Mechanical Properties of Carbon Fibers under a Combined Stress

Abstract

Fracture strengths of commercial carbon fibers were measured under a combined torsion-tension deformation, where a carbon fiber was stretched after twisting many turns around the fiber axis. The tensile fracture strength decreased parabolically towards zero with increasing number of turns, exhibiting incline 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 reaches a critical stress. This fact is consistent with the fact that the fiber has an almost isotropic amorphous-like structure. The results for a PANbased carbon fiber (M60J from Toray Co.) consisting of anisotropic graphite layers show a deviation from the theoritical curve indicating an anisotropic strength of the fiber.