A Study On Nonlinear Visco-ealstic Plastic Tensile Properties Of Architectural Woven Membrane Material

Along with membrane structure construction development and membrane material widespread application, the mechanical performance of architectural membrane material is more and more emphasized. Because of simple technology and moderate prices, the architectural woven membrane material composed of plain or panama fabric is very popular. The architectural woven membrane material differs from general architectural material, it is a flexible composite material which is made up of fabric in the middle and coating on both sides, its mechanical performance has its own distinguishing features. The membrane material needs to sustain load or defonnation depending on a specific tensile shape in construction, therefore, on the sufficient understanding and grasping mechanical performance of membrane material in tension or in the course of using is important to instruct the membrane structure the design. This study focuses on tensile nonlinear visco-elastic-plastic performance of woven membrane material.Through the tensile nonlinear visco-elastic-plastic performance study of woven membrane material in warp and weft direction, the objectives of the study are as follows:(1) to present rational plastic model under the conditions of different stresses and time; (2) to present rational nonlinear visco-elastic-plastic element model under creep conditions; (3) to present rational plastic model under the conditions of different strain and time; (4) to present rational nonlinear visco-elastic-plastic element model under stress relaxation conditions; (5) on the basis of considering plastic defonnation, to present rational nonlinear visco-elastic-plastic integral model.The creep tests are carried out on the architectural woven membrane material along the warp and weft direction at different stresses and time, stress critical value of plastic deformation is found. According to a function of several variables from the product of both the stress difference (stress-stress critical value) power function and the time power function, the experiment datum are studied by using a multiple regression analysis. It is concluded that the model has better fitting results. Then the instantaneous and transient parts of the plastic deformation are analyzed by using this model, it is showed that the instantaneous plastic deformation is higher than transient part, as far as the transient plastic deformation is concerned, the plastic deformation at an initial stage is higher than the one at a later stage, namely:the increment of the transient plastic strain will be a significant decrease.The creep tests for architectural woven membrane material are carried out along the warp and weft directions at five different stresses. By mean of four linear viscoelastic element models:Burgers fluid one with four parameters, a solid one with three parameters, a generalized Kelvin-Voigt solid one with five parameters and a generalized Kelvin-Voigt solid one with seven parameters, the experimental datum are analyzed. The results revealed that there is better fitting effect for the generalized Kelvin-Voigt solid model with seven parameters, and as far as the membrane material of the same kind and direction is concerned, three parameters of or relating to retarded time keep unchanged at various stresses. Based on these properties, the linear viscoelastic element model with seven parameters was modified into a nonlinear viscoelastic element one with seven parameters and a nonlinear visco-elastic-plastic element one with fourteen parameters. Then six parameters of or relating to retarded time are amalgamated into three ones and eight parameters of or relating to stress are amalgamated into four ones for the nonlinear viscoelastic element model with fourteen parameters. Considering that the three retarded time of the two nonlinear models were the same as the linear model, that the nonlinear factors are come from four parameters of or relating to stress, to assume between the four parameters of or relating to stress and different stresses meets 4th degree polynomial function, the creep test datum at five different stresses are fitted and then the fitted polynomial function is compared to a creep test at three other stresses and 1,200s.It is concluded that this method can predict the creep performance of different stresses well.The stress relaxed tests are carried out on the architectural woven membrane material along the warp and weft direction at different strain and time, strain critical value of plastic deformation is found. According to a function of several variables from the product of both the strain difference (strain- strain critical value) power function and the time power function, the experiment datum are studied by using a multiple regression analysis. It is concluded that the model has better fitting results. Then the instantaneous and relaxed parts of the plastic deformation are analyzed by using this model, it is showed that the instantaneous plastic deformation is higher than relaxed part, as far as the relaxed plastic deformation is concerned, the plastic deformation at an initial stage is higher than the one at a later stage, namely:the increment of the relaxed plastic strain will be a significant decrease. The stress relaxed tests for architectural woven membrane material are carried out along the warp and weft directions at five different strain. By mean of four linear viscoelastic element models:a solid one with there parameters, a generalized Maxwell one with three parameters, a generalized Maxwell one with five parameters and a generalized Maxwell one with seven parameters, the experimental datum are analyzed. The results revealed that there is better fitting effect for the generalized Maxwell model with seven parameters, and as far as the membrane material of the same kind and direction is concerned, three parameters of or relating to relaxed time keep unchanged at various strain. Based on these properties, the linear viscoelastic element model with seven parameters was modified into a nonlinear viscoelastic element one with seven parameters and a nonlinear visco-elastic-plastic element one with fourteen parameters. Then six parameters of or relating to relaxed time are amalgamated into three ones and eight parameters of or relating to strain are amalgamated into four ones for the nonlinear viscoelastic element model with fourteen parameters. Considering that the three relaxed time of the two nonlinear models are the same as the linear model, that the nonlinear factors are come from four parameters of or relating to strain, to assume between the four parameters of or relating to strain and different strain meets 4th degree polynomial function, the stress relaxed test datum at five different strain are fitted and then the fitted polynomial function is compared to a stress relaxed test at three other strain and 1,500s.It is concluded that this method can predict the stress relaxed performance of different strain well.The creep and recovery tests for architectural woven membrane material are carried out along the warp and weft directions at six different stresses and respective 9,000s. Considering possible instantaneous and transient plastic deformation under the condition of creep, the plastic deformation is combined into nonlinear viscoelastic integral Schapery's constitutive equation. The material constants and nonlinear stress depended parameters of the new model are solved with the aid of test datum, and these constants and parameters substitute the creep and recovery equation. It is found that this method can predict the creep and recovery datum well.