Strain-Rate-Dependent Tensile Behavior of Carbon and Aramid Fiber Bundles: Experiments and Constitutive Modelling

Fiber-reinforced composites are widely used in lightweight structures, making the strain-rate-dependent behavior of reinforcing fibers essential for reliable design. This study investigates the tensile response of TZ300 carbon and P240 aramid fiber bundles over a strain-rate range of 10−4–102 s−1 using quasi-static tensile, pneumatic impact, and split Hopkinson tensile bar tests. The results are analyzed with a Weibull statistical damage model and an explicit saturation-form R–O-type model. Carbon fiber bundles exhibit negligible strain-rate sensitivity: tensile strength remains between 3375 and 3496 MPa, while the failure mode changes from progressive to nearly simultaneous fracture. Aramid fiber bundles show pronounced positive strain-rate strengthening: tensile strength increases from 1950 to 3129 MPa (about 60.5%), failure strain decreases from 0.046 to 0.020, and the failure mode transitions from quasi-ductile splitting to brittle fracture. Weibull parameters remain nearly constant for carbon fiber bundles but vary markedly with strain rate for aramid fiber bundles. The Ramberg–Osgood model fits all stress–strain curves well; a unified parameter set describes carbon fiber bundles, whereas the aramid parameter K increases exponentially with the logarithm of the dimensionless strain-rate ratio ε˙/ε˙0. At the strength-prediction level, this description is more accurate for aramid than the Johnson–Cook law and physically more informative than the Cowper–Symonds law. These findings indicate that, within this strain-rate range, the fiber strength rate effect can be neglected for carbon fiber bundles, whereas aramid fiber bundles require explicit rate-dependent constitutive models in dynamic design. These results provide a reference for the dynamic design of their composites, where matrix and interface effects should be considered separately.