The effect of exfoliation corrosion on the fatigue and fracture behaviour of thin aluminium alloys
The present study is concerned with the interaction of stress and corrosion in high strength aluminium a11oys in sheet form used in the aircraft industry. Many of these alloys are susceptible to exfoliation corrosion under service conditions. This type of corrosion was reproduced in the susceptible a11oys using the Exco test method. The fracture toughness of an A1 - Zn - Mg - Cu a11oy (7178 - T6) was assessed using R - curves. The interaction between exfoliation corrosion and the thinning effect of exfoliation on the fracture toughness are discussed. Fatigue crack growth behaviour in two naturally aged A1 - Cu ~ Mg alloys, 2024 - T351 and 3L7O (2014 - T4) was evaluated under conditions of low and high humidity using constant amplitude loading. A similar assessment of the effect of humidity on the propagation rates in the higher strength 7178 - T6 was carried out. Additionally, the effect of the presence of exfoliation corrosion on crack growth in 2024 - T351 and 7178 -- T6 was examined. Both humidity and exfoliation corrosion enhanced fatigue crack growth rates in the alloys, though to differing extents, with 7178 - T6 being affected more. The fatigue resistance of 7178 - T6 diminished even further in the presence of humidity and corrosia together. Finally, a mechanism to explain the way by which exfoliation corrosion affects the fatigue and fracture behaviour of thin aluminium sheets is proposed.