PREDICTION OF TENSILE STRENGTH OF UNIDIRECTIONAL CARBON FIBER COMPOSITES CONSIDERING THE INTERFACIAL SHEAR STRENGTH

The tensile strength of unidirectional fiber composites is expected to increase according to improved interfacial shear strength due to increased load transfer capability of the matrix. Experiments, however, have demonstrated that optimum interfacial shear strength exists for maximum tensile strength, i.e., beyond the optimum point, the tensile strength of fiber composites decreases. This can be explained by the multiple fracture, i.e., when a fiber is broken, it accompanies concurrent breakage of surrounding fibers due to increased load transfer capability. Even though this 'multiple fracture' phenomenon is important to determine the tensile strength of fiber composites, few theoretical researches have been carried out. In this study, the tensile strength of unidirectional carbon fiber composites is predicted considering the interfacial shear strength. First, the effect of interfacial shear strength on the load transfer to surrounding fibers (i.e., local stress concentration) when a fiber is broken is analysed using finite element method, determining the stress concentration factor of each surrounding fiber. Based on the stress concentration factor, the number of the simultaneously broken fibers (i.e., the number of 'multiple fracture') is predicted using stochastic fiber strength distribution. Using the multiple fracture number, the tensile strength of unidirectional fiber composites is predicted, the validity of which is investigated using carbon fiber/nylon 6 composites.