Failure of mounting bolt of helicopter main gearbox support strut

The mounting bolt of helicopter main gearbox support strut was fractured into two pieces during the flight mission. The fracture occurred at the root of the first engaged thread of the bolt-nut assembly. Visual inspection of the fracture zone revealed cracks, formed by interlinking of corrosion pits, at the almost all thread roots of the bolt. Energy dispersive spectroscopy disclosed significant damage of the cadmium plating as well as the presence of large amounts of corrosion products at the bolt threads. Through the fractography and metallography analysis, it was found that the mounting bolt failed due to hydrogen-induced intergranular stress corrosion cracking (HI-IGSCC). The finite element (FE) analysis confirmed that the crack origin was located at the area with the maximum tensile stress in the bolt.