Stress corrosion cracking of aluminium alloy 7075(w) under tensile and compressive loading

The effect of compressive loading on stress corrosion cracking of aluminium alloy 7075(W) was studied using specimens containing simulated fastener holes and the results were compared with the behaviour of double cantilever beam specimens with tensile loading. Crack lengths were measured with an eddy current bore probe and confirmed by optical metallography. Intergranular stress corrosion was observed in both tensile and compressive loading. Cracks initiated readily in 3·5%NaCl solution with tensile loading and the alloy was much less susceptible in compression, with cracking being detected only after 14 day exposure to EXCO solution. No cracking was observed in unstressed specimens, even after 56 day continuous exposure to EXCO solution. In both cases, SCC was explained by an active path mechanism due to the galvanic interaction between grain boundary precipitates and adjacent precipitate free zones. Compressive loading produced a positive strain at the crack tip, which ruptured the protective corrosion product film, maintaining the path of preferential corrosion.