Metallurgical examination of failed hub Plate Lower of a helicopter

A prematurely failed hub plate lower made of titanium alloy belonging to the tail rotor assembly of a helicopter was sent to the laboratory for analysis. Examination revealed that a fatigue crack initiated at the web radius region and propagated progressively through thickness. Non-uniform loading was observed on the blade-side surface of the hub plate around the bolt hole corresponding to blade-G. Damages in the form of rubbing were also observed on the inner surface of the hub plate indicating misalignment of this component with the tail rotor shaft. It appears most probable that this misalignment has resulted in assembly stresses leading to fatigue crack initiation. There were no evidences of metallurgical or mechanical abnormalities at the fatigue crack origin. The microstructure of the hub plate lower was found to be satisfactory