The Aviation and Missile Command requested the US Army Research Laboratory to perform a metallurgical examination of a main rotor sub-assembly from an attack helicopter as vibration was noticed during a test flight after replacement of the main rotor head. This article presents the details of the metallurgical examination, the mechanical properties of and chemical analysis on the failed component, and the techniques adopted for the examination such as scanning electron microscopy, X-ray mapping, energy dispersive spectroscopy, and residual stress analysis. The discussion includes the details of crack origin, fatigue progression, and the circumstances which caused the point contact between a MP35N pin and a PH 13-8 Mo component. The failure of the main rotor strap pack outboard pitch cone bolt was attributable to galling. Several of the cracks may have propagated by fatigue and converged beneath the bolt inner diameter or adhesive wear may have induced a sub-surface crack origin.

This article deals with an investigation to determine the root cause of the differences noted in the fatigue test data of main rotor spindle assembly retaining rods fabricated from three different materials. The US Army Research Laboratory - Materials Directorate (ARL) subjected the failed tie rods to visual examination and light optical microscopy and then performed dimensional verification and measured the respective surface roughness of the rods in an effort to identify any discrepancies. Next, mechanical testing (hardness, fatigue, and tensile) was performed, followed by metallography, and chemical analysis. Finally, the ARL performed laboratory heat treatments at the required aging temperature. The results suggested that the difference in performance could not be directly linked to chemical composition, dimensional intolerance, surface roughness or any metallographic variance and that the likely explanation for the difference observed in the mechanical performance of materials lies within a variation of the heat treatment.