Viscoelastic Properties Of PTFE Coated Fabrics

Coated fabrics are typical polymer composites,which performs obviously nonlinear,inelastic,anisotropic and viscoelastic.Their mechanical properties are affected significantly by external environment,stress level,loading history and others.In actual engineering,with time passed,the creep and stress relaxation behaviors will reduce the prestress of membrane surface and result in the relaxation of membrane surface and decreasing of structural stiffness.Therefore,it is necessary to study the viscoelastic properties of coated fabrics,which can be references for studying the long-term performances and safety of membrane structures.In this paper,the viscoelasticity of PTFE coated fabrics are studied,including stress relaxation properties,creep properties and cyclic tensile properties.The effects of tensile rate,loading history,temperature and stress level were discussed.The anisotropic constitutive relations of coated fabrics are proposed.The main contents are as follows:(1)The stress relaxation and creep tests were carried out under different conditions,in which the effects of temperature,stress level and initial tensile rate were analyzed.The transformation relationship between creep behaviors and stress relaxation behaviors is derived.Several common viscoelastic models are used to fit the experimental curves,and the fitting accuray of the models was compared.The propoer constitutive relation model was proposed to describe the viscoelastic behavior of coated fabrics.(2)The uniaxial cyclic tensile tests were carried out under different conditions,and the main parameters such as loading and unloading elastic modulus,ratcheting strain,hysteresis loop area were obtained.The effect of bias angle,tensile rate and stress amplitude on the ratcheting behaviors was analyzed.The strain of membrane material was decomposed,and the modified Burgers model can be used to make a good prediction of the cyclic tensile curves.(3)A series of off-axial tests were carried out,including cyclic tensile test,stress relaxation test and creep test.Based on the theory of orthotropic composites,the relationship between elastic modulus,relaxation modulus & creep compliance and bias angle is derived.Based on the plastic theory of anisotropic material and flow rule,the anisotropic constitutive relation was obtained by introducing the creep potential function,the effective stress and the effective creep rate.