Caustic Embrittlement Cracking on a Helicopter Tail Rotor Output Shaft

A tail rotor mast, from a helicopter tail rotor gearbox assembly, was found to have multiple circumferential cracks along a machined outer diameter during an inspection. Intergranular features and a discoloration to a shallow depth were observed on each of the cracks that were opened for examination. The features were a result of caustic embrittlement that formed during manufacturing from residual stresses induced during machining and a subsequent black oxide treatment.  The study includes testing that was conducted to determine the amount of localized residual stresses necessary to produce similar shallow depth cracks and intergranular features.