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The elastic constants are approximately determined using a multi-step linear approximation method for more accurate material nonlinear analysis of coated fabric membrane structures. The approximate determination is performed within each quadrilateral or triangular surface defined on the surfaces of the biaxial elongation property formed with the data of stress-strain curves measured by biaxial extension testings. Two types of 40 × 40 cm square flat membrane model of a PTFE coated plain-weave glass fiber fabric are analyzed applying finite element method adapting a 20-steps linear approximation method. One of the models is an inplane deformation model subjected to biaxial extension under the condition of stress ratio of 1:1. The stress-strain response in the central region of the membrane is analyzed. And the other model is a membrane deformed by the application of air pressure. The relation between the central deflection and the pressure is analyzed. The results of analysis showed excellent coincidences with experimental results.

Coated plain-weave fabrics are used as a membrane material of membrane structures including architectural membrane structures such as clear span air-supported membrane structure, flexible containers and airships. Uni/biaxial extension properties of coated fabrics are vary important for the design of membrane structures. In the present paper, the authors first presented SEM photographs of the surface and cut section to understand the structure of this material. Then unidirectional tensile tests were done up to failure. The stress-strain curves showed nonlineality and anisotropy. Some discussion was done on these properties. Poisson's ratio showed extreme nonlinearity at the stress region up to 20% of the failure stress which is the design stress. Specimens used here were a PTFE-coated grass fiber fabric and a PVC-coated polyester fabric which are widely used in the field of architectural membrane structure.

The membrane materials are generally in various states of biaxial deformation in membrane structures, and also have visco-elasticity in addition to the peculiar nonlinearity. The biaxial extension curves, therefore, should be measured under the experimental conditions of iterative extension and various tensile speeds. In this paper, the authors selected PTFE coated plain-weave glass fiber and PVC coated plain-weave polyester fiber fabrics as the specimens, and measured the uni/biaxial extension curves up to the design stress (assumed to be 20% of the uniaxial breaking stress) under the experimental conditions of iterative extension and two different tensile speeds. The nonlinearity, the slope, and the iteration time up to convergence of the extension curves were studied. The dependences of these factors on tensile speed were also discussed. The main result of discussions was that the slope of extension curves depended upon tensile speed.

The crack-tear-strength properties of coated plain-weave fabrics, specimen A (PTFE-coated glass fiber fabric) and specimen C (PVC-coated polyester fiber fabric), were investigated by uni/bi-axial tensile testing using the specimens which had the breadth of 20-40cm and the initial crack length of 2a=0.5-5.0cm at the center. The crack-tear-strength, Tp, to 2a=1cm, where Tp was defined as the maximum tensile stress measured during the crack propagation, decreased abruptly to less than 50% of the uniaxial tensile breaking stress. Tp in the warp of specimen A with 2a=0.5-5.0cm on the biaxial stress condition was lower than that on the uniaxial stress condition, but it was opposite for specimen C. The coating materials did not affect the value of Tp. Further more, it was discussed that either the authors' or Racah's equation could be used as the mathematical expression of 2a-Tp curve measured.

The biaxial stress-strain curves of PTFE-coated glass fiber (specimen A) and PVC-coated polyester fiber (specimen B) plain-weave fabrics were measured by maintaining the ratio of warp stress/fill stress at 1/1, 2/1 and 1/2. The extension curves, especially that of specimen A, showed peculiar high nonlinearity which was evident in the fill extension curves consisting of the parts of very small slope in the lower stress region and of steep slope in the stress region later. For the precise and practical structural analysis of membrane structure, the author presented a method of multi-step-linear approximation to the extension curves measured on many stress ratio conditions corresponding to a variety of biaxial deformation states in the real structure. The least square method was applied, and the error, which results from the approximation of elastic constants in the constitutive equation for each secant line through the extension curve, was made to be minimum in this method. By applying the method, the three-step-linear approximation was conducted on the specimens and the errors were evaluated. According to the result of the approximation, the accuracy was good for the practical application but was not so sufficient about the fill extension curve of specimen A. However, it can be generally said that the present method can be applied for the elastic structural analysis of membrane structures.

Method for deformation analysis of coated plain- weave fabrics which have nonlinear mechanical properties has been reported on this journal [1]. Following this, in this paper, biaxial tensile tester developed by the authors and the crucial specimen for the tester are introduced.And convergence properties of solution given by application of the method on the crucial specimen model of calculation are studied concerning to the coarseness of subdivision of finite ele ment and to the number of steps of iterative calculation.Finally, the methods are verified comparing biaxial stress-strain curves given by the calculation and the measurement.As a result, it is concluded that the method can be progressed toward the application to deformation analysis of membrane structures.