HLH Rotor Box Beam Fatigue Test.
ARMY RESEARCH AND TECHNOLOGY LABS FORT EUSTIS VA
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The purpose of this program was to evaluate the fatigue properties of a fiberglassepoxy box beam specimen which was similar in design to the Heavy Lift Helicopters HLH rotor blade spar. The fiberglassepoxy box beam was designed and fabricated by Boeing Vertol using a ply layup scheme similar to the rotor HLH blade spar but with a different preimpregnated fiberglass tape. The box beam was tested at a fatigue load level sufficient to cause failure within a reasonable number of fatigue cycles 5 x 1000,000 to determine fatigue strength and observe failure modes and rates. Premature fatigue failure occurred in the box beam as the result of chordwise wrinkles occurring during fabrication. The Integral Spar Inspection System ISIS, which depends on a pressure differential between the sealed spar and outside ambient pressure to detect leaks resulting from cracks or flaws, indicated that a leakage had occurred at approximately the same time the failure was observed. It was concluded that the ISIS appeared to be a viable technique for signaling a structural failure of fiberglass spars. The presence of the wrinkles in the spar which resulted in premature failure, however, precluded any conclusions with regard to the fatigue strength of the preimpregnated fiberglass tape used. Author