Type of Document Dissertation Author Namilae, Sirish Author's Email Address firstname.lastname@example.org URN etd-02202004-181704 Title Deformation Mechanisms at Atomic Scale: Role of Defects in Thermomechanical Behavior of Materials Degree Doctor of Philosophy Department Mechanical Engineering, Department of Advisory Committee
Advisor Name Title Namas Chandra Committee Chair Keywords
- Grain Boundaries
- Carbon Nanotube
Date of Defense 2004-02-16 Availability unrestricted AbstractThe primary focus of the thesis is to understand the role of defects and interfaces in the deformation of nanoscale structures and systems. Various nanoscale systems such as symmetric tilt-grain boundaries (STGB) in aluminum, topological defects in carbon nanotubes (CNT), hybridization defects in carbon nanotubes and nanoscale interfaces in CNT based composites are investigated using molecular dynamics and statics. In order to further explore the effect of nanoscale interfaces on the macroscopic behavior of CNT based composites a multiscale model, which hierarchically employs molecular dynamics and the finite element method is developed.
Carbon nanotubes are cylindrical structures of carbon wrapped from a planar hexagonal mesh of atoms. Topological defects are planar irregularities in this hexagonal mesh, while hybridization defects are formed when changes in bonding cause out of plane disturbance. The deformation characteristics of CNTs in presence of both these types of defects are modeled using Brennerís potential. The other material systems studied in this work are symmetric interfaces in aluminum. Symmetric tilt-grain boundaries are a type of grain boundaries with restricted degrees of freedom due to symmetry. The sliding behavior, energetics and effect of magnesium doping in these grain boundaries is investigated using embedded atom method (EAM) potentials in the molecular dynamics setting.
Study of deformation has been traditionally under the purview of continuum mechanics; concepts such as stiffness, strength, damage, and fracture are best studied using continuum stress and strain measures. Because of the discrete nature of atoms, these concepts are not clearly understood in atomistic simulations. In this work, different stress measures are employed for Brennerís potential and the criterion for applicability in various conditions is examined. A new methodology to evaluate strains for nanotubes is developed. Local and global deformation characteristics in elastic and inelastic regimes in nanotubes with defects are examined and compared with defect-free nanotubes. It is found that there is a decrease in stiffness of nanotubes in presence of topological defects. The local elastic moduli are found to reduce to 60 % of that of defect-free nanotube. A simple model is developed to predict the reduction in stiffness in presence of a number of defects. In the case of hybridization defects caused by attachment of hydrocarbon functional groups, the elastic modulus is found to improve marginally. In addition, the onset of inelasticity and fracture occur at lower strains in functionalized nanotubes.
Interfaces in composites affect the key mechanical properties such as stiffness, strength and fracture toughness. In this work, interfaces in nanotube based composites are modeled as hydrocarbon chemical attachments between the matrix and CNT. Molecular dynamics simulations of fiber pullout tests are then employed to understand the load transfer behavior and quantitatively determine the interface strength. These results are used to generate traction-displacement constitutive relation for a continuum description of interfaces in terms of cohesive zone model. A multiscale methodology is formulated using the atomically informed cohesive zone model to represent interfaces in a finite element formulation. Application of this approach is demonstrated by examining the effect of interface strength on the stiffness of nanotube based composites.
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