Numerical Simulation Of Static And Dynamic Mechanical Behaviors Of Helical Auxetic Yarn

Textile composite materials are widely used for protection against dynamic loading.Introducing negative Poisson’s ratio effect into these materials can enhance their shear resistance,indentation resistance,fracture toughness and energy absorption capability,hence further improving their protection performance.In recent years,a special type of composite yarn with negative Poisson’s ratio has received much attention.Being constructed by helically wrapping a high modulus yarn on a low modulus yarn,it expands in the transverse direction upon stretching in the longitudinal direction,and hence is vividly called helical auxetic yarn(HAY).Experiments have shown that fabric made of this composite yarn has excellent blast and ballistic resistance.However,the existing literature on HAY is limited to quasi-static research.In this thesis,by using finite element simulation and starting from the quasi-static study,we carried out for the first time a study on the dynamic behavior of shock loaded HAY.In the quasi-static analysis,we solved the problem of nonconvergence of the calculation results caused by a variety of factors.The results verify the radial expansion phenomenon of HAY under quasi-static longitudinal stretching.The engineering elastic moduli under different stretching degrees were obtained,and the change of the tensile stress-strain relation with the loading degree was studied.On this basis,the influence of the material properties and geometric parameters on the auxetic effect was discussed.The dynamic simulation results show that under longitudinal shock loading,two longitudinal waves(combined waves in the strict sense)are produced in the wrap yarn and the core yarn,respectively,and between the two wave fronts,there is an amazing phenomenon of periodic transverse expansion.The energy of the HAY is mainly distributed in the region behind the longitudinal wave front in the core yarn which plays a dominant role in energy absorption.The wave speed of the wrap yarn increases with increasing loading speed,while that of the core yarn decreases with increasing loading speed.Material properties and geometric parameters have a great influence on the wave speeds.Under transverse impact,two longitudinal waves and an observable transverse wave are produced in the HAY;the structure of the longitudinal waves is similar to the longitudinal shock case.Due to the interaction between the core yarn and the wrap yarn,the angle γ of the core yarn which characterize the transverse wave decreases,and the influence of the bending wave is reduced,which is conducive to increasing the ballistic limit speed of the core yarn.The quasi-static simulation results in this thesis can provide some guidance for the optimal design of HAY.The dynamic simulation results,through preliminarily revealing the mechanism of HAY’s shock resistance,can provide some reference for the application of this structure in the protection against dynamic loading.