A study of crack closure using the optical method of caustics and consequences for the use of ΔKEFF as a fatigue crack driving force

Using the optical method of caustics, measurements of the crack-tip stress field have been used to experimentally determine stress intensity factors in 2024-T3 centre-cracked panel aluminium alloy specimens during fatigue loading. The aim of the work was to directly, remotely and non-destructively measure crack-tip shielding as a result of closure, and so determine Delta K-eff, under conditions of both constant and variable amplitude loading. Results show that this novel technique clearly detects crack closure under steady-state constant amplitude loading. However, although transient changes induced by, say, tensile overloads modify the extent of closure and so have a dramatic effect on crack growth rate, they do not change the crack-tip stress field in the K-dominant zone. Consequently, it is concluded that the successful application of Delta K-eff as the fatigue crack driving force can only be justified on phenomenological rather than physical grounds. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.