| Architectural membrane material is a flexible composite, which is made by coating polymer materials onto the surface of fabrics. It is a tensile resistant material with minor resistance to compression and bending loads. In the present study, efforts are made to predict tensile properties of coated woven fabric as membrane material through an investigation of the changes in fabric structural geometry and tensile property. It is demonstrated that, once the relationship of tensile properties of woven fabric before and after coating is known, so as the fabric parameter such as weave density and yarn count, the tensile performance of the membrane can be predicted if the amount of stretch of woven fabric during coating process can be controlled. The results of the study would provide a basic knowledge for one who is studying other mechanical properties of membrane material than the stress-strain behavior. The results would also be useful for relevant business such as designing and manufacturing departments of tensile structures.A nonlinear elastic constitutive model is developed under the assumption that the stress is uniformly distributed at the nodes in the plane of the membrane material in the direction of one yarn system and the strain in the perpendicular direction resulting from crimp interchange is also uniform. In the model the curved yarns and the coating polymer are replaced by straight bars, of which the tensile behaviors obey Hooker's law. The shear deformation and the bending resistance of the membrane are neglected. The model accounts for the basic mechanisms of crimp interchange, yarn extension and the extension of coating polymer. Polyester woven fabric coated with PVC (PVC/PES) is used to evaluate the prediction results of the model. Although the comparison between the theoretical predictions and experimental results shows an acceptable agreement between them at lower strain levels, a significant deviation is observed when at higher strain levels. The greater the strain is, the larger the deviation would be.By observing tensile curves of yarn and woven fabric it is noticed that both show non-linearity with distinct turning points. Before and after the turning points the slope of the curves changes significantly. In the present study, eight sample curves in warp or fill direction are chosen from a population of tensile curves. By regression fitting curves as well as the turning point are obtained. By means of the least square method, the nonlinear tensile curve of fabric is approximated by multi- linear sections. The slop of the linear section represents the elastic constant of the sample within the section. It is confirmed by W-Test that the sample obeys a normal distribution. The confidence interval of a population's mean is solved at 5% significant level. By examining the cross sections of woven fabric before and after coating it is well demonstrated that, during coating process, the degree of crimp in warp yarns reduced due to the processing tension, while that in fill direction increases by crimp interchange. The crimp interchange would affect the prediction of tensile properties of membrane in relation to those of woven fabric before coating.By considering the crimp interchange and extension of fabric during coating process, a nonlinear elastic-plastic constitutive model for evaluating tensile performance of membrane material is developed and the tensile performance of the membrane is simulated. The effects of fabric extension during coating process in the tensile performance are discussed. In the simulation the tensile deformation mechanisms of either crimp interchange or tensile deformation of fabric during coating is analyzed. PVC/PES coated fabrics are used to evaluate the prediction results of the model. It is demonstrated that, once the tensile deformation of fabric during coating process is controllable, the tensile performances of coated fabrics can be predicted from the tensile behavior of fabrics before coating. |
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