Atomistic Modeling Predictions Of Crack Tip Behavior In Silicon Carbide

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This dissertation composes three papers detailing work intended to model the crack tip behavior in silicon carbide (SiC). While having impressive thermal, mechanical, and electrical properties, the utility of SiC as a structural material is often limited by its low fracture toughness. My studies focus on the application of direct molecular dynamics (MD) simulations with empirical potentials, analytic models and electronic structure calculations to illuminate key crack tip mechanisms at low temperature, high temperature and under the influence of impurities. First, I use a multifaceted modeling approach, including electronic structure calculations, MD simulations and analytic models, to illuminate the transition between crack tip mechanisms of single crystal SiC across a range of temperatures and strain rates that govern the intrinsic brittleness of SiC. Then the atomistic predictions are critically examined and compared with published experimental data. The strengths and deficiencies of these analysis tools for predicting crack tip mechanisms in complex covalent materials are discussed in details. Second, I adapt Kohn Sham Density Functional Theory (KSDFT) modeling approach to simulate fracture in SiC under pure mode I and mixed mode loading. In both cases, the crack propagates higher than the Griffith's toughness with a modest lattice trapping effect. Using KSDFT framework to compute the cohesive force across the cleavage plane, the lattice trapping effect as a function mode mixity can be explained. Throughout the work, atomistic simulation predictions are critically compared with published experimental data. Plausible crack tip mechanisms at low temperature and the deficiencies of empirical potential used in MD simulations for predicting crack tip response in SiC are discussed in detail. Third, I perform a series of KSDFT calculations and atomistic simulations to predict the crack tip behavior in the presence of various impurities at a range of impurity concentration. To mimic the impurity effects on decohesion and slip, a strategic bond shielding approximation is employed by modifying the interactions between the crack tip atoms. Depending on impurity type and concentration, I show that impurities can influence several important crack tip mechanics, including cleavage, dislocation kink nucleation and dislocation kink propagation, that ultimately govern the macroscopic fracture toughness of SiC.
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Warner,Derek H.
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Hennig,Richard G.
Zehnder,Alan Taylor
Zabaras,Nicholas John
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Civil and Environmental Engineering
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Ph. D., Civil and Environmental Engineering
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Doctor of Philosophy
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