Original paper(Vol.48 No.3 pp.269)

Compressive Damage of Carbon Fibers and Their Crystalline Structures

Tagawa Tetsuya; Kouketsu Tomotaka; Miyata Takashi

Abstract:Carbon fiber is one of the high performance fibers employed in advanced composites. However, the use of carbon fibers as a structural component is often limited owing to their poor compressive properties. In the present work, compressive damage of carbon fiber monofilaments was investigated for various type of fibers. The tensile recoil technique and the SEM observation for looped fiber were conducted to evaluate compressive damage. The compressive strengths evaluated at the tensile recoil test and the loop test were smaller in the higher tensile modulus fibers. However, whether the compressive damage (kink) was occur or not in the loop test was well dependent on the elastic modulus ratio between in tensile and in torsion. The torsional modulus for each fiber was also measured with the torsional vibration method.
For the evaluation of the crystalline structures, the grazing incident X ray diffraction method was performed. Even in similar tensile modulus fibers, the polyacrylonitrile based fiber, which showed the higher compressive strength, had the more disordered graphite crystalline phase in the fiber surface region than the mesophase pitch based fiber. The disordered crystal in the surface layer might locally lead the more isotropic mechanical properties than ordered crystal and this isotropic properties in the surface layer could increase the compressive strength of the carbon fiber. It was suggested the possibility of the optimum crystalline structure for the compressive strength, maintaining the tensile properties.

Key Words:carbon fiber, compressive strength, tensile recoil test, loop test, X-ray diffraction, graphite crystal