Nonlinear constitutive equation for vapor-grown carbon nanofiber-reinforced SC-15 epoxy at different strain rate

In this study, tensile tests were performed on 0, 1, 2, and 3 wt% vapor-grown carbon nanofiber (CNF)-modified SC-15 epoxy at strain rates ranging from 0.00033 to 0.033 s−1. Experiment results showed that both the elastic modulus and the tensile strength of the materials increased with higher strain rates, but the failure strain decreased with higher strain rates, indicating that the composite is a strain rate-dependent material. Experiment results also showed an even distribution of CNFs in the 1 and 2 wt% systems and an agglomeration of CNFs in the 3 wt% system. Therefore, the 2 wt% CNF-infusion system exhibited maximum enhancement, compared to other systems. Based on the results, a nonlinear constitutive equation was established to describe the strain rate-dependent stress-strain relationship of neat and nanophased epoxy. The parameters in this model are tensile modulus E, stress exponent n, and stress coefficient σ*. The stress exponent n, which controls the strain rate-strengthening effect and the strain rate hardening effect of the composite, is independent of both strain rate and CNF content. The stress exponent σ*, however, varies with both strain rate and CNF content.