LAUSR

Abstract This paper describes the failure investigation of in-flight failures of bleed air ducts in jet fighters. Severe rupture had occurred at the bleed air duct during the flight that had caused the emergency landing situation. The tubular duct was manufactured using commercially pure titanium. An inspection revealed that all cracks occurred in the central portion of the bleed air duct. Examination of the fractured surfaces by using electron microscopy revealed that cracks initiated at multiple corrosion pits in the inner surface of the duct and these propagated by brittle cleavage cracking with occasional areas of fatigue striations induced by in-service cyclic pressure. Fractography, chemical analysis, and metallographic analysis confirmed that brittle fracture involved the ingress of hydrogen into a duct surface. The hydrogen effectively decreased the ductility of the duct which contributed to brittle fracture. The final destructive rupture of the duct can be explained by fracture mechanics. The critical crack size and stress induced by conditions at or near the operating load were estimated based on computational fracture analysis to explore the fracture mechanism. As a consequence, a novel nondestructive inspection method has been developed to prevent such failures in future.