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Brittle Ductile Transition in Carbon Nanotube Bundles

C.F. Cornwell, J.B. Allen, C. Marsh, T. Carlson, P. Stynoski, B. Newcomb, B. Masters, R. Ebeling, C.R. Welch
U.S. Army Engineer Research and Development Center, US

Keywords: nanotube, brittle, ductile, defects, elastic moduli, failure analysis, statistical methods, tensile strain, mechanical properties, molecular dynamics, molecular mechanics, thermodynamic properties, Van der Waals forces


Carbon nanotubes (CNTs) attract considerable interest because of their extremely high stiffness, strength, and electrical and thermal conductivity. While the strength and stiffness of CNTs can be extremely high, bundles composed of aligned CNTs are far weaker then the constituent CNTs. To understand why this occurs, we performed simulations of neat bundles of aligned CNTs to determine the relationship of the stress-strain properties of the bundles. We ran a series of simulations to investigate the effects of CNT length and cross-link distribution on the tensile response of parallel-aligned CNT bundles. The simulations showed that including cross-link atoms between the CNTs in the strands increased the load transfer between the CNTs by an order of magnitude, preventing them from slipping past one another. The elastic modulus and strength of the bundles were found to increase with the density of the interstitial bonds and the length of the CNTs making up the bundle. Analysis of the post-yield behavior of the bundles indicates that there is a transition from ductile to brittle behavior going from bundles constructed from shorter CNTs and low cross-link counts to bundles constructed from longer CNTs and higher cross-link counts, or, as the number of cross-links per CNT increases.
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