We present several multiscale quantum mechanical (QM), molecular mechanical (MM), and continuum mechanical (CM) schemes to study the strength properties of carbon nanotubes (CNTs) and graphene sheets. A bridging domain method based on overlapping domain-decomposition schemes using the Lagrange multipliers field is developed to couple an atomistic domain with a finite element domain. The ONIOM is used to couple the quantum mechanical model with the molecular mechanical model. We have also developed a minimal overlap QM/MM coupling scheme, where the part of the region that is treated by quantum mechanics does not require MM calculations, which makes the method suitable for systems containing localized regions that cannot be modeled by available empirical potentials. This method is called quantum to molecular mechanical overlapping domain (QtMMOD) method. In these methods, we have scaled the MM potential by a scaling factor which depends on the applied strain and matches the MM energy to the QM energy to avoid certain unphysical behavior and match the QM and MM models more closely. Fracture properties of CNTs with large defects have been calculated and it is suggested that these large defects were the reason behind the low experimental strengths of CNTs. It was found that even at nanoscale, the fracture strengths of materials decrease as the flaw size increases and agree relatively well with the Griffith theory. We have also developed an extended-finite element method for studying dislocation motion in CNTs to model the super-plasticity phenomenon observed experimentally at high strain and temperatures.

Last modified

• 09/06/2018

Creator

• Roopam Khare

DOI

• https://doi.org/10.21985/N2WB2S

Subject

• Mechanical Engineering

Keyword

• Engineering
• Mechanical

Date created

• 2008-04-23

Resource type

• Dissertation

Rights statement

• In Copyright