Failure Analysis of a Main Rotor Pitch Horn Bolt Located on the AH-1 CobraHelicopter

摘要

A comprehensive metallurgical examination of the pitch horn bolt was conducted todetermine the probable cause of failure. The component is part of the main rotor hub assembly and had failed while in service. Light optical microscopy revealed evidence of corrosion pitting in regions adjacent to the fracture. Chemical analysis verified that the part was fabricated from 4340 steel. It was determined by metallographic examination that the microstructure was tempered martensite. Hardness measurements taken on transverse cross sections of the bolt near the fracture indicated that the material had been hardened to the upper limit of the specified range. The surface finish was measured along the upper shank and conformed to the requirements of the engineering drawing. Fractographic examination utilizing the scanning electron microscope (SEM) revealed multiple crack origins which assumed a decohesion. Many of these crack sites were initiated from corrosion pits. Energy dispersing spectroscopy (EDS) performed on areas within the crack initiation site showed the presence of chlorides. Beyond the thumbnail zone fast fracture occurred in a ductile manner, which was confirmed by a dimpled topography. The failure was attributed to stress corrosion cracking (SCC).