The stress intensity of mixed mode cracks determined by digital image correlation

A generalized approach for determining the stress intensity factors (SIFs) K-I and K-II for any mode mixity directly from displacement fields obtained by digital image correlation is presented using a centre fatigue cracked aluminium plate as an example problem. It was found that the crack-tip position could be determined on average to within 50 per cent of the displacement vector spacing (60 mu m). The approach has been shown to be fairly robust, both in terms of the stability of the SIFs thus obtained and their sensitivity (less than 0.07 MPa root m) to the above uncertainty in the crack-tip location. Differences between the nominal and experimentally determined K-I and K-II values were found to be comparable (around 1.0 MPa root m) for values of K-I ranging from 0 to 15 MPa root m and K-II from 15 to 0 MPa root m, and may have arisen because the nominal values are not representative of those acting at the real crack tip.