Analysis of stressed membrane structures

Abstract

A growing use of tensioned membrane structures requires insight and understanding of the mechanical behaviour of the structure and the membrane material. In current practise the complex material behaviour is simplified due to the uncertainties that exist around the material behaviour. Testing and modelling the membrane material may provide a more accurate tool for structural analysis. Research has been performed on PTFE coated fibreglass, a widely used architectural textile. Non-linear material properties have been identified as well as other material characteristics. Different aspects of bi-axial fabric testing have been researched. A bi-axial test protocol must provide accurate measurements that will provide insight in the non-linear stress-strain relation of PTFE coated fibreglass. Various bi-axial tests have been performed in the Stevin Lab at the Faculty of Civil Engineering, Delft University of Technology. Accurate displacements measuring devices have been designed and created to record the strains of the fabric. These tests supplied insight in the non linear material properties. Bi axial tests at preset stress ratios have been performed on six identical cruciform test samples of PTFE coated fiber glass (Verseidag B18089). Various approaches have been made to model the fabric’s stress-strain behaviour. Modelling the stress strain relation by creating a best fit surface through the experimental data did not result in a useable model. A different approach where the nonlinear stress strain relation is linearized and where Hooks law was applied did not result in an accurate model. The best results were obtained by a non linear model based on the fibres’ geometry. This model showed good resemblance with the experimental data. The model is calibrated by three parameters, fibre diameter, fibre spacing and Young Modulus of the fibres. The material model is programmed in FORTRAN language and is linked to the general purpose finite element software Ansys. Several test cases have been analysed, based on the material model. It appears that non linear geometric behaviour combined with non linear material behaviour demands an accurate description of the Jacobian matrix for convergence purposes. The performance of the model is not yet at the level of industrial application. Calculation times exceed the reasonable for desktop application. Various recommendations have been made in order to improve the model’s performance. Additional tests on other fabric qualities may contribute to the applicability of the model. These tests must turn out whether the model can easily be adapted to other fabric qualities.