Fatigue Behavior and Fracture Mechanism of a Hot Rolled AA7020 Aluminum Alloy

Abstract

Fatigue tests of smooth specimens and CT specimens of a hot rolled AA7020 Aluminum alloy have been performed in
laboratory at ambient temperature. Fatigue strength and fatigue crack propagation (FCP) characteristic were
evaluated and fracture mechanism was discussed on the basis of crack initiation, small crack growth, and fracture
surface analysis. The growth behavior of small and large cracks has been investigated on 7020 alloy.The FCP
resistance was found significantly lower than that of other aluminum alloys. The crack growth measurements were
performed in CT specimens at constant load ratios ranging from R = 0.1–0.5. The fatigue strength at 104 cycles was
110 MPa that led to a considerably fatigue ratio of 0.1, fatigue failure did not occur. There existed two different
modes of crack initiation depending on applied stress level. Above 200 MPa, cracks initiated at the specimen surface
in transgranular or intergranular manner due to cyclic slip deformation, while below that stress subsurface crack
initiation took place. The growth of small cracks initiated at the surface coincided with the FCP characteristic after
allowing for crack closure for large cracks, but the operative fracture mechanisms were different between small and
large cracks.