Experimental and analytical characterization of fracture behavior of a PAN-based carbon fiber under combined tensile-torsional loading

The clarification of the fracture behavior of CFRP under multiaxial loading has become a research focus; therefore, understanding of the properties of carbon fiber is essential. This study experimentally and analytically clarifies the effect of torsional loading on the tensile strength of PAN-based carbon fiber. Combined tensile-torsional tests were performed under proportional-loading. The results showed that tensile strength decreased as torsional stress increased. Weibull analysis, treating principal stress as a stochastic variable, confirmed that strength variation decreases as the torsional-to-tensile stress ratio increases. To investigate the cause, a defect size distribution analysis was conducted. The findings indicated that the critical defect size increases as the principal stress plane angle increases, whereas its variation decreases. Considering the anisotropy of fiber strength, a new concept of normalized principal stress was introduced. This revealed that fracture behavior conforms to the maximum principal stress theory. Accordingly, a new fracture criterion was formulated using uniaxial tensile strength, X and pure torsional strength, S as: σ=X1−τ/S2.