Failure analysis of hook end self locking nuts of an aero-engine

Repeated premature failures of hook end self locking nuts, a developmental aero-engine component, were encountered after ground test runs. The part is a component of turbine module of the engine and experiences severe temperature-loading condition during engine tests. It was manufactured using a y'-y '' precipitation hardened Ni-Fe-Cr wrought alloy. The finished cornponents were subjected to silver plating process to ensure anti-seizure. The failed hook nuts were examined in details using stereo zoom microscope, optical microscope and scanning electron microscope. Fractured surface exhibited two distinguished regions. Intergranular fracture, characteristics of brittle failure morphology, was observed on majority of the fractured surface. Shear lip formation was evidenced towards one edge indicating final fracture region. This region exhibited dimples, typically seen in case of overload failure. The intergranular fracture was concluded to be due to hydrogen embrittlement of the part. The electroplating process on the finished hook end self locking nuts revealed that no de-embrittlement treatment was carried out on the components after silver plating. The electrolysis process resulted hydrogen entrapment during plating. Omission of de-embrittlement treatment caused the entrapped hydrogen to diffuse into the material thereby making the material to be brittle. This resulted brittle failure of the hook end self locking nuts.