Failure analysis of main rotor blade fork bolt (big) of a helicopter

A failed main rotor blade fork (big) of a helicopter was forwarded to this laboratory for analyzing the cause of failure. Examination revealed that the bolt has failed by fatigue. There was multiple fatigue crack initiation on the threads of the bolt. After initiation, a few of the cracks joined together to form major fatigue crack fronts which then propagated progressively over about 95% of the cross section before culminating in static overload failure. Evidences suggest that there was relative movement (slip) between the nut and the bolt which resulted in thinning of the threads on the bolt by wear process. Because of reduction in cross section, the load carrying capacity of the threads reduced drastically leading to initiation of a number of fatigue cracks on the threads. The material, microstructure and hardness of the bolt were found to conform to design specification. No metallurgical abnormalities were found responsible for the fatigue crack initiation