0 MPam1/2); 2) Under the same ΔK = 3.0 MPam1/2, the cracks grow more when Kmax = 3.0 MPam1/2 (R = 0) than when Kmax = 1.5 (R =-1). Comparing the features of modecrack growth obtained from the crack propagation tests under cyclic shear stress and the RCF cracks, we conclude that the increase in values of stress intensity factors of mode II explains that the cracks under frictional contact are promoted by friction