A STUDY OF CRACK CLOSURE USING THE OPTICAL METHOD OF CAUSTICS AND CONSEQUENCES FOR THE USE OF DELTA-K-EFF AS A FATIGUE-CRACK DRIVING-FORCE

Using the optical method of caustics, measurements of the crack-tip st ress field have been used to experimentally determine stress intensity factors in 2024-T3 centre-cracked panel aluminium alloy specimens dur ing 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 v ariable amplitude loading. Results show that this novel technique clea rly detects crack closure under steady-state constant amplitude loadin g. However, although transient changes induced by, say, tensile overlo ads modify the extent of closure and so have a dramatic effect on crac k growth rate, they do not change the crack-tip stress field in the K- dominant zone. Consequently, it is concluded that the successful appli cation of Delta K-eff as the fatigue crack driving force can only be j ustified on phenomenological rather than physical grounds. (C) 1998 Pu blished by Elsevier Science Ltd. All rights reserved.