| Kevlar fiber is widely used in many fields,such as aerospace industry,shipbuilding industry,automobile industry,sporting goods,etc.due to its high strength,low density and high temperature resistance.The military protective equipment made of Kevlar materials not only has tough "armor",but also the mobility and flexibility inconsistent with its appearance,which has saved countless soldiers’ lives on the battlefield.The smart structure based on the characteristics of twisted winch has advantages of simple structure,fast response,no lag,etc.,which makes it become one of the hotspots in the field of smart structure.In 2014,Haines et al.published research results on artificial muscle preparation by twisted in Science,and pointed out that the research on mechanical and thermal characteristics of twisted structure is the foundation of understanding the excellent performance of such smart materials.Subsequently,many researchers focus on the prediction of the equivalent Young’s modulus and the analysis of the coefficient of thermal expansion of this kind of structure,and have obtained a series of important research results.However,as to the twisted structure,there is usually a kind of untwisting phenomenon during the process of external tension or temperature rising,which reduces the mechanical properties of the whole structure and affects its mechanical response and output.This master’s degree thesis is aimed at Kevlar fibers and its twisted structure with wide engineering application.Based on clarifying the mechanical characteristics of Kevlar fibers,the experiment and numerical simulation of the untwisting phenomenon of twisted structure in the process of tension and temperature rising are carried out.The main works are as follows:Firstly,the mechanical properties of Kevlar fiber and fiber bundle are tested at room temperature and under 120℃.It is found that Kevlar 29 fiber is brittle material.With the increasing of temperature,the modulus of Kevlar 29 fiber will not change obviously,but its tensile strength will decrease with the increasing of temperature.Under the experimental conditions in this paper,it is observed that Kevlar 29 fiber will undergo secondary strengthening when the strain is 1.6%.For Kevlar fiber bundle,it is found that the modulus decreases with the increasing of temperature.Secondly,the torsion behavior of the Kevlar twisted structure during the heating process has been measured by experiments and been simulated by finite element method.The Kevlar fiber bundle produced by DuPont is used to make second-stage and third-stage twisted structures as the experimental samples in this paper.An experimental platform is designed for the real-time measurement of the twist angle change of the twisted structure during the heating process,and the reliability of the experimental platform is verified.Based on the experimental platform,the torsion behavior of Kevlar first-stage,second-stage and third-stage twisted structures with the increasing of temperature has been measured,and the trend of torsion angle with the change of temperature is obtained.The results show that as the temperature rises,the first-stage twisted structure will be untwisted,the second-stage twisted structure will be twisted,and the third-stage twisted structure will untwist.And it is also found that the three structures have the same torsion trend when the temperature is decreasing and increasing.The first-stage and second-stage twisted structure models are established,and the untwisting behavior of the Kevlar twisted structure during heating is calculated by finite element numerical calculation.It is found that the change of the twist angle of Kevlar twisted structure is a follow-up relationship with the temperature without considering the interaction between the fibers,and the Kevlar twisted structure appears the untwisting behavior with increasing temperature.The qualitative explanation of the twisting phenomenon observed in the second-stage twisted structures experiment is given.Finally,the finite element numerical calculation and analysis are carried out for the untwisting behavior of the twisted structure in the process of tension.According to the existing experimental results of tension and torsion of the twisted structure in our group,the corresponding beam-shell structure models are established,and the numerical results are compared with the experimental measurement results.It is found that the numerical calculation results are in good agreement with the experimental measurement results.The untwisting angle of per unit length of the three-strand structure will increase linearly with the increasing of the axial tension.For the three-strand structure with the same diameter,when the axial tension is the same,the larger the initial helix angle is,the smaller the untwisting angle per unit length is.The influence of Young’s modulus on the untwisting behavior has also been analyzed. |
PDF Full Text Request