In this work, epoxy resins were modeled using all atom representations in nanoscale simulation boxes. Tetrafunctional epoxy and corresponding multifunctional amine were chosen as model materials. Algorithms of constructing interconnected network structures were invented developed to properly account for the chemical structures and computational cost. Monomers were generated in diamond lattice and crosslinked to model complex epoxy multifunctional network. The initial configurations were relaxed and equilibrated using molecular dynamics and suitable force field. Physical, thermal and mechanical properties resulting from equilibrated simulation box are in good agreement with experimental results. Possible impact of chemical degradation was studied by adopting oxidation and hydrolysis algorithms. Mechanism of degradation was based on bonds reaction probability and chemical structures of epoxies. Both oxidation and hydrolysis were found to decrease materials performances by reducing number of crosslinking points. Elastic modulus of materials was directly related to crosslinking density. Interfaces between two types of epoxies were constructed to study interactions at interfaces. Covalent bonds linking two components play an important role in interfacial strength. Free volume calculation helps to identify and monitor nucleation of crazes and voids within materials. It was found voids and cracks prefer to initiate and grow at 2 interfaces and lead to failures. Additional compatibilizer layers can improve overall composite performances by preventing void growth at interfaces. Diffusion pattern of water in epoxy resins was studied by tracking displacement of single molecules during certain time intervals. The characteristic of water diffusion in epoxies was interpreted by free volume theory. Reactive force field was introduced to study thermal degradation behavior of epoxy resins. Number of molecules and variation of different types of covalent bonds during heating processes were tracked and analyzed to uncover the degradation mechanism of epoxy resins.

During high speed cutting processes, metals are subject to high strains and strain rates. The dynamic nature of the deformation during high speed cutting makes it difficult to detect atomic scale deformation mechanisms experimentally. Atomic scale plasticity behaviour is often studied using various micromachining techniques such as micropillar compression testing, nanoindentation, and nanoscratching. However, strain rates in micromachining experiments are far lower than those seen during high speed cutting. Atomistic simulations can be used to study high strain rate plasticity at nanometre length scales. In this thesis, we present results from molecular dynamics simulations of plasticity in nanostructures. Results from simulations of uniaxial strain of both bcc and fcc nanopillars are presented. We find that the outcomes of these uniaxial strain simulations depend sensitively on the initial configurations of the systems. In particular, the choice of crystallographic surfaces on the faces of the pillars and the means by which strain is implemented in the simulations can affect the simulation results. We find that the twinning anti-twinning asymmetry in bcc materials causes nanopillars to deform by dislocation glide in compression and by twinning in tension. This explains the compression tension asymmetry reported experimentally in bcc micropillars. We find that deformation is mediated by glide of shockley partials in fcc pillars for compressive and tensile strains. Simulations of pure shear of nanocrystalline Fe are also presented. We find a change in deformation mechanisms for this system when at high temperatures. At low temperatures, plasticity is mediated in part by dislocation glide and twinning. However, at temperatures above 1200K the deformation is dominated by grain boundary sliding, recrystallization, and amorphization.

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published_or_final_version / Physics / Doctoral / Doctor of Philosophy

published_or_final_version / Mechanical Engineering / Doctoral / Doctor of Philosophy

published_or_final_version / abstract / Mechanical Engineering / Doctoral / Doctor of Philosophy

No description available.

No description available.

No description available.

No description available.