Several stainless steel bolts used on a Titan Space Launch Vehicle broke at the shank and failure was attributed to stress-corrosion cracking. But results could not be duplicated in the laboratory with salt-solution immersion tests until the real culprit was established: the secondary effect of galvanic coupling, hydrogen embrittlement.

The purpose of this investigation was to determine the root cause of the differences noted in the fatigue test data of main rotor spindle assembly retaining rods fabricated from three different vendors, as part of a Second Source evaluation process. ARL performed dimensional verification, accessed overall workmanship, and measured the respective surface roughness of the rods in an effort to identify any discrepancies. Next, mechanical testing was performed, followed by optical and electron microscopy, and chemical analysis. Finally, ARL performed laboratory heat treatments at the required aging temperature and follow-up mechanical testing.

The failures of two aircraft components, one from a landing gear and the other from an ejector rack mechanism, were investigated. Both were made from PH 13-8 Mo (UNS S13800) precipitation-hardening stainless steel which had been heat treated to the H1000 and H950 tempers respectively and then chromium plated. The parts were characterized metallographically and mechanically and were found to be compliant. Detailed fractographic examination revealed that the first stage of both failures was similar: subsurface initiation of numerous cracks with a wide range of orientations and cleavage like features. The cracking was followed by fatigue in one case and catastrophic failure in the other. Hydrogen embrittlement was identified as the most likely mechanism of failure.