Doctor of Philosophy (PhD)
Civil and Environmental Engineering
In material science, size effects is described as the variation of material properties as the sample size changes. In this dissertation, the size dependency of the material strength is addressed as size effects. The size effects underlying mechanisms depend on the nature of the considered material. In the case of crystalline metals, size effects in crystalline metals are governed by the dislocations, as the primary deformation mechanism, and their interactions with one another and other defects such as grain boundaries. In this dissertation, the size and strain rate effects of fcc metallic samples of confined volumes are investigated during the nanoindentation and pillar compression experiments using large scale atomistic simulations. Examples of possible benefits include better understanding, controlling, and accelerating the development in new micro- and nano- technology such as microelectromechanical systems (MEMES), nano-coatings, thin films, nanocomposites, ultrafine grain bulk materials, and multilayer systems. First, the effects of different boundary conditions on the simulation of nanoindentation are investigated using Molecular Dynamics (MD). Next, the available theoretical models of size effects during nanoindentation are evaluated using atomistic simulation. In the next step, the MD simulation is incorporated to investigate the governing mechanism of size effects in a nanoscale single crystal Ni thin film during indentation. The effects of grain boundary (GB) on the sources of size effects are then investigated during the nanoindentation test. In the next step, the different mechanisms of size effects in fcc metallic samples of confined volumes are studied during high rate compression tests using large scale atomistic simulation. Different mechanisms of size effects, including the dislocation starvation, source exhaustion, and dislocation source length effect are investigated for pillars with different sizes. Furthermore, the size and strain rate effects are then investigated using the observed dislocation length distribution. Finally, the hardening mechanisms in fcc metallic structures during high rate deformations are studied by incorporating the dislocation network properties.
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Yaghoobi, Mohammadreza, "Modeling of Size Effects in Metallic Samples of Confined Volumes" (2017). LSU Doctoral Dissertations. 4487.
Voyiadjis, George Z.