Structural Analysis And Experimental Study On Double-layer ETFE Foil Cushion

This paper takes the structural analysis method of double-layer ETFE cushion and the mechanical behaviors of ETFE foils as the main research contents. With the numerical computation programs, the numerial calculation analysis, the material properties and the model test have been employed to carry out deeply research on the form finding method, stress analysis method, the structural characters of double-layer ETFE cushion.In order to determine the initial stress of pneumatic membrane structures in form-finding analysis on the condition that the internal pressure is given, the Taylor Expansion of a multi-variable function was used to establish the volume differential equation. With the nonlinear finite element method based on Updated Lagrangian formulations, this paper presents a new approach to calculate the nodal displacements and initial stress in pneumatic membrane structures form-finding analysis. A numerical procedure has been developed and two air supported membrane structures were selected as the analytic models. By comparing the numerical results with the corresponding theoretical results, the validity of the presented approach was shown.Based on Updated Lagrangian formulations, the nonlinear finite element method is employed to develop a program for form-finding analysis and stress analysis of double-layer ETFE cushion. Five types of cushions with different shapes are selected as the analytic models. The rise, the internal pressure and the initial stress are taken as the parameters in form-finding analysis and the relationships among these parameters are given by calculation. In stress analysis, the changes of the volumes and the internal pressure, and the principle stresses of the ETFE cushions are calculated in which six load cases are considered. According to the numerical results, the mechanical properties of the different cushions are presented. The working state of ETFE cushions can be decided by the principle stress of up-layer membrane on condition that the internal pressure changed less than 5%.Uniaxial tensile tests of ETFE foils are described in this paper. Three kinds ETFE foils with different thickness are used in the tests and the specimens are cut from foils along the machine direction (MD) and the transverse direction (TD). The tensile strength, the tensile strain at break and the stress-strain curve are obtained from the tests. A method to determine the two different turning points of the stress-strain curve is given. According to the curve of stress and strain, the yield stress, the yield strain and the Young's modulus are calculated. By means of high resolution digital photography, the Poisson ratio of ETFE foil is measured. From the test results, it is found that the tensile properties of the foils along the machine direction and the transverse direction are almost the same, which means the ETFE foils used in the tests can be taken as a type of isotropic material.In order to study the structural behaviors of ETFE foil cushion, two experimental models with different ratios of rise to span have been used in the experiments. After checking the upper layer shapes of the ETFE foil cushions, by means of increasing the internal pressure with and without sands paved on the upper layer foil, the loading tests have been carried out and the displacements of the upper layer have been measured. The yield strength, the tangent and the secant Young's modulus of ETFE foil used in the experimental models are obtained from the tensile test. With the Young's modulus of ETFE foil from the tensile test and the computation procedure based on nonlinear finite element method, the numerical analysis of model tests has also been performed. It has been found that the numerical results are much close to the results from the model tests when the secant Young's modulus of ETFE foil is used in calculation.