Modeling of viscoelastic response in epoxy fiberglass plastic under cyclic high temperature and load

Object of Research: Glass fiber reinforced epoxy composites (GFRP) subjected to cyclic thermomechanical loading, simulating operational conditions of structural elements such as chimneys, gas ducts, and related infrastructure. Purpose: To investigate the stress-strain state (SSS) of GFRP under repeated heating and cooling cycles, varying initial levels of mechanical stresses, and different cycle durations. The objectives include conducting experimental studies of viscoelastic properties at various temperatures and developing an enhanced viscoelastic model that accounts for the material’s memory effects. Methodology: Utilized a previously proposed and refined multi-element model based on the three-element Kelvin–Voigt framework, featuring sequentially “switchable” elements that activate and deactivate at specific temperatures. This approach accounts for viscoelastic memory effects and the accumulation of residual stresses. A Python script was developed to calculate the SSS under multiple heating-cooling cycles. Mechanical parameters (E₁, E₂, η) were determined from stress relaxation curves at different temperatures. Experimental investigations were conducted on GFRP samples within the temperature range of 30–180°C. Additionally, the glass transition temperature of the epoxy polymer was evaluated to be approximately 130°C. Results: Experimental data revealed that pure epoxy polymer specimens accumulate significant residual tensile stresses at low initial stress levels. In contrast, GFRP composites exhibit substantially reduced residual stress buildup due to glass fiber reinforcement, which limits thermal expansion and maintains higher stiffness at elevated temperatures. Nonetheless, under certain conditions—such as prolonged hold times at peak temperatures and varying cycle durations—a slight accumulation of residual stresses was observed in GFRP samples. Notably, despite exceeding the glass transition temperature of the matrix, GFRP maintained adequate stiffness even at 180°C, thereby expanding its operational temperature range and demonstrating its potential for use in load-bearing structures under elevated temperature conditions. Conclusions: The developed model and conducted experiments confirm that viscoelastic memory effects and the accumulation of residual stresses in GFRP are significantly influenced by material composition, initial mechanical stress levels, and thermal cycle parameters. GFRP exhibits notably lower residual stresses compared to pure epoxy polymer, and its high stiffness above the matrix’s glass transition temperature supports its application for long-term use in a broader temperature range. Furthermore, the experimental studies of viscoelastic properties at various temperatures provided critical data for more accurate modeling and prediction of the stress-strain state in polymer composites. The proposed modeling approach and computational tools offer valuable means for predicting the reliability and service life of GFRP structural components subjected to cyclic thermomechanical loading.

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