Brittle and Ductile Behavior in Carbon Nanotubes

The field of carbon nanotubes has seen an explosive growth in recent years due to the substantial promise of these quasi-1D structures for potential uses as highstrength, light-weight materials, super-strong fibers, novel nanometer-scale electronic and mechanical devices, catalysts, and energy storage media. Despite the potential impact that new composites based on carbon nanotubes could have in many areas of science and industry, a full characterization of their mechanical properties, and ultimately of their strength, is still lacking. Carbon nanotubes have already demonstrated exceptional mechanical properties: Their excellent flexibility during bending has been observed experimentally and studied theoretically [1 ‐ 3]. Their high stiffness combines with resilience and the ability to buckle and collapse in a reversible manner: even largely distorted configurations (axially compressed, twisted) can be due to elastic deformations with virtually no atomic defects involved. [1,2,4,5] In this Letter we focus on the occurrence of mechanical failure in carbon nanotubes under a tensile load, which leads to the emergence of novel, unforeseen patterns in plasticity and breakage. Because of its hexagonal symmetry, a graphite sheet (graphene), the basic constituent of carbon nanotubes, has three equivalent directions with respect to the application of an external planar tension. We call “longitudinal” the tension that is applied parallel to one of the C-C bond directions, and “transverse” the one that is applied normal to it. Once the planar sheet is rolled into a nanotube, the case of the transverse tension corresponds to the application of tensile strain to an armchair tube, while the longitudinal case corresponds to the application of tensile strain to a zigzag tube. Our study, based on the extensive use of classical, tight-binding and ab initio molecular dynamics simulations, shows that the different orientations of the carbon bonds with respect to the strain axis lead to completely different scenarios: ductile or brittle behaviors can be observed in nanotubes of different symmetry under the same external conditions. Furthermore, the behavior of nanotubes under large tensile strain strongly depends on their symmetry and diameter. Several modes of behavior are identified, and a full map of their ductile-vs-brittle behavior is presented. Beyond a critical value of the tension, an armchair nanotube in “transverse” tension releases its excess strain via spontaneous formation of topological defects. A transverse tension finds a natural release in the rotation