Second- and Third-Order Elastic Constants of Filaments of HexTow^® IM7 Carbon Fiber

Single filaments of HexTow^® IM7-12K carbon fiber were subjected to tensile measurements on a device which applies a known stress σ, and measures the resulting strain ε, and the change in resistivity Δρ. Young’s modulus E, the resistivity ρ, the piezoresistivity Δρ/ρε, and the nonlinearity in the stress-strain relation δ, were determined to be 264.1 ± 16.0 GPa, 1.5 ± 0.1 × 10⁻³ Ω cm, 1.3 ± 0.1, and −4.96 ± 0.23, respectively. The values obtained for Young’s modulus and the resistivity of the fiber are in reasonable agreement with the values reported by the manufacturer. To the best of our knowledge, this is the first report of a measurement of a third-order elastic constant of a single filament of HexTow^® IM7-12K. Given the high elastic strains attainable in these fibers and the negative value of δ, the usual calculation of E from a linear fit to the stress-strain data leads to an incorrect higher value of E. According to the accepted thermodynamic definition of the elastic constants, one must use the initial slope of the stress-strain curve to evaluate E. We also observed that the glue used to secure the fiber has an influence on the apparent modulus of the fiber.