Prediction of tensile and flexural strength of unidirectional CFRP considering the interfacial shear strength

Abstract

The tensile strength of unidirectional fiber composites is interpreted as an initiation and propagation of crack inside, and the crack propagation is the result of fiber fracture and load transfer to surroundings. After the fiber fracture the load is carried by matrix in shear loading, so the load transfer capacity is expected to increase according to improved interfacial shear strength (IFSS). In theoretical study the extreme IFSS can make enhanced property, reaching to rule of mixture, however experiments have demonstrated that optimum interfacial shear strength exists in tensile strength. This can be explained by the effect of multiple fracture. When a fiber is broken, it induces concurrent breakage of surrounding fibers due to stress concentration. This ‘multiple fracture’ phenomenon is important to determine the tensile strength of fiber composites. In this study, the tensile and flexural strength of unidirectional carbon fiber composites were 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 was analyzed using finite element method, determining the stress concentration factor of each surrounding fiber. Based on the stress concentration factor, the ‘multiple fracture number’ was calculated using statistical prediction approach. 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.