Abstract

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.

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