Verification and Optimization of the Strain Limits for Carbon Fiber Multifilament Tensile Modulus Calculation

Carbon fibers and their composites are widely used in aviation, aerospace, automotive, electronics, and other fields. In this work, morphological, chemical, wettability, and crystalline characterizations, as well as multifilament tensile tests, are performed on CCF300, T300, CCF800H, T800H, CCF800S, and T800SC carbon fibers. The crystalline characteristics and multifilament tensile modulus of the carbon fibers are thoroughly studied. According to XRD, Raman, and TEM, T800-grade carbon fibers (with grain sizes around 2.15 nm and I_D/I_G around 1.75) possess larger grains and a higher graphitization degree than T300-grade carbon fibers (with grain sizes around 1.75 nm and I_D/I_G around 2.50). Subsequently, the multifilament tensile modulus calculated by different strain limits is compared with each other to reveal its changing rules. Combining the crystalline structure analysis and mechanical properties, the evolution model and relevant effects of the multifilament tensile process are summarized. Furthermore, regarding the structural and mechanical results implying that T800-grade carbon fibers have longer linear elastic ranges, a new distinguishing method of the strain limits for multifilament tensile modulus calculation decided by the strain at failure of carbon fibers is proposed to optimize the measurement of multifilament tensile modulus of T800-grade carbon fibers. The optimized calculating method increases the measured modulus of T800-grade carbon fibers by around 7% while making it closer to the nominal modulus value from the producers.