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UBC Theses and Dissertations

Nonlocal continuum shell models for torsion of single-walled carbon nanotubes Khademolhosseini, Farzad


Carbon nanotubes (CNTs) have attracted much attention from scientists and engineers because of their relevance to a wide range of applications. Various approaches have been used for the characterization of CNT properties, among which continuum modeling has generated much interest due to computational efficiency. However, at the nanoscale the dimensions of a system are comparable to the inter-atomic or inter-molecular spacing of that system, and the material cannot be modeled as a continuum. This is known as the “size-effect”. To overcome the limitations of classical continuum mechanics, modified continuum models have been proposed, among which models based on the concept of nonlocal elasticity have proven effective in quantifying the size-dependent mechanical response of CNTs. This thesis investigates the “small-size” effects in the torsional response of single walled carbon nanotubes (SWCNTs) by developing a modified nonlocal continuum shell model for their torsion. The purpose is to facilitate the design of devices based on CNT torsion by providing a simple, accurate and efficient continuum model that can predict the torsional buckling loads, the frequency of torsional vibrations and the propagation speed of torsional waves. To this end, Eringen’s equations of nonlocal elasticity are incorporated into the classical models for torsion of cylindrical shells given by Timoshenko and Donnell. In contrast to the classical models, the nonlocal model developed here predicts non-dimensional buckling loads that depend on the values of certain geometric parameters of the CNT, allowing for the inclusion of size-effects. In the case of torsional vibrations and propagation of torsional waves, the classical and nonlocal models predict non-dispersive and dispersive behavior, respectively. Molecular dynamics simulations of torsional buckling, axial buckling and torsional vibration of various SWCNTs are also performed, the results of which are compared with the classical and nonlocal models and used to extract consistent values of the nonlocal elasticity constant. Interestingly, the nonlocal elasticity constant depends on the existence of circumferential and/or longitudinal modes in the deformed shape of the CNT. In all loading cases the superiority of the nonlocal model over the classical elasticity model in predicting the size-dependent mechanical response of SWCNTs is established.

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