Size effect on tensile strength of carbon fibers

Tensile strength of carbon fibers exhibits statistical Weibull type distribution and significant size dependence. In the present work, ten types of polyacrylonitrile based and mesophase-pitch based carbon fiber monofilaments were tested for two or three different gauge lengths. Size effect in both axial and radial directions were analyzed based on the two parameters Weibull statistics. It was found that the size effect in axial direction was almost similar for all fibers tested. This result suggests that the tensile strength obtained for a certain gauge length is a meaningful measure as a representative strength of the fiber strength. In radial direction, the size effect of the tensile strength was larger than that in axial direction. The tensile strength of the carbon fibers seemed to have unisotropic statistical characteristics. Size dependence in diameter was numerically simulated with an assumption of unisotropic distribution of Reynolds-Sharp type defects.