Nonlinear Constitutive Model Of Coated Fabric Membrane And Deformation Analysis Of Inflatable Membrane Structur

As a new type of space structures,membrane structure buildings have obvious advantages in economic and artistic benefits compared with traditional buildings.Inflatable membrane structure is an important part of the membrane structure system,in which the coated fabrics is the main material of the membrane structure in the engineering.Due to the differences in yarn weave parameter and mechanical properties of coatings,the coated fabrics show complex nonlinear mechanical behaviors under loads.However,the understanding of these complex nonlinearities is not enough,which restricts the full utilization of material’s properties in the design and analysis of membrane structure.Meanwhile,the stress-state significantly affects the mechanical properties of the materials.Therefore,it is necessary to study the material nonlinearities and constitutive model considering the different stress-states of the materials.The air-supported membrane structure is a typical inflatable membrane structure,and the nonliear deformation of the structure under loads shows the coupling effect of the internal pressure,the stress of membrane and the external load.The complex nonlinear mechanical characteristics of the materials contribute to the complex nonlinear response of the air-supported membrane structure under loads.Therefore,it is beneficial to consider the nonlinearity of the materials concerning the engineering design and in-service safety assessment of air-supported membrane structures.The research contents of this paper is showed as followed:Firstly,an experimental study of the warp-knitted fabric PVDF8028 subjected to biaxial loads was performed to expose the detailed mechanical behaviors and determine proper elastic parameters for the fabrics under multiple stress ratios.The least-square method was adopted to calculate the elastic parameters for different stress states,and response surfaces of strain and elastic parameters were used to reveal the mechanical behaviors in detail.Comparison between coated plain-woven and warp-knitted fabrics were used for exhibiting the influences of microstructures and deformation mechanisms on the macroscopic mechanical properties of materials.The results show that the stress-strain behaviors exhibit significant nonlinearities,and could be characterized by appropriate response surfaces.The elastic stiffness response surfaces of loading and unloading processes could form an unbalanced X-shaped cross,and detailed elastic parameters in those two processes could be obtained by corresponding response surfaces.Compared with plain-woven fabrics,the warp-knitted fabrics could exhibit more obvious nonlinear characteristics due to the existence of their coiled yarns and lower Poisson’s ratios because of special non-crimp yarn structure.The differences in macroscopic mechanical properties for these two materials result from the corresponding differences in microstructures and deformation mechanisms.Secondly,the paper investigates the mechanical properties of coated polyester fabrics in biaxial monotonic tensile tests.The nonlinear behaviors of the constitutive parameters within the full-stress range were detailedly analyzed,and a new constitutive model based on the specific coupling relationships between the uniaxial and biaxial flexibilities was proposed to describe the complex tensile behaviors of coated fabrics under plane stresses.Finally,the accuracy of the nonlinear model in numerical simulations was verified,and special attention was devoted to the finite element implementation of the model as an ABAQUS user material(UMAT)subroutine.It was found that the constitutive parameters of the material varied noticeably with the stress levels and ratios.The stiffnesses in the larger-stress direction are nearly stress-domination,and the stress ratios exert a more significant effect on Poisson’s ratios than that on stiffnesses in the lower-stress direction.The proposed constitutive model,considering the material nonlinearities,is validated by the good correlations between the MATLAB calculated and experimental results.Additionally,the FE simulation results using the UMAT are consistent with the experiments which reveals that the constitutive model proposed and the results obtained in this work are reliable.Thirdly,a biaxial constitutive model was proposed based on the results above and the effect of wrinkles for the materials and was applicable to analyze the mechanical behaviors of the materiels under tensile,wrinkled and relaxed.The biaxial constitutive model as an ABAQUS user material(UMAT)subroutine was used to predict the nonlinear loaddeformation response of inflatable tubes,and the structural deformation of inflatable tube was discussed considering the the positions of loads and the change of internal pressure.The results show that the proposed constitutive model can correctly characterize the mechanical response of the materials,and the predict value of the bearing capacity in the finite element analysis was relatively safe based on the constitutive model.Besides,the higher the values of internal pressure,the greater the bearing capacity of the inflatable tubes.Meanwhile,the positions of loads have little effect on the ability of the structure to bear the bending moment,but showed a significant impact on the structure beared the concentrated force.Finally,finite element numerical models of the air-supported membrane structures were established based on the engineering.Considering the materials’ nonlinearities,the effects of the direction for the fabrics’ yarns and the values of internal pressure on the structural deformation were studied,including the coupling effect of the internal pressure,the stress of membrane and the external loads.It was found that the direction of the fabrics’ yarns has a significant effect on the stress-strain response of the membrane for the structure under fixed internal pressure,and a reasonable setting of the orientation for the material can effectively reduce the stress of the membrane.The wind load caused a distinct stress-strain redistribution phenomenon on the membrane of the structure,and the high stress area of the membrane changed with the increase of internal pressure.