Multiaxial characterisation and nonlinear thermoviscoelastic isotropic and orthotropic constitutive models of ETFE membranes

Ethylene-tetra-fluoroethylene (ETFE) is a stiff and ductile polymer widely employed in cladding elements for roofs and façades, formed in pneumatic cushions or tensioned foils. The material’s high thermoviscoelastic effects require a comprehensive time and temperature-dependent multiaxial characterisation and accurate constitutive model to foster the design of lightweight ETFE tensioned construction and their standards. A multiaxial experimental campaign is performed to assess the material response in a wide range of temperatures, from − 20 ° C to 60°C, strain rates, from 0.01%/s to 1%/s, and mechanical loading conditions typical of ETFE buildings. Uniaxial and biaxial bulge tests on circular and elliptical samples are carried out to assess constant strain rates, pressure rates and temperatures, unloading, cyclic, creep and relaxation conditions until the material yield point. Two nonlinear thermoviscoelastic plane stress constitutive models, one isotropic and one orthotropic, are developed using the Eyring stress shift factor to capture material nonlinearities, in the framework of the Boltzmann and the time–temperature superposition principles. A freezing of the Eyring shift factor during unloading has been introduced to represent ETFE relaxation and cyclic conditions. The numerical implementation of the consecutive models within finite element software is made available open source and has enabled the their validation on independently acquired data. The results show high accuracy between measurements and predictions from both isotropic and orthotropic formulations, thus paving the way for their use in structural design to capture highly nonlinear time and thermal effects, such as multiaxial prestress loss after installation or creep caused by variable loads. • Nonlinear thermomechanical characterisation of ETFE until the yield point. • Uniaxial and biaxial bulge tests on samples with different shapes and orientations. • Assessment of strain rate, creep, relaxation and cyclic effects with temperature. • Development of isotropic and orthotropic nonlinear viscoelastic plane stress models. • Freezing of the Eyring shift factor employed to predict unloading conditions. • Open-source implementation of the material models for finite element applications.