Mechanical Properties of Ceramic Fibers under a Combined Stress

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Fracture strength of commercial carbon fibers was measured under combined torsion-tension deformation, where a carbon fiber was stretched after twisting many turns around the fiber axis. The tensile fracture strength decreased drastically with increasing number of turns, exhibiting inclined fracture surface. The results of a low modulus pitch-based carbon fiber (XN-05 from Nippon Graphite Fiber Co.) agree quite well with the theoretical relation derived on the assumption that the fracture occurs when the local tensile stress reached a critical stress. This fact is consistent with the fact that the fiber has an almost isotropic amorphous-like structure. The results for a high PAN-based carbon fiber (M60J from Nippon Carbon Co.) consisting of anisotropic graphite layers show a deviation from the theoritical curve indicating an anisotropic strength of the fiber.