Simulation of mechanical response of carbon nanotube yarns under uniaxial tensile loading

Abstract

Carbon nanotube (CNT) yarns (CNTYs) are microscopic assemblies of carbon nanotubes in the form of continuous fibers that have significant technological applications. Their applications as individual fibers or embedded in polymer composites demand tensile load bearing, but the mechanical response of CNTYs is yet not well understood. A sequential, three-step, multiscale finite element (FE) modeling approach is proposed to predict the stress-strain response of CNTYs under uniaxial tensile loading. The modeling approach is based on experimental observations, which motivates an idealized concept of the hierarchical structure of CNTYs discretized into three hierarchical levels: (i) nanoscale model of bundles comprising CNTs, (ii) mesoscale model of CNT fibrillars formed by bundles, and (iii) microscale model of the CNTYs comprising twisted CNT fibrillars. The results from each level feed the simulation of the upper level. The inherent randomness in the plexiform structure of CNTYs is addressed through the Monte Carlo method, and the effect of porosity and twist level are also included for the first time in modeling the CNTY. The simulations predict that the elastic modulus of the bundle is fairly independent of its diameter but the yield stress and strength decrease with increasing bundle diameter. The aspect ratio of the individual CNTs comprising the bundles is also shown to be a key factor determining the CNTY strength. The simulation results show that considering an orthotropic response at the microscale level provides stress-strain curves that match the experimental results. This model of the stress-strain response of CNTYs elucidates the factors affecting that response and provides critical information towards their use in a variety of applications.