Deformation And Tear Damage Mechanical Properties Of Biaxial Warp Knitted Fabrics

Large-span membrane structure is a new type of space structure.Because of its outstanding advantages in energy saving,environmental protection,applicability and construction speed,it has gradually developed into an important form of large public buildings such as gymnasiums,exhibition halls and storages.Especially,in recent years,with the deterioration of air quality and the improvement of people’s health concept,the inflatable membrane structure which can isolate haze or other pollution and intelligently purify air has gradually been favored.Therefore,the research on the structure design,construction and mechanical properties analysis of inflatable membrane structure has become a hot spot.Inflatable membrane materials are essentially fabric reinforced flexible composites.At present,the fabrics of flexible composites are mainly woven fabrics and warp-knitted membranes account for a very small proportion.However,compared with woven fabrics,warp-knitted membranes have obvious advantages in the stability,stiffness and strength of tensile and shear deformation.At present,warp-knitted membranes have increasingly become the mainstream materials for inflatable membranes in Europe,America and China.In the process of practical application,membrane materials inevitably will be subjected to multiple forces,such as tension,shear and so on.At the same time,membrane structure is also damaged by accidental piercing,wind pulling,inherent damage of materials and splicing welds.The mechanical properties of materials are important indexes to be considered in structural design.Therefore,it is of great significance to study the deformation and tearing properties of biaxial warp-knitted membrane materials for accurate design and safety evaluation of membrane structures.The biaxial warp-knitted membrane material used in inflatable membrane structure was taken as the research object,and its deformation and tearing damage properties were studied by combining experimental and numerical simulation.The main contents include a series of mechanical properties tests,such as uniaxial tension,biaxial tension,biaxial shear,trapezoidal and central tear were completed with digital image correlation(DIC).The mechanical properties involved were discussed in depth with theoretical and numerical analysis,and the deformation and tear damage properties of warp-knitted membranes were obtained.Firstly,the uniaxial tensile test of biaxial warp-knitted membrane material was carried out.The tensile properties of the membrane at seven off-axis angles(0,15,30,45,60,75 and 90 degrees)were studied.The characteristics and laws of tensile strength,fracture elongation and elastic modulus of the membranes at different off-axis angles were analyzed detailedly.The results show that the stress-strain curve under biaxial tension load can be divided into three typical stages.The non-linearity of the curve varies gradually with the offaxis angles.There is a “M” shaped relationship between the Poisson’s ratio and the off-axis angles.At the same time,the “W” strength criterion is proposed to predict the tensile strength of the membrane at different off-axis angles.Secondly,the biaxial loading tests of biaxial warp-knitted membranes under 11 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 carried out.The strain characteristics,elastic modulus and Poisson’s ratio of the membrane were obtained by processing the cyclic data.The warp and weft strain response surfaces were drawn,and the response characteristics of membrane materials under various stress ratios were analyzed.The elastic parameters under different stress ratios were calculated,and their characteristics and laws were analyzed.The strain field of the membrane in the target area was measured by DIC,and the results were compared with those measured by extensometer.The results show that the warp and weft strain response surface is gentle in the high stress area and fluctuates in the low stress area.The effect of high stress ratio on the elastic parameters is more obvious in warp,and the more stress ratio is selected,the more stable the elastic parameters are.Thirdly,the biaxial shear tests and analysis of biaxial warp-knitted membranes with four shear stress ranges under three stress medians were carried out,the shear deformation characteristics and shear stress-shear strain curve data were obtained.The effects of stress levels and shear stress ranges on shear and tensile deformation were compared and analyzed.The variation rules of shear strain,shear hysteresis loop and shear modulus were also discussed.The results show that the residual shear strain in the first cycle is the largest,and the increment of shear strain between the second and third cycles decreases significantly.With the increase of shear stress range,the area of hysteresis loop increases and the shear modulus decreases.With the increase of the median stress,the increase of hysteresis loop area decreases gradually,while the shear modulus decreases gradually,and finally all tend to be stable.Finally,trapezoidal tearing and central tearing tests were carried out on biaxial warpknitted membranes,and the tearing failure characteristics,strength and deformation laws of the membranes were analyzed.For these two kinds of tear failure tests,the effects of crack length,crack inclination and off-axis angles on tearing properties of specimens were analyzed,and the failure law and crack failure mechanism of membrane under different crack length and crack inclination were analyzed with finite element method.The results show that the tearing failure mode is related to the off-axis angles of the specimens.The tear will be destroyed along the weft and warp direction when the off-axis angle is less than 45 degree and greater than 45 degree.The tear strength of membrane is mainly controlled by the bearing yarn,and the tear strength increases with the increase of the number of unbroken bearing yarns.