Fatigue crack growth from environmentally induced damage in 7075 alloy

Ageing aircraft fleets commonly experience environmentally induced degradation. This requires an accurate assessment of fatigue performance including any influence of corrosion damage. Since the majority of the life of a fatigue crack is spent in propagation of small cracks, the predictive capabilities of crack growth laws must ideally capture small and intermediate crack behaviour as well as long crack growth for fatigue life assessment. Here, studies were conducted on corroded coupons including “dome nut hole” coupons with intergranular corrosion (IGC) representative of a critical location in the RAAF AP-3C Orion aircraft. The examination of cracks grown from corrosion with quantitative fractography revealed that fatigue cracks can nucleate from corrosion sites and are strongly affected by the morphologies and geometries of the corrosion damages. Further, the locations of the crack origins and the properties of the local microstructure can have a significant impact on the initial crack propagation. The presence of an intergranular corrosion (IGC) region at the dome nut hole can influence the fatigue crack paths such that multiple cracks originating from corrosion sites can be segregated by the IGC. Beyond the depth of the IGC, coexisting cracks coalesce and propagate as a single main crack. The experimental crack growth measurements and predictive techniques investigated herein confirms that a Hartman-Schijve variant of the NASGRO equation can accurately capture small, intermediate and long crack growth, and therefore allow for the total fatigue life to be assessed.