AN INVESTIGATION OF CRACK CLOSURE AND THE PROPAGATION OF SEMIELLIPTICFATIGUE CRACKS IN Q1N (HY80) PRESSURE-VESSEL STEEL

The results of an experimental investigation of the effect of crack cl osure on the propagation of semielliptical fatigue cracks are presente d. Load-shedding fatigue threshold tests were carried out at stress ra tios of 0.2, 0.35, 0.5 and 0.7. Crack closure was measured at the surf ace and depth positions using backface strain gauges, near-tip gauges, and a clip gauge. Differences between the surface and depth growth be haviour are explained by considerations of the effects of the transiti on from plane stress conditions at the surface to plane strain conditi ons at the depth. The effects of stress ratios are attributed largely to differences in the crack opening displacement, which result in aspe rities coming into contact to induce roughness-induced crack closure.