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An off-axis flexure (OAF) test has been presented to measure ply strength under multiaxial stress states. This test involves unidirectional off-axis specimens loaded in bending, using an apparatus that allows these anisotropic specimens to twist as well as flex without the complications of a resisting torque. A three-dimensional (3-D) finite element stress analysis verified that simple beam theory could be used to compute the specimen bending stresses at failure. Unidirectional graphite/epoxy specimens with fiber angles ranging from 90° to 15° have combined normal and shear stresses on their failure planes that are typical of 45° plies in structural laminates. Tests for a range of stress states with AS4/3501-6 specimens showed that both normal and shear stresses on the failure plane influenced cracking resistance. This OAF test may prove to be useful for generating data needed to predict ply cracking in composite structures and may also provide an approach for studying fiber-matrix (F-M) interface failures under stress states typical of structures.
composite, ply strength, test, multiaxial, off-axis flexure (OAF)
Senior researcher, NASA Langley Research Center, Hampton, VA
Research scientist, Analytical Services and Materials, Inc., Hampton, VA