Failure investigation of hydrogen induced cracking of stabilator skin in jet aircraft

In this study, a 1 m long crack found in the stabilator skin of a jet aircraft has been investigated. SEM observation revealed that the crack initiated from the free edge of the inboard side where fatigue striations and ratchet marks were observed, then propagated transversely through the skin in the manner of a brittle crack. Predominantly intergranular fracture with quasi-cleavage morphologies was observed on the entire fractured surface. Fractography with hydrogen measurement as well as hardness testing indicated that hydrogen induced cracking (HIC) was the dominant cracking mechanism. Visual observation identified undesirable loads induced by several improper maintenance activities. Finite element analysis was used to correlate the experimental observations with these identified abnormal load cases. The location of the stress concentration determined by finite element analysis showed good correlation with the actual cracking location. It was concluded that an undesired assembly force induced a high residual stress, and operational aerodynamic loads combined with these maintenance loads contributed to exceeding the HIC threshold stress in the degraded metal due to hydrogen embrittlement.