Characterization and modelling of the nonlinear material behaviour of coated fabrics for architectural membrane structures II

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Textile membrane structures made of coated fabrics are nowadays a firmly established constructive form in civil engineering, in particular for the unsupported roofing of large areas. The adequate incorporation of material stiffness forms the basis for a predictive modeling of the membrane in Finite Element models and thus for the calculation of mechanical load and displacement in the membrane and in the substructure, ensuring damage-free constructions, safety and profitability. Coated fabric membranes show an anisotropic and highly nonlinear stress-strain-behaviour in biaxial tension tests. As a result of external loading, softening of the material is observed leading to permanent strains which increase with cyclic loading and which show a saturating behaviour under fixed maximum load. The additional strains resulting from this preconditioning are essential to be considered when dimensioning a membrane structure, which, however, is not the case in nowadays practice due to a lack of suitable models. Moreover, the nonlinear stress-strain-behaviour is currently approximated by linear elastic formulations which yield an insufficient accuracy in the prediction of stresses and displacements. This shows that there is a fundamental knowledge gap with respect to the characterization and modeling of the nonlinear material response, which is planned to be closed in this research project. Central goal of this project is to develop concepts for the incorporation of load- and structure-dependent nonlinear material properties of textile membranes for the structural computation of membrane constructions. The main innovations and thus research goals are as follows: 1. Development of a procedure for biaxial tension tests to identify structure- and load-dependent material behaviour of textile membranes, which is adapted to the real load situation and which account for the prestress and preconditioning process under external loading. 2. Development of two methods for finite element calculations for the consideration of the nonlinear material properties of textile membranes base on: a. Load- and structure-dependent fictitious elastic constants and b. Continuum-mechanical material model for the description of the nonlinear inelastic material properties. 3. Validation of the methods developed in 2. based on experimental large-construction experiments mainly focusing on two structure types (plane and saddle-shaped form) for the assessment of displacements and stresses in real membrane structures. 4. Extension of the method using fictitious elastic constants to further structural shapes (e.g. high point membrane and 4-point sail) by comparative analysis with the nonlinear model.