Investigation Of Dynamic And Quasi-static Mechanical Properties Of Carbon Nanotube Fibers

As a new one-dimensional material,carbon nanotube fibers have attracted much attention due to their lightweight and strong mechanical properties.Recent studies have shown that carbon nanotube fibers have potential applications in the fields of military defense,electronic engineering and bionic medicine.As carbon nanotube fiber materials inevitably need to be under complex loading conditions in engineering applications,high requirements are put forward for their strength,stiffness,crash-resistance and mechanical stability.Therefore,this paper studies the tensile deformation and failure behavior of carbon nanotube fibers with different helical degrees under different loading strain rates.Based on in-situ SEM and micro Hopkinson bar devices,analyzing the influence of surface microstructure and loading mode on the ductile and brittle conversion mechanism and fracture mode of carbon nanotube fibers.Firstly,a quasi-static continuous tensile test was carried out on carbon nanotube fibers with different surface twist angles by using a micro-scale tensile testing machine with a constant loading rate.The effect of surface twist angle on the elastoplastic conversion mechanism and failure mode of carbon nanotube fibers was analyzed by comparing the stress-strain curves of the fibers.Meanwhile,with the aid of stress relaxation experiment,the time-varying mechanism of fiber viscosity was analyzed,and the time-dependent behavior of fiber surface microstructure in the process of deformation evolution was verified by means of in-situ SEM tensile device.In addition,the strain rate-sensitive behavior of carbon nanotube fibers was studied in the range of low strain rate(10-5s-1?0.1 s-1).The results showed that the larger the surface twist angle of the fibers,the lower the tensile modulus,the greater the degree of stress relaxation,and the more obvious the strain rate effect,which had obvious twist angle-dependent strain rate effect.Secondly,an improved 6mm Hopkinson bar system suitable for micron scale fiber materials was set up in this paper to study the impact resistance of carbon nanotube fibers.The dynamic tensile failure behavior of single-stranded carbon nanotube fiber was tested,and the strain rate effect and ductile-brittle conversion mechanism were analyzed by comparing with the quasi-static experimental results.The interfacial interaction of the fiber was discussed by stretching the twin-stranded carbon nanotube fiber.Finally,based on the experimental results,the empirical formula related to the twisting angle was established,and fitting the statistical results of the fracture stress and strain of the fiber.The competitive effects of densification strengthening and off-axis weakening caused by twist were analyzed theoretically.Taking into account the surface twisting angle,time dependent behavior of microstructure and strain rate effect,a three-component viscoelastic model was established by means of numerical method,and a stress-strain equation was obtained to predict the fracture strength,deformation behavior and nonlinear response of carbon nanotube fiber.