Tensile Performance Of Woven Membrane Materials Under Uni-axial, Bi-axial And Multi-axial Loads

Lightweight structures,such as tension structures and air-supported structures, have been applied widely in the past thirty years.Woven membrane materials as roof materials used for this kind of structures play an important role in the application of lightweight structures.As a kind of flexible material,membrane materials have virtually little bending stiffness.To sustain a shape,the membrane materials must be in tension.Mechanical properties of the membrane materials in tension are therefore very important for structural design,installation and maintenance.Therefore,this study focuses on the tensile performance of woven membrane materials under uni-axial,bi-axial and multi-axial tensile loads.The aim of this study is to discuss the testing conditions under bi-axial and multi-axial loads and tries to find out something important for the establishment of experimental standards of membrane materials.The objective of this study is to get a better understanding of the tensile performance of flexible membrane materials and try to guide the design and the application of the membrane structure.The study focuses on the tensile performance of woven membrane materials under uni-axial loads.One group of off-axis tensile experiments,with off-axial angles of 0°,15°,30°,45°,60°,75°and 90°,are conducted under low tensile stresses.The anisotropic behaviors on elastic constants of the membrane materials are analyzed with the application of off-axial constitutive response of the orthotropic and elastic materials.It shows that the experiments agree well with the prediction results.It is proved that woven membrane materials could be regarded as orthotropic and elastic materials within 20%of the ultimate tensile stress.To analyze the anisotropic behaviors at the tensile failure stage,another group of off-axial tensile experiments with the same off-axial angles are carried out.Three types of failure mechanisms, pure tensile failure,pure shear failure and a mixed failure of tensile and shear,have been observed by analyzing the fracture configuration of the specimens under each bias tensile loading.For the prediction of the anisotropic failure strength of woven membrane materials,Tsai-Hill strength criterion is used.The criterion has shown its great efficiency,except that in the off-axial direction of about 15°,which is due to the difference on the yarn crimp between the warp and the fill directions resulting from weaving and coating processes.Factors that might affect test results of membrane material under uni-and bi-axial tensile loads are examined.After series of tensile tests on PVC-coated membrane materials,it is demonstrated that to measure the strains in the two perpendicular directions,the contact method by the needle extensometer does not interfere the correct data recording;the positions where the strains are measured on specimens have a great influence on the test results of the stiffness and Poisson's ratio in warp direction under uni-axial load;to perform bi-axial tensile tests the size of the cruciform specimen in bi-axial tensile test can be much smaller than those suggested in the literature.It is also shown that the residual strains of woven membrane materials are affected not only by the properties of the constituent yams and woven structure but also by loading conditions during the coating process.To find a feasible method to estimate the elastic modulus for woven membrane materials under bi-axial loads,modeling equations has been deduced.During the model development,woven membrane materials are treated as orthotropic,elastic and linear.Based on the constitutive relationships for orthotropic and elastic materials, two equations sets are deduced for the estimation of the elastic modulus in the warp and the fill direction respectively when suffering from bi-axial loads.From the comparison of the experiments and theoretical prediction,it can be noticed that the model can achieve ideal results when it comes to the estimation of the modulus in the main direction,which is the direction with a higher stress than the other one. Therefore,based on the hypothesis of orthotropic,elastic and linear materials,there is an easy way to estimate the bi-axial tensile properties of woven membrane materials in the main direction only through the uni-axial tensile tests.The study also focuses on the experimental investigation of tensile performance of membrane materials under multi-axial tensile loads.Factors,such as specimen configuration and loading speed,which could affect the correct interpretation of testing results,are investigated.The configuration of the specimen for the multi-axial tensile tests is identified as a gear-shape with large arm widths.A loading speed of lower than 20 mm/min is suggested to obtain the tensile properties of membrane materials under multi-axial loads.The tensile performances of membrane materials under uni-,bi-and multi-axial loads are compared.It shows that the tensile strength and elastic modulus under bi-and multi-axial loads are much greater than those under uni-axial loads.Therefore,for the application of membrane materials in lightweight structures,bi-axial or multi-axial loading conditions will be necessary.Experiments on the specimens with an initial crack in the center under multi-axial loads show that,by comparison with other loading directions,the tensile properties in warp direction of woven membrane materials play an important role in the failure performance and crack propagation under multi-axial loads.To eliminate the dependence on the mechanical properties in warp direction,the balance of the two principle directions of membrane materials should be improved.