Non-linear and time-dependent viscoplastic material properties of ETFE films as membrane elements with multiaxial stress states in civil engineering

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In membrane structures, foils made of ethylene tetrafluoroethylene (ETFE) are mostly used on prestigious buildings as roof or façade systems, whereby they impress with their high translucency and low dead weight, enabling large, column-free spans. In the design of ETFE foil structures, the multi-axial stresses in the ETFE foils due to the applied loads are simplified into biaxially aligned orthogonal main stresses. Although the multiaxial load bearing behaviour is decisive, the design is based on monoaxially determined material properties. This leads to deficient approaches with respect to the material properties and the load bearing behaviour of the structure. To counteract this situation, the material and load bearing behaviour of ETFE foils in multiaxial stress conditions needs to be investigated. Currently, only the biaxial material behaviour is determined by means of cross-shaped specimens, whereby due to the slotted cross-shaped specimen geometry always a specimen and no material failure occurs and the actual overall biaxial material behaviour cannot be determined. In STR 482/6-1, the applicant was able to show that the multiaxial, non-linear viscoelastic to viscoplastic material behaviour of ETFE foils cannot be determined by biaxial test rigs, but must be investigated in multiaxial test rigs. In preliminary multiaxial tests it could be shown that in the multiaxial stress state, compared to the monoaxial stress state, non-negligible material-strengthening and -stiffening effects occur. The aim of this project is to analyse and characterise the viscoplastic material behaviour of ETFE foils as well as the strengthening and stiffening behaviour in multiaxial simple and cyclic loading. Material reserves are to be identified in order to expand the fields of application of ETFE foils in membrane structures of the building industry and to be able to design these foil structures more safely and at the same time more economically. In this context, the current structural forms - anticlastic and synclastic - as well as influences from varying strain rates, varying test temperatures, different foil thicknesses and foil products on the multiaxial load bearing behaviour of ETFE foils are to be investigated. The analytical models, correlations and methods for the description of the mono- and biaxial material behaviour established in STR 482/6-1 are to be further developed on the basis of multiaxial tests to be carried out and numerical parameter studies, so that the multiaxial, visco-plastic material behaviour can be modelled analytically and material hardening, material strengthening and saturation effects due to cyclic loads can be taken into account.