Corrosion Fatigue in 7075-T6 Aluminum: Life Prediction Issues for Carrier-Based Operations

Fatigue experiments performed on 7075-T6 AI alloy at various frequencies and stress levels and under alternating wet and dry environments indicate that simultaneous action of corrosion and fatigue substantially accelerates crack initiation and growth rates when compared to pure fatigue (dry air) conditions. In experiments performed under alternating wet and dry conditions, fatigue crack growth rate was observed to increase rapidly in the presence of mildly corrosive (salt solution) and decrease sharply when subjected to noncorrosive dry air. Crack arrest of various durations was observed at transition points between dry and wet cycles. Lowering the frequency of fatigue loading significantly reduced crack initiation and overall life time. S-N curves showed a continuous downward trend without reaching a plateau or threshold. These observations led to the conclusion that aircraft structural integrity can be seriously compromised even under mildly aggressive environments and at subcritical stress levels when cracks are present regardless of changing of environments and/or test frequency.