Full Scale Fatigue Testing of Advanced Fiber Composite Components

The importance of full-scale testing in the development process for fiber-reinforced composite helicopter dynamic components is discussed, and illustrated by means of several examples drawn from Sikorsky experience in the use of composites in a wide variety of structural applications. Laboratory fatigue testing is conducted at Sikorsky on all flight-critical dynamic components in order to determine structural adequacy. The intention of this testing is to generate fracture modes by testing with loads accelerated above the levels encountered in service. The fracture mechanisms exposed to date in this testing on composite components have not been fiberrelated, but rather matrix-related (interlaminar shear or disbond). These fracture mechanisms are related to design details such as ply endings, holes, curved sections, bonded joints, or bolted joints. Test methods and fracture mechanisms for the following components are discussed: Cross Beam Tail Rotor Spars, Main Rotor Blades, UH-60A External Stores Support and S-76 stabilizer. Structural analysis of these components and fracture modes is not presented since the primary mechanism - interlaminar shear - is difficult to treat analytically. This emphasizes the importance of the empirical approach as a necessary complement to analysis in the development of critical composite components used in fatigue loaded applications.