Introducing the compliance ratio concept for determining effective stress intensity

Abstract

Local crack tip strain measurements show that the conventional determination of ΔK effective may be misleading, especially when opening loads are relatively high as is often the case in the near-threshold regime. The conventional method assumes that K-opening can be directly related to a deviation in the linearity of a load vs crack mouth displacement curve (`compliance offset method'). Although this deviation in linearity can be attributed to closure, local strain measurements show that significant crack tip strain can occur below the K-opening load. The redistribution of stresses associated with a partially open crack and the effect of this redistribution on the stress field in front of the crack cannot be determined simply on the basis of a change in the slope of the load–displacement curve. However, a recent addition to ASTM E-647 (Recommended Practice for Determination of Fatigue Crack Opening Load from Compliance) implies that it can. An alternative `compliance ratio' technique based on local crack tip strain is presented. A modification of this technique allowing the use of remote compliance measurements to account for crack tip strain below the opening load is suggested.