Novel thermo-viscoplastic characterization method for glass fiber-reinforced thermoplastics

This work develops a thermo-viscoplastic characterization framework for a glass-fiber-reinforced polypropylene (GFPP) composite for forming applications. Uniaxial tensile tests were conducted at three temperatures (20, 70, and 120 °C) and three strain rates to evaluate anisotropic and temperature-dependent behavior. In-plane biaxial tests on cruciform specimens were then performed to reach higher equivalent strains and reproduce multiaxial deformation paths. An elasto-viscoplastic constitutive model based on the modified G'Sell–Jonas formulation was calibrated through inverse finite-element identification using experimental forces and DIC strain fields. The model reproduced the deformation response with identification errors of 2.5–7.6 % across the temperature range. Temperature-dependent parameters exhibited an approximately linear reduction of 60–80 % with increasing temperature. The resulting model provides a consistent input for thermoforming simulations of GFPP components.