STRESS STATE‐RELATED FATIGUE CRACK GROWTH UNDER SPECTRUM LOADING

Abstract

Abstract— The fatigue crack growth behaviour in aluminium alloy sheets of 2024‐T3 and 7475‐T761, subjected to standardized spectra (TWIST and FALSTAFF), was investigated using centre‐cracked specimens. A strip crack closure model was used to interpret experimental data, and to make predictions for the crack growth.The strip model is based on the Dugdale concept, but modified to keep plastically‐stretched materials on the crack surface so that the crack opening load can be determined, and the fatigue crack growth can be analysed according to Elber's crack growth assumption. Differing from other models of the same kind, a variable constraint factor was introduced to account for the gradual transition of stress state at the crack tip resulting from the crack growth. It has been shown that the transition of stress state at the crack tip causes the unusual behaviour of the fatigue crack growth in sheets. Both experiments and predictions show that a crack may grow faster at a low load than at a higher one in a certain applied load range due to the crack tip stress state transition. The crack tip stress state also contributes to the thickness effect observed for the crack growth in sheets. In agreement with experimental results, it has been shown that a plane stress state will prevail at the crack tip in a thin sheet compared to that in a thick sheet. The plane stress state results in a higher crack opening level which leads to a longer fatigue life for thin sheets.