Study On The Biaxial Stress Relaxation Properties Of PTFE Coated Fabrics

Due to flexible shapes and high transmittance,membrane structures have been widely used in large space structures.The commonly used building membrane materials are all typical polymer materials with obvious viscoelastic properties,causing stress relaxation and creep.These mechanical behaviors lead to the loss of pretension and continuous deformation of the structure,and affect the safety and aesthetics of the structure.In order to investigate the biaxial viscoelastic properties of membranes,a biaxial tensile testing machine was developed.In the process of research,the key problems,such as the flexibility of loading mechanism,the stability of low stress level and the requirements of vertical loading,were solved step by step.Finally,the design of biaxial tensile test equipment was established.Horizontal loading mode can complete multiple loading schemes,such as proportional loading and unloading,alternate loading and unloading,and stress maintenance.Based on the equipment,tensile tests were carried out under different cruciform specimen sizes,deformation measurement positions and loading modes.The results showed that the size of specimen and the location of deformation measurement have little influence on the test results,while different loading modes have great influence on the tensile curve.These conclusions were used to determine the loading scheme of subsequent stress relaxation test.In this paper,a series of biaxial stress relaxation tests of PTFE coated fabrics were carried out on different tensile processes.The effects of tensile rate,tension order in warp and weft direction,ratios of initial pretension on viscoelastic properties of warp and weft stress relaxation were discussed.Several of viscoelastic models,such as generalized Maxwell model,fraction model and fractional exponential model were adopted to simulate the stress relaxation modulus.Then the precision of those models was discussed.The results showed that the fitting accuracy of the generalized Maxwell five-element model was better.Combined with practical engineering construction scheme,biaxial stress relaxation tests were carried out by means of secondary tension,stage tension and cyclic tension,respectively.Through investigating the stable stress level of coated fabric under different tension methods,the effective construction tension scheme was determined toreduce the influence of stress relaxation for membrane structures.It showed that secondary tension construction scheme was more effective.Whereafter,the curved surface was formed by means of horizontal and vertical loading of cruciform membrane.And secondary tension method was adopted to investigate the change of stress relaxation on membrane surface and the feasibility of the construction scheme was verified.Based on the generalized Maxwell five-element mode,the stress relaxation analysis was conducted though the numerical program.It showed that the numerical results were near to the experimental data.The above research results provided theoretical basis for the construction tension scheme.