Comitti, Alessandro; (2025) Thermoviscoelastic characterisation and modelling of ETFE foils for tensile structures. Doctoral thesis (Ph.D), UCL (University College London).

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Text Alessandro Comitti - PhD Thesis - Thermomechanical characterisation and modelling of ETFE foils for tensile structures.pdf - Accepted Version Download (79MB) | Preview

Abstract

ETFE is a stiff and ductile polymer employed in membrane structures as a tensioned foil or shaped into inflated cushions. The membrane has a strong architectural connotation as it is highly transparent and well adapts to complex building envelopes, hence it could represent an alternative to the use of glass with a more lightweight, sustainable cladding technology. The material’s mechanical properties strongly depend on both time and temperature effects, however, the lack of a comprehensive understanding of the mutual influence of these variables prevents an optimal design and wider exploitation of ETFE in sustainable lightweight construction. In this work, an experimental characterisation was performed using a quasi-static universal electromechanical machine, a custom biaxial inflation setup and a dynamic mechanical analyser in order to delve into ETFE behaviour. Uniaxial and biaxial properties were investigated across a range of temperatures spanning from -20 to 60° C, at different strain rates, together with creep, cyclic and relaxation procedures. ETFE revealed a peculiar response that strongly depends on temperature and less on the strain rate, as well as showing a stress-dependent nonlinearity. Constitutive modelling of the mechanical behaviour of ETFE was performed utilising the compliance data of low-stress creep tests, combined to form a master curve thanks to the application of the Time-Temperature Superposition Principle. The material law was implemented in MATLAB and Finite Elements through the use of a recursive algorithm, allowing its validation on the other experimental data. This model could predict accurately the foil behaviour in its linear viscoelastic domain, in every load condition. Both isotropic and orthotropic models were developed to capture the orthotropy of the base material. A unified material model was obtained by extending the linear viscoelastic constitutive equations in the nonlinear domain through an Eyring stress shift factor, which modifies the material relaxation behaviour analogously to the temperature. Furthermore, the thermal behaviour was included in the constitutive relations. The unified model is proven to accurately predict the effects of time and temperature on ETFE, as it was validated against independently acquired data. The implemented model has a high potential in obtaining an accurate prediction of the ETFE behaviour, resulting in safer structural designs, as it was further proven through some application studies on the typical shapes and loading conditions of ETFE foils in building envelopes.

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