Fracture Properties and Strength Evaluation of Carbon Fiber under Tension-Torsion Loading

Recently, carbon fiber reinforced plastics (CFRPs) have been used as high-pressure container materials for fuel cell vehicles and other applications. When used as structural components, carbon fibers in a CFRP are subjected to multiaxial loading. As is well known, multiaxial strength and fracture behavior are significantly different from those under uniaxial loading. However, research on the multiaxial strength evaluation of carbon fibers is insufficient. To date, there have been a few reports on studies under tension-torsion loading, but these have hardly clarified the fracture criterion. In this study, the tensile-torsional fracture strengths of PAN-based carbon fibers were obtained by proportional loading tests. As a result, it was found that the tensile strength along the fiber axis decreased largely with increasing shear stress at the fiber surface under tension-torsion loading. The changes in the experimental results were well explained by adding a correction term to Tsai-Hill criterion, the theory for anisotropic materials. Furthermore, since PAN-based carbon fibers showed extreme differences in strength between uniaxial tension and pure torsion, it was finally concluded that the carbon fibers follow the maximum principal stress theory when using normalized stresses. ©2024 The Society of Materials Science, Japan.