Doctor of Philosophy (PhD)
Engineering Science (Interdepartmental Program)
This work addresses the micro- and nano-behaviors in metals through constitutive modeling and experiments. The size effect encountered during nanoindentation experiments, which is known as indentation size effect (ISE), is investigated for metal materials. The ISE is believed to be related with the strain gradient at small indentation depths. Geometrically necessary dislocations (GNDs) are formed in order to accommodate the strain gradient, which results in the increase in the material hardness. The grain boundaries in polycrystalline materials play an important role on the material hardness during nanoindentation experiments as there is a soften-hardening segment in the curve of hardness as a function of the indentation depth. The grain boundaries act as barriers of the movement of dislocations that leads to an increase of the dislocation density. The increasing dislocation density gives the additional increase in material hardness, resulting in the hardening phenomenon. The classical continuum mechanics has to be enhanced with the strain gradient plasticity theory in order to address the ISE. Using the strain gradient plasticity theory, a material intrinsic length scale parameter is incorporated. The length scale bridges the gap between the behaviors in macro-scale and the micro-/nano-scale. The ISE of different materials can be characterized using this length scale parameter. The length scale parameter is determined through the strain plasticity model, the nanoindetation experiments using continuous stiffness measurement (CSM) mode in determining the fitting parameters and the finite element method (FEM) in determining the equivalent plasticity strain in the expression of the length scale. The rate dependency of ISE is investigated and it shows that the hardness increases with the increasing strain rate. The length scales at different strain rates are determined and they decrease with the increasing strain rate, leading to the increase in the hardness. The grain boundary effect on the hardness is verified as there is no hardening-softening phenomenon in single crystalline materials as there is no grain boundary in them. The grain boundary effect is further isolated by using bicrystalline materials. The single grain boundary is investigated through nanoindentation experiments from different distances to the grain boundary. The results show that the hardness increase as the distance decreases, providing a new type of size effect.
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Zhang, Cheng, "Constitutive Modeling and Experiments for the Micro and Nano Behaviors in Metals" (2015). LSU Doctoral Dissertations. 2422.
Voyiadjis, Geroge Z.