| As a novel type of building structure,the memebrane structure in the large span buildings,such as gymnasiums,exhibition halls and warehouse buildings,has been widely used due to excellent economics,large span,short construction period,convenient construction,good applicability,outstanding safety,and environmental protection of nodes.The membrane material carrys the primary load and determains the safety of membrane structure,as the main constituent material.Fabric is the most widely used membrane material in membrane structure engineering.Currently,most of the matrix of flexible composites employ the plain weave structure,however,the biaxial warp-knitted fabric has increasingly become the primary material for the air-supported membrane structure in Europe,America and China due to excellent mechanical properties,stability,strength,stiffnessof tensile shear deformation 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.Therefore,it is of great significance to investigate the tearing property of the membrane materials.The biaxial warp-knitted fabric membrane materials used in the membrane structure were taken as the research object,and they were studied by the method of experiment,numerical simulation and physical modeling The main contents include: Completed the central tearing and trapezoid tearing experiments of the biaxial warp-knitted fabric membrane.Established the physical models of central tear and trapezoid tear to investigate the factors that influence the tearing forces and conducted mesoscopic modeling analysis through numerical simulation.The failure mechanism of central tear and trapezoidal tear of the biaxial warp-knitted fabric membrane and the influence of different factors on the tearing performance were studied.Firstly,the central tearing tests under different slit lengths((20mm,30mm))and different bias angles(0°,15°,30°,45°,60°,75°,90°)were conducted.Meanwhile,DIC method was used to investigate the tearing properties of the membrane material under on-axis angles.And the trapezoid tearing tests were conducted under different slit lengths(5mm,15 mm,25mm,35mm))and different bias angles(0°,15°,30°,60°,75°,90°).Results show that the central tearing strength and fracture displacement gradually decrease with the increase of the initial slit length,and the tearing strength shows a "W"-shaped pattern with the increase of the bias angle of yarns.The strain of yarns and shearing effect are closely related to the distance from the slit.On-axist rapezoid tearing strength slightly increases with the increase of the slit length.The off-axis tearing strength decreases with the increase of the slit length,and shows a inverted "V"-shaped pattern with the increase of the bias angle of yarns.Secondly,the physical models of tearing resistance were established for off-axis central tear and off-axis trapezoidal tear,and the material nonlinearity was introduced into the model.And the change of tear bearing capacity and the characteristics of tearing mechanism of the biaxial warp-knitted fabric membrane under two types of tearing forms are analyzed in detail.Results show that the initial bias angle could affect the extent of necking of the central tear.The initial off-axis angle of the yarns and the slit length directly determine the numbers and distribution area of the main loading yarns.The yarns whose ends are fixed(type A area)are important parts of the overall tear resistance.Tear strength is mainly related to factors such as yarn elastic modulus,weaving density,and off-axis angle.Thirdly,numerical studies under series of parameters were conducted on a typical biaxial warp-knitted envelope fabric,and in theses numerical models microscopic structure of yarns and nonlinear characteristics of envelop materials were considered.Using the numerical models verified by corresponding experiments,the bias tearing failure of the fabric material were exposed.Furthermore,the effects of different slit lengths,cutting angles and bias angles on the mechanical behaviors and tear strengths of envelop materials were discussed in detail.Shearing effects are introduced to establish the prediction theory of bias central tearing strengths of envelop materials.Results show that the shape and extent of stress concentration zone in the vicinity of the slit tip are vital factors influencing the slit propagation direction and tear strength,and the evolution of the ellipse shaped zone of stress concentration is obviously affected by the slit lengths and initial bias angles of yarns.The slit lengths and initial bias angle of yarns could interfere the out-of-plane deformation of the membrane surface and the yarns stress level.Factors such as clamping boundary conditions,yarns weaving pattern and matrix wrapping play an important role in the central tearing performance.Finally,numerical studies under series of parameters were conducted on a typical biaxial warp-knitted envelope fabric,and in theses numerical models microscopic structure of yarns and nonlinear characteristics of envelop materials were considered.Using the numerical models verified by corresponding experiments,the bias tearing failure of the fabric material were exposed.Furthermore,the effects of the slit lengths and bias angles on the mechanical behaviors and tear strengths of envelop materials were discussed in detail.Shearing effects are introduced to establish the prediction theory of bias trapezoid tearing strengths of envelop materials.Results show that the shape and extent of stress concentration zone in the vicinity of the slit tip are vital factors influencing the slit propagation direction and tear strength,and the evolution of the airfoil shaped zone of stress concentration is obviously affected by the slit lengths and initial bias angles of yarns.As the slit length increases,the fracture displacement decreases significantly,whereas the on-axis tear strength increases due to the changes of out-of-plane deformation of the membrane surface,slit tip shape and yarns’ stress level.The maximum off-axis tear strength is reduced by 34.4% As the yarns direction could exert complex effects on the appearance of airfoil zone and yarns’ stress level,an inverted “V”shaped relationship is presented between the tear strength and yarn bias angle. |
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