Fracture Toughness and SCC Behavior of 7075 Aluminum Alloy under Mixed-Mode I-II Loading

摘要

Under mode I, II and mixed-mode I-II loading, the fracture toughness and SCC response of aluminum alloy 7075-T6 plate have been studied in the S-L and L-S orientations. The fracture toughness in S-L under mixed-mode loading coincides with K_(1c) for K_2/K_1 <1 and K_(2c) for K_2/K_1 >1, where K_(1c) and K_(2c) are the fracture toughness under mode I and mode II respectively, and K_2/K_1 is the stress intensity ratio. The former fracture occurs due to a preferential intergranular delamination controlled by a normal stress component on grain boundaries, while the latter fracture proceeds in a trangranular shear mode. The SCC growth rate under the mixed-mode loading decreases with an increase of K_2 component, and under the pure mode II loading SCC growth becomes more difficult. Based on the experimental results and discussions of stress-induced hydrogen accumulation at crack tip the intergranular SCC of 7075-T6 can assumed principally controlled by hydrogen embrittlement, and an anodic dissolution reaction involved in the mode II SCC process plays an assisting role in lamellar crack joining in transgranular fracture.