Constitutive Model Of Building Coated Fabrics

The mechanical properties of building coated fabrics show significant nonlinearity,because of the complex mechanical mechanism of multi-component materials and complex weaving process,The nonlinearity mainly includes the material nonlinearity of the multi component materials,the geometric nonlinearity of the crimp interchange.process and the friction contact nonlinearity during the stretching process.The coated fabircs are always assumed to be linear and elastic,which is different from the actual engineering.It may lead to the uncertainy and the unsafety in actual engineering.In order to thorough grasp the nonlinear mechanical behavior of coated fabrics,the mechanical properties and constitutive model of coated fabrics are studied by means of mechanical test,theoretical analysis and numerical simulation.The main contents are as follows:(1)Off-axial tensile behaviors and failure modes of building coated fabircs.A series of uniaxial off-axial tensile tests were carried out to study the mechanical properties of coated fabrics under different tensile rates.The following parameters are considered in the tests including eleven in-plane directions(0°,5°,15°,25°,35°,45°,55°,65°,75°,85°and 90°)and six tensile rates(10mm/min,25 mm/min,50 mm/min,100 mm/min,200 mm/min,and 500 mm/min).The variations of main mechanical parameters of PTFE coated fabrics were analyzed and the corresponding tensile failure modes and mechanisms were studied.(2)Central tearing behaviors of building coated fabrics with initial notch.A series of uniaxial tensile tests with different initial notches are carried out,in which the effects of notch shape,notch size,specimen size and loading rate on the central tearing behaviors are studied.Then,a tearing analysis model for coated fabrics with initial notch is proposed,and the tear strength of specimens with initial slit or circular notches are predicted.Finally,the comparisons between experiment data and the predictions of four models are presented,including the proposed tearing model,the Theile's empirical theory,the critical stress theory and the stress intensity factor theory.(3)The load-dependent constitutive relation models of building coated fabrics.Based on the fundamental mechanical tests of coated fabrics,a macroscopic non-linear and orthotropic constitutive model is proposed,in which the strains were chosen as the variables.The polynomial functions are used to describe the material stress-strain relationships under uniaxial tension and in-plane shear loading,and the variations of material stiffness during the unloading phase.The proposed constitutive model was implemented into the ABAQUS software by UMAT,and validated by the physical tests.Results show that it can describe the material mechanical behaviors during the loading and unloading phase,as well as the complex loading history.Besides,it can also make a good prediction on the failure modes and tensile strength.The mechanical behavior of uniaxial specimens and membrane structure can be well predicted by the proposed constitutive model.(4)The hyperelastic constitutive relation model of building coated fabrics.An anisotropic hyperelastic constitutive model for coated fabrics was developed based on fiber reinforced continuum mechanics theory.The strain energy per unit volume in the hyperelastic model is additively decomposed into two parts,representing the tensile energy from warp and weft yarns elongation,shearing energy from interaction between warp and weft yarns,respectively.The direction of yarns is recorded through the direction vector,and then a general expression for engineering application of the offaxis tensile of coated fabrics is derived.The method of determining the material parameters was given and then the off-axial tensile curves were verified with different angles.The developed constitutive model was introduced into the FEM software ABAQUS.The constitutive model was validated by a static loading test with the offaxial 45 degree specimen.The predictions are in good agreement with the experimental results.