Breaking Load Method Evaluation of Sheet AA7075

Increasing fuel economy is a high priority of the automotive industry due to consumer demand and government regulations. High strength aluminum alloys such as AA7075-T6 can be used in strength-critical automotive applications to reduce vehicle weight and thus improve fuel economy. However, these aluminum alloys are known to be susceptible to stress corrosion cracking (SCC) for thick plate. The level of susceptibility to SCC must be determined before a material is implemented. ASTM standards exist that generate semi-quantitative data primarily for use in screening materials for SCC. For the purposes of this work ASTM G139 (breaking load method) has been used to evaluate sheet AA7075-T6 for use in automotive applications. A tensile fixture applying a constant strain was used to quantitatively measure residual strength of the material after exposure to a corrosive environment. The material studied showed negligible loss of strength when loaded to 50% of the yield strength after 40 days of exposure to a corrosive environment and a statistically significant loss of strength after 30 days in the corrosive environment when loaded to 95% of the yield strength. This approach appears to be successful for screening high strength aluminum alloy sheet, which would allow automakers to evaluate different material compositions and tempers in terms of their stress corrosion cracking resistance.