Relation between interfacial shear strength and tensile strength of carbon fiber resin composite strands

The dependence of the tensile strength of unidirectional carbon fiber/epoxy resin composite strand on the fiber-matrix interfacial shear strength was investigated. The interfacial shear strength was changed by applying different levels of liquid-phase oxidations to the carbon fibers. The tensile strength of the composite strands did not increase monotonically with increasing interfacial shear strength but showed a maximum at a certain level of the interfacial shear strength. The results were interpreted based on a fracture model which took account of accumulation of fiber fractures due to distribution of the fiber strength, stress transfer to the fibers via the interfacial shear stress, and multiple fracture of fibers due to stress concentration. It was shown that the decrease in the tensile strength of the composite strand at a larger interfacial shear strength is caused by the increase in the coefficient of a multiple fracture. The dependences of the multiple fracture on the interfacial shear strength and the matrix modulus were compared.