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A helicopter tail rotor blade spar failed in fatigue, allowing the blade to separate during flight. The 2014-T652 aluminum alloy blade had a hollow spar shank filled with lead wool ballast and a thermoset polymeric seal. A corrosion pit was present at the origin of the fatigue zone and numerous trails of corrosion pits were located on the spar cavity's inner surfaces. The corrosion pitting resulted from the failure of the thermoset seal in the spar shank cavity. The seal failure allowed moisture to enter into the cavity. The moisture then served as an electrolyte for galvanic corrosion between the lead wool ballast and the aluminum spar inner surface. The pitting initiated fatigue cracking which led to the spar failure.

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Richard H. McSwain, Fatigue Fracture of a Helicopter Tail Rotor Blade Due to Field-Induced Corrosion, Handbook of Case Histories in Failure Analysis, Vol 2, Edited By Khlefa A. Esaklul, ASM International, 1993, p 30–32,

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