Analysis of Sub-Critical Cracking in a Ti-5Al-2.5Sn Liquid Hydrogen Control Valve

A liquid hydrogen main fuel control valve for a rocket engine failed by fracture of the Ti-5Al-2.5Sn body during the last of a series of static engine test firings. Fractographic, metallurgical, and stress analyses determined that a combination of fatigue and unexpected aqueous stress-corrosion cracking initiated and propagated the crack which caused failure. The failure analysis approach and its results are described to illustrate how fractography and fracture mechanics, together with a knowledge of the crack initiation and propagation mechanisms of the valve material under various stress states and environments, helped investigators to trace the cause of failure.