Experimental And Numerical Study On The Tearing Mechanical Behavior Of Warp-knitted Fabric Under Tensile And Shear Stress

Membrane structures,as an innovative form of large-span spatial structure,have attracted considerable attention in the past few decades,which have achieved outstanding economic and artistic benefits due to the short building period and beautiful shapes.The primary construction materials in modern membrane structures are mostly coated fabrics,which are composed of the base cloth layer resisting external load and coating layers on the surface enhancing the durability performance.In this respect,polyester fabric with Polyvinylidene Fluoride(PVDF)coating or Polyvinylchloride(PVC)coating and glass-fiber fabric with Polytetrafluoroethylene(PTFE)coating are currently two familiar composite materials.Among them,the PVC coated plain weave fabric has been frequently considered as research object to explore the mechanical properties of architectural membrane materials.However,due to the unique woven structure,the utilization of the physical properties of woven yarns and tensile strength for the PVDF coated warp-knitted fabric is significantly higher than that of the plain weave fabrics.Especially in the central tearing test,the warp-knitted fabrics are obviously stronger than the plain weave fabrics in terms of the tearing strength owing to the yarn gathered.Therefore,the coated warp-knitted fabrics have gradually become an important body material for large-span inflatable membrane structures.Due to natural aging and windborne debris,some tiny cracks and holes would be inevitably generated on the fabrics’surface.Under the initial prestress and extreme conditions,crack propagation can be easily induced from the stress concentrates in the vicinity of the crack tips and eventually causes the overall failure.Catastrophic failure on account of crack propagation is one of the primary failure modes for architectural membrane structures.Furthermore,there is a certain off-axial angle between the principal axis and the yarn orientation in the actual engineering applications.Accordingly,it is necessary to deeply investigate the mechanical properties of coated warp-knitted fabrics in the different off-axial states.In this paper,a typical warp-knitted fabric was taken as the study object,and its mechanical properties were investigated by combining experimental and numerical simulation methods.The off-axial tensile tests of warp-knitted fabric were conducted.Based on the digital image correlation technique(DIC),the distribution of strain field under critical failure state was obtained.The central tearing failure mechanism of warp-knitted fabric was explored by combining DIC tests and numerical simulation methods.Based on the microscopic numerical models of biaxial central tearing,the effect of tensile and shear stress on the tearing mechanical properties was analyzed.The main works of article include:Firstly,the warp-knitted fabric was selected as the experimental material,and its uniaxial tensile tests under seven typical off-axial angles were conducted.The effects of off-axis angles on mechanical properties and failure modes were investigated.Based on the VIC-2D system of DIC technology,the distribution of the strain field of the warp-knitted fabric under different off-axial angles was explored.The failure mechanisms and failure modes of the warp-knitted fabric were studied.The results show that the yarn orientation of warp-knitted fabric has a significant effect on mechanical properties.For on-axial specimens,the distribution of the strain field of the warp-knitted fabric in the lateral direction is uniform,and the area of maximum strain occurs at the edge of the specimen.For small off-axial specimens,the direction of distribution of the strain field is consistent with the direction of the off-axial angle.The strain field presents a banded distribution along the weft yarn direction.For large off-axial specimens,the strain field of the specimens presents two banded distributions related to the 450 off-axial angle.Secondly,a series of uniaxial central tearing tests for seven off-axial angles,including 0°,15°,30°,45°,60°,75°,and 90°,and the development of microscopic numerical models were performed on a typical warp-knitted fabric.The variations of failure morphologies and tearing strengths of the warp-knitted fabric with the increase of off-axial angles were systematically analyzed.And combined with experimental and numerical results,the influences of off-axial angles on tearing strength and the micro-level load-carrying capacity and deformation mechanisms of yarns were further investigated.Based on the DIC technology,the distributions of the strain and displacement field were investigated.The results show that the yarn orientation of warp-knitted fabric has a significant effect on the direction of slit propagation and opening shape.As off-axial angles increase,a "W" shaped relationship between critical tearing strength,ultimate tearing strength and off-axial angle is presented,and an inverted "V" shaped relationship between ruptured displacement,failure displacement and off-axial angle is revealed.The yarn orientation of warp-knitted fabric also has a significant influence on the range of stress concentration and stress distribution.During the tearing failure process,the phenomenon of stress concentration in the vicinity of the crack tip is obvious.Finally,a microscopic numerical model was established to simulate biaxial central tearing failure of warp-knitted fabric under different loading stress ratios,including 1:3、1:2、1:1、2:1、3:1(warp:weft),and different slit angles,including 0°,30°,45°,60°,and 90°.The failure mechanisms and the direction of slit propagation under different loading stress ratios and slit angles were explored.The effects of tensile and shearing stress on the tearing mechanical properties and tearing strength were discussed,and a parametric analysis was carried out.The results show outstanding feasibility and accuracy of the established FE model on simulating the tearing behaviors of warp-knitted fabric.The direction of slit propagation is always perpendicular to the direction of the maximum tensile stress of the yarn.Besides,the load-carrying capacity of warp and weft yarns is closely concerned to the direction of the slit propagation.The tensile and shearing stress has a significant effect on the tearing mechanical properties.