Surface Crack Growth in Si3N4 under Cyclic Shear Load, and under Rolling Contact Fatigue (Mode II Surface Crack Growth of Si3N4)

Abstract

Surface crack growth behavior from pre-indentations of HIP-Si3N4 was observed under Rolling contact fatigue (RCF), and also under cyclic shear load. 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 the cracks grew in Mode II by shear stress (ΔKII=3.0MPa·m1/2, KIImax= 1.5MPa·m1/2 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 when the ΔKII=2.4MPa·m1/2 was lower than that of RCF. We found the following three features concerning Mode II crack growth. (1) The cracks grew by cyclic shear stress in Mode II (ΔKII=2.4MPa·m1/2). (2) Under the same ΔKII=2.4MPa·m1/2, the cracks grow more when KII max=2.4 MPa·m1/2 (R=0) than when KIImax=1.2 (R=-1), (3) The cracks tend to stop growing when ΔKII is lower than 1.2 MPa·m1/2 (R=0). Comparing the features of Mode II crack growth obtained from the torsion fatigue tests and the RCF cracks, we conclude that the surface cracks are propagated in mode II (ΔKII=3.0 MPa·m1/2) under rolling contact fatigue.