Characterisation of the nonlinear viscoelastic material behaviour of ETFE and ECTFE foils for membrane structures

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As an umbrella term, membrane structures comprises structures made both of technical textiles and technical foils dealt with in the present application. Both have a very low dead weight, which means that they are used for wide, unsupported spans of surfaces in the roof area and in the façade. So far, only one type of film has been used for membrane structures made of foils: foils made of ethylene-tetrafluoroethylene, ETFE for short. ETFE foil stuctures are a comparatively young type of construction. Another possible foil material is ethylene-chlorotrifluoroethylene, or ECTFE for short, which has not yet been used in membrane structures. ETFE foils exhibit nonlinear viscoelastic material behavior in both mono- and biaxial tensile tests. At present, however, basic knowledge of the mechanical behaviour under varying boundary conditions such as temperatures or load speeds is lacking both for the already used foil material ETFE and for the new foil material ECTFE.The central research objective of this proposal is to close fundamental gaps in knowledge regarding the characterisation and modelling of mechanical material behaviour. Therefore, the stress-strain behaviour of ETFE and ECTFE foils in mono- and biaxial short-time tensile tests has to be fundamentally studied and recorded, including all material parameters (moduli of elasticity, poisson’s ratios, yield stresses and strains, tensile strengths, elongation at break, etc.). In addition, the mono- and biaxial creep and relaxation behaviour of both types of foil are to be investigated in long-term tension tests. Since membrane structures are always multi-axially tensioned, the biaxial material behaviour is decisive for the design. Since biaxial material characterization is associated with considerable effort, one research goal is also to develop a correlation between the mono- and biaxial material behaviour and to describe it analytically.In order to achieve the research objectives, both the tensile load behaviour (mono-/biaxial) and the creep and relaxation behaviour (mono-/biaxial) are examined in detail, taking into account the influencing parameters of material thickness, specimen geometry, temperature, strain rate, stress ratio in the biaxial stress state and load history. Materials from different manufacturers are included. Characteristic values as quality features and strength values as design relevant values for synclastic, anticlastic and plane structures are determined on the basis of the experimental results. Analytical models to describe the nonlinear, viscoelastic stress-strain as well as the creep and relaxation behaviour in the mono- and biaxial state are developed. The developed methods and models will also be validated in large scale component tests on an existing large component test stand and extended by numerical FE parameter studies.