Ultra-low cycle fatigue life of aluminum alloy and its prediction using monotonic tension test results

Ultra-low cycle fatigue (ULCF) life of ductile metal is closely correlated with monotonic tension coupon test results. This paper aims to propose a novel approach to evaluate crack initiation of aluminum alloy under ULCF loading only with monotonic tension coupon test results. ULCF tests on 15 specimens made of aluminum alloy 6061-T6 were conducted, and numerical analyses using a previously proposed cyclic void growth model indicated that the ULCF life of aluminum can be greatly underestimated by the model. A new fracture model based on the concept of different dislocation structures is thus proposed, which classifies damage into kinematic hardening correlated and isotropic hardening correlated. A material constant is employed to consider the relatively low damage induced by the kinematic hardening compared with the isotropic hardening one. The newly proposed fracture model can well simulate the instants of crack initiation for the specimens. A process to evaluate the ULCF life of aluminum alloy based on both monotonic coupon test results and simple numerical analysis is presented.