Failure of Unidirectional Composites in Tension

Two approaches were used to predict the tensile strength of unidirectional fibrous composites. The first, based on a criterion of maximum tensile traction on the failure plane, was applied to predominantly brittle systems. The second, based on the maximum distortional energy concept, was applied to predominantly ductile systems which manifest yield before fracture. In both cases, it was found that only one material parameter is required to predict the strength-versus-fiber orientation relationship, provided the same failure mode prevails. Tests performed on brittle and ductile glass-epoxy composites agreed favorably with the stated theoretical formulations under uniaxial tension and flexure. The applicability of the theory was not affected by the strain-rate factor, which does have a substantial effect on the yield strength of the ductile system. A similar approach can be used to predict the tensile strength of unidirectional composites under more general cases of plane stress.