Fail-safe Approach for the V-22 Composite Proprotor Yoke

The V-22 proprotor yoke is fabricated from filament-wound unidirectional glass/epoxy belts that react to centrifugal force (CF) loads and beam and chord bending loads. Fiberglass tape is laid up at off-axis orientation between the unidirectional belts to react shear loads. The structural criteria for both metallic and composite V-22 dynamically loaded rotor components specify that these components be fatigue tested and analyzed to show a "safe life" of 30 000 h. Based on the fatigue test results of prototype and production proprotor yokes, a safe life of 30 000 h for the composite proprotor yoke could not be substantiated. However, testing of the yoke was continued beyond delamination initiation. The V-22 proprotor yoke continued to carry load in the fatigue tests with significant delaminations within the composite structure. These results, in addition to similar test results produced by Bell Helicopter Textron on composite proprotor yokes for commercially produced aircraft, indicate that it will be possible to substantiate this component using a "fail-safe" methodology.