Measurement of CTOD along a surface crack by means of digital image correlation

Abstract

The experimental measurement of fracture toughness and crack driving force are the main steps for the application of fracture mechanics methods in structural integrity assessment. Crack Tip Opening Displacement (CTOD), defined as the opening of a defect at its crack front upon loading a test specimen or structure, is one of the most important crack driving force parameters. However, due to its subsurface nature direct measurement of CTOD is very cumbersome. CTOD is commonly measured using single or double clip gauge techniques and applying a plastic hinge model. However, clip gauge CTOD calculations merely provide information related to the center of the defect. For the case of a finite-length surface breaking defect, where CTOD is variable along the defect front, exact knowledge of CTOD over the entire front is not possible by the clip gauge measurement techniques. In the present paper a novel technique is proposed based on full field three-dimensional profile measurement of the notched surface by means of stereoscopic Digital Image Correlation (3D-DIC) to measure the CTOD over the entire crack front. The proposed method is verified by measurement of silicone replicas cast inside the notch in addition to a supporting Finite Element (FE) investigation.