Investigation On The Static And Dynamic Mechanical Properties In Twisting Carbon Nanotube Ribbon

Twisted carbon nanotube(CNT)fibers could be used as actuators and artificial muscles owing to their excellent mechanical properties,which have potential applications in intelligent medical and safety protection fields.However,the twisting influences on the mechanical properties of CNT fibers and their composites are still unclear,especially on the evolution of microstructure and timedependent properties.In this work,combined with the numerical and theoretical methods,the dynamic and static mechanical experiments were designed to systematically investigate the effect of twisting on the mechanical properties of CNT ribbon.The in-situ scanning electron microscope(SEM)mechanical device was used to study the static tensile properties of CNT assemblies,and the microstructure deformation mechanism of the stiffening and toughening effect of the twisted fiber were revealed.The surface angle of twisting CNT fiber would rotate due to the non-uniformity in the tensile process,and the torsion energy generated during twisting and stretching would be released completely after the fracture of fiber.The adhesive increases the load transfer efficiency,modulus,and failure strength of the composite CNT fibers.In order to investigate the high strain rate behavior of CNT assemblies,the fibers were tested by micro-dynamic tensile equipment and high-speed photography.The experimental results show that the untwisted CNT ribbon revealed a higher strain rate sensitivity than that of the twisted CNT fiber.This is mainly due to the inhomogeneity structure and the interlaced CNT network.The increasing strain rate will improve the failure strength and reduce the failure strain.Adhesive tends to block the pull out of CNTs,resulting in brittle fracture mode of composite CNT fiber under dynamic loading.The rate sensitivity of the adhesive and the denser structure also affect the rate sensitivity coefficient of the composite CNT fiber.The numerical model of twisting thin film was established,and the results show that different twisting boundary conditions will lead to different mechanical parameters.The film gradually shrinks while twisting under the free axial displacement boundary,and under the fixed boundary the internal stress gradually increases with twisting due to the absence of spatial contraction.At the initial twisting stage,the interface friction does not play an important role because the inner walls of film do not contact each other,however,at the later twisting stage,the larger friction coefficient will enhance the interface interaction and lead to higher strain energy.The enhancement of interface friction can improve the stability of the microstructure in the tensile process.Moreover,the simulation results considering plasticity and failure behavior also indicate that the shrinking axial displacement during twisting inclines to increase the tensile toughness of fibers.The rate-dependent of twisting fibers mainly depends on the twisted microstructure and the mechanical behavior of film.The microstructure adjustment during twisting and stretching has a significant time-dependent behavior and causes variable internal stress and failure modes.The internal stress of fixed axial displacement twisting film was theoretically calculated considering elastic mechanical behavior,which shows good repeatability with that of numerical simulation results.The relationship between the axial internal stress and the shrinkage displacement under two different twisting boundary conditions was obtained.At the same time,the relationship between the axial internal stress and the surface angle during the twisting process was derived considering the elastic/plastic mechanical behavior of the film.Furthermore,based on the Frenet frame,the helical structure model was simplified and the tensile stress-strain constitutive relation of the twisted structure was calculated.Single and multiple stress relaxation experiments were designed to investigate the time-dependent properties of untwisted CNT ribbon and CNT fiber.The results show that the stress relaxation degree of CNT ribbon is higher.The viscoelastic model is consistent with the experimental curve.The timedependent tensile and relaxation stress of the twisted fiber were analyzed,and the stress-strain relationships of the CNT ribbon/fiber were obtained.The stress relaxation behavior of multi-CNT fibers would be limited due to more interface constraints.