Fatigue cracking from a corrosion pit in an aircraft bulkhead

For aircraft, the repair and management of corrosion can be a major through life cost driver as well as an availability degrader. A common form of corrosion in aircraft structural alloys is pitting corrosion. This paper describes fatigue cracking in a laboratory environment that had initiated from a significant corrosion pit in a locally shot-peened aluminium alloy 7050-T7451 bulkhead of an F/A-18 Hornet aircraft. Extensive study of this material under Hornet type loading has shown that if the initial equivalent pre-crack size is determined, then the fatigue crack growth follows the lead crack form, as was the case for this corrosion-pit initiated crack in the absence of retardation affects. This is a valuable observation which should allow the growth of cracks from corrosion pits to be determined through fatigue crack modelling without the need to account for the potential of corrosion-assisted fatigue effects, which are postulated here to be insignificant for combat-type aircraft. The main outcome of the investigation was that whilst the physical size of the corrosion pit was large (442 μm deep) its effect as a fatigue crack initiator i.e. its effective pre-crack size (EPS), was significantly smaller (i.e. approximately 10 μm deep).