The Young's modulus, strength, and toughness of nanostructures are important to proposed applications ranging from nanocomposites to probe microscopy, yet there is little direct knowledge of these key mechanical properties. Atomic force microscopy was used to determine the mechanical properties of individual, structurally isolated silicon carbide (SiC) nanorods (NRs) and multiwall carbon nanotubes (MWNTs) that were pinned at one end to molybdenum disulfide surfaces. The bending force was measured versus displacement along the unpinned lengths. The MWNTs were about two times as stiff as the SiC NRs. Continued bending of the SiC NRs ultimately led to fracture, whereas the MWNTs exhibited an interesting elastic buckling process. The strengths of the SiC NRs were substantially greater than those found previously for larger SiC structures, and they approach theoretical values. Because of buckling, the ultimate strengths of the stiffer MWNTs were less than those of the SiC NRs, although the MWNTs represent a uniquely tough, energy-absorbing material.

http://science.sciencemag.org/content/sci/277/5334/1971.full.pdf

Nanobeam mechanics: Elasticity, strength, and toughness of nanorods and nanotubes

Carbon nanotubes subject to large deformations reversibly switch into different morphological patterns. Each shape change corresponds to an abrupt release of energy and a singularity in the stress-strain curve. These transformations, simulated using a realistic many-body potential, are explained by a continuum shell model. With properly chosen parameters, the model provides a remarkably accurate ``roadmap'' of nanotube behavior beyond Hooke's law.

/pdf/nanomechanics-of-carbon-tubes-instabilities-beyond-linear-c39bdv9fp7.pdf

Nanomechanics of carbon tubes: Instabilities beyond linear response.

The curling of a graphitic sheet to form carbon nanotubes produces a class of materials that seem to have extraordinary electrical and mechanical properties. In particular, the high elastic modulus of the graphite sheets means that the nanotubes might be stiffer and stronger than any other known material, with beneficial consequences for their application in composite bulk materials and as individual elements of nanometre-scale devices and sensors. The mechanical properties are predicted to be sensitive to details of their structure and to the presence of defects, which means that measurements on individual nanotubes are essential to establish these properties. Here we show that multiwalled carbon nanotubes can be bent repeatedly through large angles using the tip of an atomic force microscope, without undergoing catastrophic failure. We observe a range of responses to this high-strain deformation, which together suggest that nanotubes are remarkably flexible and resilient.

Bending and buckling of carbon nanotubes under large strain

We report high resolution electron microscope (HREM) observations and atomistic simulations of the bending of single and multi‐walled carbon nanotubes under mechanical duress. Single and multiple kinks are observed at high bending angles. Their occurrence is quantitatively explained by the simulations, which use a realistic many‐body potential for the carbon atoms. We show that the bending is fully reversible up to very large bending angles, despite the occurrence of kinks and highly strained tube regions. This is due to the remarkable flexibility of the hexagonal network, which resists bond breaking and bond switching up to very high strain values.

/pdf/structural-flexibility-of-carbon-nanotubes-3phmvtqn2y.pdf

Structural flexibility of carbon nanotubes

We have conducted pulling and bending tests on individual carbon nanotubes in-situ in a transition electron microscope. Based on our observation of the force required to break the tube, a tensile strength of 0.15 TPa was computed. From corresponding bending studies on such nanotubes, the Young's modulus was estimated to be 0.9 TPa (0.8 TPa after ‘sub continuum’ corrections). These results suggest a strength that is a large fraction of the elastic modulus, although previous measurements of their elastic stiffness have yielded higher modulus values, by as much as a factor of 2. The result does indicate that individual nanotubes can fail as essentially defect-free materials. Furthermore, we observed no obvious reduction in cross-sectional area prior to the failure. In addition, the bending experiments revealed a remarkable flexibility in these tubes. These unique properties support the potential of nanotubes as reinforcement fibers in structural materials.

E. Daguerre

Papers

Flexibility of graphene layers in carbon nanotubes

Dynamic bond length as an explanation for the flexibility of graphene layers