Viscoelastic Memory Effects in Cyclic Thermomechanical Loading of Epoxy Polymer and Glass Reinforced Composite: Experimental Study and Modeling under Variable Initial Stress and Cycle Durations

This article presents a study of the viscoelastic behavior of an epoxy polymer and a glass-reinforced composite based on it under cyclic thermomechanical loading. The goal is to model and explain the experimentally observed stress state formation, including the accumulation of residual stresses at various initial mechanical stress levels and durations of heating and cooling cycles. An improved material model implemented as a Python script is used, allowing the consideration of memory effects on thermomechanical loading depending on the level and nature (mechanical or thermal) of initial stresses. A Python script was developed to determine the viscoelastic parameters of mate-rials (elastic modulus E₁, elastic parameter E₂, viscosity) for the three-element Kelvin–Voigt model. The parameters of the polymer and the glass-reinforced composite used in the main modeling were determined at different temperatures. This makes the approach different from the one we previously used [1], where the parameters were determined only at one temperature. The accumulation of stresses under different ratios of mechanical and thermal stresses was also con-sidered differently. The experiment showed that high levels of residual stresses could form in the pure epoxy polymer, depending on the viscoelastic properties of the material, the parameters of the temperature cycles, and the initial stress state. In the case of the glass-reinforced composite, the effect of residual stress accumulation is significantly weaker, which may be due to reinforcement and high residual stiffness even at elevated temperatures (the studies were conducted from 30 to 180 °C for the composite and from 30 to 90 °C for the polymer). The modeling results qualitatively and quantitatively agree satisfactorily with the experimental data, offering a possible explanation for the observed effects. The proposed approach and tools can be used to predict the stress-strain state of polymer composite structures operating under cyclic thermomechanical loads.