Experimental And Numerical Study On Central Tearing Mechanical Properties Of Warp Knitted Fabrics

Membrane structure is a new type of building structure.Because of its advantages of good economy,meeting the needs of large-span,short construction period,convenient construction,strong applicability,strong safety,node environmental protection and so on,membrane structure has been widely used in gymnasiums,exhibition halls and storage buildings.Membrane material is the main material of membrane structure,which bears the main bearing capacity of membrane structure and determines the safety of membrane structure.Fabric membrane material is the most widely used membrane material in membrane structure engineering.At present,as the base cloth of flexible composite material,plain fabric is the main one.However,warp.knitted membrane has become the mainstream material of air supported membrane structure in Europe,America and China because of its obvious advantages in deformation stability,rigidity,strength and other mechanical properties such as tension and shear.In the process of using membrane structure,once the membrane material fails,the whole structure may collapse.Crack propagation is one of the main failure modes of membrane materials.Because of internal defects or external damage,membrane materials will inevitably produce cracks.Once the crack tip expands and tears the membrane materials,the whole structure will be in danger of failure.In practical application,the cracks of membrane materials appear randomly in every position of membrane materials,but the probability of appearing in the edge part is relatively low,and the cracks are more likely to appear in the central area of membrane materials.Center tearing,trapezoidal tearing and tongue tearing are common forms of tearing.In practical application,most of the cracks in the membrane occur in the interior of the membrane,and the central tearing is more consistent with the real tearing phenomenon.Therefore,it is of great significance to study the central tearing property and crack propagation of membrane materials.The central tearing properties of the warp-knitted membrane materials used in the membrane structure were taken as the research object,and their were studied by the method of experiment and numerical simulation.The main contents include:completing a series of mechanical properties tests such as uniaxial and biaxial tensile tests and tearing tests and so on,and analyzing the meso modeling through numerical simulation,in-depth analysis of the central tear failure mechanism of warp-knitted membrane materials and the influence of different factors on the central tear performance of membrane materials.First of all,the uniaxial tensile test and biaxial tensile test were carried out.The mechanical properties of the membrane materials under different stress ratios(0:1,1:0,1:1,1:2,1:3,1:5,2:1,2:3,3:1,3:2,5:1)were obtained by cyclic loading on the membrane materials.The data were analyzed and programmed into matlab program to obtain the characteristics of the strain response surface and the elastic parameter response surface of the membrane materials.These parameters can be applied to the numerical study of subsequent tearing model.Secondly,the uniaxial central tear tests were carried out on the films with four kinds of slit angles(0 °,30 °,45 °,60 °)and seven kinds of off-axis angles(0 °,15 °,30 °,45 °,60 °,75 °,90 °).The influence of different cutting angle and different off-axis angle on the tear performance of the membrane center was analyzed.And the strength characteristics were summarized.The results show that the cutting angle does not affect the crack propagation process of the membrane material,and the change of the off-axis angle will change the crack propagation process of the membrane material,and will have a great impact on the rigidity and the fracture extension displacement of the membrane material.Thirdly,in the commercial finite element software ABAQUS,the micro model of the uniaxial central tear of warp-knitted membrane was built,and the model size was modeled according to the test size of uniaxial tearing.By comparing the numerical results with the experimental results,the influence of the slit length,the slit angle,the off-axis angle,reinforcement and coating defects on the tear performance of the membrane were analyzed from the micro aspect.The results show that the strength of the membrane can be effectively improved by adding reinforcing film at the cutting seam of the membrane;in addition,the random defects in the coating will have a certain impact on the strength of the membrane,but the impact is small.Finally,the biaxial tear test with the stress ratio of 1:1 was carried out for the warp.knitted membrane,and the micro model was built in the commercial finite element software ABAQUS according to the test size.The numerical results were compared with the test results,and the influence rules of the slit length,the slit angle,the stress ratio,reinforcement and shear stress on the tear performance of the two-way center of the membrane were analyzed from the micro aspect.The results show that the stress at the cutting tip of the membrane is higher than that in other regions when the tear propagation occurs at the cutting tip.In addition,when there is shear stress in the membrane,Tsai-Hill criterion can be used to predict the tensile shear strength relationship of the membrane.