| Membrane material is a new composite,which has been a representative of novel material due to its low modulus,soft touch,high elasticity,high tensile strength,light quality,flexibility design,complex shape and rational stress mechanism.There is an increasing trend concerning the use of membrane structures within architectural design,storage,transportation,civil engineering construction and so on.Membrane structures is made of soft fabric and coated with various resins.The common fabric is plain fabric and made from polyester and glass fiber,which only contains orthogonal wrap and weft yarns.However,theoretically,multi-axial warp knitted fabric(MWKF)should easy to make complex stress structure because it is non-crimp,multi-axis and multi-layered reinforcements.There are some problems to await to settle in the traditional produce ETFE,PTFE and PVC membrane materials for instance high cost,low yield,the complex craftwork and serious waste pollution and so on.The curtain coating technology of polyethylene and polyurethane has solved these shortcomings.Hence,it is necessary to investigate on the preparation process and the mechanical properties of membrane materials strengthened by polyester MWKF.This paper reviews the recent developments of membrane materials,describes in details manufacturing methods and technologies of curtain coating polyester MWKF.The polyethylene(PE)and polyurethane(PU)membrane materials strengthened by polyester MWKF were successfully prepared via curtain coating.Experimental and theoretical investigations on anisotropic mechanical behavior of PE-MWKF and PU-MWKF were conducted under small deformation,big deformation,reversed cyclic loading and heavy load creep conditions.The geometrical model built in the scale of representative volume unit was applied to anisotropic elastic mechanical behavior of membrane materials.And a non-linear visco-elastic-plastic mathematical model was established to understand the anisotropic creep behavior of PE-MWKF.They were both justified by mechanical experiment.The main contents can be summarized as follows:Firstly,knitting technology of quadri-axial warp knitted fabric using the polyester yarn,and curtain coating technology of composite are presented in detail.Knitting technology of quadri-axial warp knitted fabric,by using the polyester yarn and DTY materials,on Copentra Multi-axial knitting machine,is introduced in this section.The equipment parameters,raw material,warping processing,yarn layer placement and knitting process are also elaborate in this section.In addition,the fabric parameters and tensile property were tested.And the processing,structure,work principle and characteristics of a newly equipment for curtain coating MWKF were described,meanwhile,the main parameters of this equipment were provided.Secondly,the elastic mechanics behavior of PE-MWKF and PU-MWKF were analyzed through a microcosmic representative volume element(RVE).From microcosmic point of view,a RVE was established.The volume fraction of each fiber beam was obtained by measuring the 3D structure parameters.The matrix and fiber was connected using the bridge matrix.It was assumed that all kinds of micro elements in RVE were isotropic materials,the flexibility matrix was expressed.Using MATLAB to calculate the flexibility matrix of macroscopic materials.So the elastic modulus at 0°,22.5°,45°,67.5°and 90° were predicted.Finally,the experimental results were compared with the predicted results,it was found that the predicted results were in good agreement with the experimental results.Thirdly,anisotropic elastic mechanical behavior of PU-MWKF was studied under small deformation condition.In practice,the PU-MWKF was available in many shapes and sizes,not just rectangle and flat.In this section,the tensile characteristics of PU-MWKF were investigated.Five groups of tensile experiments,with off-axial angles of 0°,22.5°,45°,67.5°and 90°,were conducted under constant velocity.The failure mechanism was explored by analyzing the damaged specimens.The initial modulus of 0° and 22.5°,45°,67.5°,90° were obtained by a linear fit and double line fit,respectively.Additionally,the deformation behavior of PU-MWKF in three regions was investigated by digital image correlation(DIC)system,and an orthogonal anisotropic model was used to predict the modulus and Poisson's ratio of 22.5°and 67.5°.The results showed that the apparent modulus of PU-CMWKF strongly depended on the cut of directions.And the failure mechanism under in-plane direction loading suggests that tensile and shear failure act together.The analytical model was validated along five directions in the representation of elastic constant under corresponding small strain.Fourthly,the anisotropic mechanical behavior of PE-MWKF was studied in three elastic stages.In order to investigate the anisotropic tensile behaviors of PE-MWKF,one group of specimens were cut out,with off-axial angles of 0°,22.5°,45°,67.5°and 90°,which were experimented under uniaxial tension.Meanwhile,the deformations of these specimens were recorded by the DIC system.Three modulus of PE-MWKF in different tensile levels under failure stress were calculated,and the modulus and Poisson's ratios in direction of 22.5° and 67.5° were estimated with the off-axial constitutive model.Another group of specimens with 20% of their ultimate tensile stress were loaded periodically with an interval of 3mins for 3 times to demonstrate periodic load's influence on the mechanical properties of the specimens.The results show that the elastic behaviors in the third phase were not the orthogonal anisotropy.By regression of the experimental data,the experimental equations of the elastic modulus were given at different load direction.Also the failing stress and the modulus of the specimens at all directions but 0° were increased significantly after periodic loads.Finally,creep deformation behavior of PE-MWKF was explored under the condition of high stress at the 20?.The behaviors of creep and strain recovery at tensile mode in five directions for PE-MWKF were thoroughly studied.The effect of stress and fiber directions on the material's creep response was studied and the viscoelastic and viscoplastic deformation data from creep and strain recovery test was analyzed through a mathematical model.The results show that the mathematical model is consistent with the experimental data,and the mathematical model can be used to predict the creep behavior of multi-axial warp knitted fabrics.The viscoelastic and viscoplastic strains of this materials are related to applied stress and time in the condition of 20?,especially along skewed the direction of fiber yarn.It is found that viscoelastic strain along parallel to fiber is larger than the non-parallel one.Further,viscoplastic strain has close relationships with property of matrix and fiber.Multi-axial warp-knitted fabric can be prepared by adjusting the operating conditions.Because the modulus of matrix is close to the one of reinforceme nts,the matrix has a significant impact on the mechanical behavior and performance of the membrane material.In elastic stage,the stress-strain curve has a three linear character.There are two turning points in the curve,that is,strengthen-point and yield-point.The first stage is shearing strain of matrix.The second stage is tension strain of reinforcing yarn,stitch yarn and matrix.The third stage is tension strain of reinforcing yarn and matrix.A microscopic representative volume element model can be used to calculate the elastic modulus of the membrane structures.The elastic constants of infinitesimal deformation PU-MWKF is satisfied to the limits of orthotropic.However,three stages of elastic constants of PE-MWKF is not satisfied to the limits of orthotropic due to different parameters and calculating methods for elastic modulus and Poisson's Ratio.The phenomenon of strength aggrandizement and nonlinearity deterioration has been observed in cyclic loading tests of PE-MWKF.The influence of skew yarn causes the non-uniform deformation in the sample of the membrane material.Deformation of the membrane material is a small part of time-dependent visco-elastic and visco-plastic and most of elastic. |
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