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

The object of research is glass fiber reinforced epoxy composites (GFRP) subjected to cyclic thermomechanical loading, simulating operational conditions of structural elements like chimneys. This study investigates the stress-strain state (SSS) of GFRP under repeated heating and cooling cycles, varying initial mechanical stresses and cycle durations. The aim is to develop an improved viscoelastic model accounting for material memory effects. Method. A refined multi-element Kelvin-Voigt model with sequentially switchable elements was used to represent viscoelastic memory and residual stress. A Python script calculated SSS under cyclic loading. Mechanical parameters were derived from stress relaxation curves (30–180°C). Experimental investigations were conducted on GFRP samples, and the epoxy's glass transition temperature was evaluated (~130°C). Results. Experimental data revealed that GFRP composites exhibit substantially reduced residual stress accumulation compared to pure epoxy polymer due to fiber reinforcement. While slight residual stress buildup was observed in GFRP under specific conditions (prolonged peak temperature holds, variable cycle durations), GFRP maintained adequate stiffness even at 180°C, exceeding the matrix glass transition temperature. This extends its operational temperature range. Viscoelastic memory effects and residual stress in GFRP are significantly influenced by material composition and thermal cycle parameters. GFRP demonstrates notably lower residual stress and sustained stiffness above the matrix's glass transition temperature, making it suitable for broader temperature applications. The proposed model and experimental data enhance SSS prediction and provide valuable tools for predicting the reliability and service life of GFRP structures under cyclic thermomechanical loading.