A fractographic study of fatigue failures in combat aircraft trailing edge flap hinge lug bores in both test and service assets

This paper presents an overview and comparison of two fractography investigations of cracking in combat aircraft trailing edge flap hinge lug bores. One, a retired service asset and the second, a full-scale durability test article in a fleet repaired configuration. Since these hinge lugs are a primary structure with the potential for fatigue failure to lead to flap separation from the wing in-flight, a detailed understanding of the root cause, crack morphology and growth were required to manage these structures in-service.

The root cause of fatigue crack nucleation and growth in the service assets was found to be corrosion whilst in the full-scale durability test it was mechanical damage or fretting associated with the application of the fleet representative bushing repair scheme. The implications of each root cause were examined along with the crack morphologies, and proposals for in-service inspection and the prevention or the delay of damage formation are discussed.