Ultra-low-Cycle Fatigue Failure of Aluminum

Abstract

Ultra-low-cycle fatigue (ULCF) life of ductile metal is closely correlated with monotonic tension coupon test results, since the dominant failure mode is the same. In this chapter, a novel approach to evaluate crack initiation of aluminum alloy under ULCF loading only using monotonic tension coupon test results is proposed. ULCF tests on 15 specimens made of aluminum alloy 6061-T6 are conducted, and numerical analyses using the previously proposed cyclic ductile fracture model indicate that the ULCF life of aluminum can be greatly underestimated. 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 tension coupon test results and simple numerical analysis is presented.