Nonlinear thermoviscoelastic modelling of ETFE foils using free volume model

Ethylene-Tetrafluoroethylene (ETFE) is gaining prominence in the building industry as a thin transparent membrane foil in tensile membrane structures. When installed on façades or roofs in inflated cushions, these thermoelastic foils, which exhibit time-, temperature- and direction-dependent behavior, are subjected to significant mechanical forces over extended periods. Recent research suggests an emerging focus on modelling the nonlinear thermoviscoelastic plastic behavior of the material. The formulation of a reliable constitutive model below the material glass transition temperature that captures the shift from linear to nonlinear elastic behavior is of significant importance for economical engineering design. This paper presents a thermomechanical characterization of ETFE foils through a viscoelastic plastic model up to yielding above its first yielding point until its second yield point around 20 % strains. This was achieved through uniaxial tensile and creep tests, which were performed at temperatures between 16 °C and 32 °C, in the material directions 0°; 45°; 90°, at a strain rate of 0.166 % / s and load level from 4 MPa on a 50 mm width specimen. The study derives a comprehensive set of material parameters for linear viscoelastic and nonlinear viscoelasto-plastic models, using the Boltzmann superposition law with Time-Temperature Superposition Free Volume Model to capture nonlinearity, Perzyna law to capture viscoplastic behavior. For an engineering approach, an isotropic material model is assumed. The model validation of the viscoelastic model through independent creep on a load level 8 MPa and 16 MPa and uniaxial tensile tests at different strain rate from 0.0083 %/s to 0.83 %/s shows promising results for engineering applications across linear, nonlinear and plastic regimes, with physically reasonable material parameters. • application of the Free Volume Model for ETFE. • extended capabilities to capture the strain field. • nonlinear viscoplastic model for ETFE. • Time Temperature Superposition Principle for ETFE.