Failure analysis of an accessory bevel gear installed on a J69 turbojet engine

Abstract The fracture of a drive bevel gear (base material: AISI 8617) used for an accessory gear box of a turbojet engine was investigated. From the visual examination of the fracture surface of the failed bevel gear it was shown that the cracking mode was fatigue. Careful observation of the fatigue crack origin using SEM (scanning electron microscopy) indicated that there were no surface defects. The fatigue crack initiated at the upper root area of the gear teeth and propagated intergranularly. The cracking route was identical with that of typical tooth bending fatigue (except) for deviations attributed to improper surface heat treatment. Microstructural analysis revealed that the fractured gear was not properly case hardened and this lowered the surface hardness dramatically. The remedial action is to pinpoint the exact process parameters that resulted in the wrong case hardening.