Modeling and Characterization of Stress Relaxation Behavior of EVA–CNT Nanocomposites Under Uniaxial Loading

Understanding the time-dependent viscoelastic behavior of polymer carbon nanotube (CNT) nanocomposites is crucial for predicting their performance, particularly in load-bearing applications where the nanocomposites can be exposed to either short- or long-time loadings. In this study, the stress relaxation behavior of ethylene-vinyl acetate (EVA)–CNT nanocomposites containing 1, 3, and 5 wt% CNTs was experimentally investigated under load- and strain-controlled uniaxial relaxation conditions and modeled using the generalized Maxwell linear viscoelastic model. The results indicate that in the linear viscoelastic domain, the EVA–CNT nanocomposites retain at least 70–80% of their instantaneous modulus, and the equilibrium modulus almost doubled with increasing nanotube content from 1 to 5 wt%, yielding a 1.70- to 1.85-fold increase depending on the initial strain and relaxation period. When the strains exceed the viscoelastic limit (10–25%), all nanocomposites relaxed to about 65–70% of the initial stress. The generalized Maxwell model accurately captured the relaxation behavior of EVA–CNT nanocomposites within the linear viscoelastic domain, providing a robust fit for experimental relaxation data with relative errors not exceeding 5%. This study expands the potential application of EVA–CNT nanocomposites in engineering by providing insights into their stress relaxation behavior under both small and large deformations while highlighting areas for future improvement.