Determination of intrinsic strength of carbon fibers

In the process of developing carbon fibers (CF) from new sources or through new production methods, it is often a matter of great concern to assess the potential ability of the fiber under development to exert the high strength through the efforts for improving the production method. In this study, such a value is called intrinsic strength and has been determined from the strength of notched CF by taking account of the orthotropy in the mechanical properties of the CF. The finite element analysis showed that the orthotropy in the mechanical properties causes marked increase in the stress concentration at the notch tip. The orthotropic mechanical properties of the CF including the tensile, transversal compression, and torsional moduli were determined on single fibers. A blunt notch was introduced on the surface of CF using a focused ion beam milling machine and the tensile tests were performed on the notched fibers. From these results, the intrinsic strength of the CF was determined based on the point stress criterion. The intrinsic strength were 24 and 36 GPa, respectively, for the high tensile strength and the high tensile modulus type CF.