The tensile strength and inter-laminar shear strength of carbon fiber -resin composites comprised of matrix resins of different tensile moduli ranging from 4.74 MN/m2 to 1170 MN/m2 were measured, and the effect of matrix resin properties on the effective tensile strength of carbon fibers in carbon fiber -resin composite strands was investigated. The inter-laminar shear strength changed in the range from 13.3 MN/m2 to 98.8 MN/m2 in correspondence with the tensile modulus of matrix resin. It was

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... bserved that the inter-laminar shear fracture occurred at the interface between fiber and resin when the matrix resin was sufficiently stiff. On the other hand, when the stiffness of the matrix resin was low, the inter-laminar shear fracture occurred in the matrix resin. The effective fiber strength in the composite strands varied in the range from 1.78 GN/m2 to 2.94 GN/m2, and it tended to increase with increasing tensile modulus of matrix resin. From the results of the observation of fracture surface, the fracture mechanism of the composite strands was considered in relation with the matrix resin properties. When the matrix resin is stiff enough, the effective fiber strength of the composite strands depends on the strength at the interface between fiber and resin. In contrast, when the matrix resin becomes soft, the effective fiber strength is governed by the shear modulus of matrix resin.