Doctor of Philosophy (PhD)
Civil and Environmental Engineering
This work presents three different studies investigating plastic deformation mechanisms in metals and alloys using crystal plasticity finite element (CPFE) modeling. The first study presents a new nonlocal crystal plasticity model for face-centered cubic single crystals under heterogeneous inelastic deformation. The model incorporates generalized constitutive relations that incorporate the thermally activated and drag mechanisms to cover different kinetics of viscoplastic flow in metals and describes the plastic flow and yielding of single-crystals using dislocation densities. The model is compared to micropillar compression experiments for copper single crystals and clarifies the complex microstructural evolution of dislocation densities in metals. The second study investigates the grain boundary strengthening effect for polycrystalline copper using crystal plasticity and cylindrical indentation simulations. A new nonlocal continuum model is developed that encompasses the heterogeneity in yield strength based on the exponentiated Weibull function and predicts the plastic properties of materials in the micron length scale. The model incorporates the heterogeneity in yield strength and captures the intrinsic size effect using the deformation gradient two-point tensor. The model also incorporates the Hall-Petch strengthening and the homogenization of anisotropic polycrystalline metals into an isotropic effective medium. The third study explores the plastic deformation mechanisms in metal-graphene nanocomposites through a crystal plasticity finite element model. The study shows that the two-dimensional shape of graphene can significantly strengthen metals by controlling dislocation motion. Nanopillar compression tests were simulated using a physics-based model that incorporated surface nucleation and single-arm source dislocation mechanisms. The model consisted of a nanolayered composite with copper grains and monolayer graphene sandwiched between them. The study quantified the accumulation of dislocations at the graphene interfaces and established a Hall-Petch-like correlation between yield strength and the number of embedded graphene layers. The results demonstrate that the two-dimensional shape of graphene can significantly strengthen metals by controlling dislocation motion, leading to the ultra-high strength of the copper-graphene composite. Overall, these studies provide a deep understanding of crystal plasticity and can lead to more accurate predictions of the mechanical behavior of materials.
Jeong, Juyoung, "Physics-Based Crystal Plasticity Model for Predicting Microstructure Evolution and Dislocation Densities" (2023). LSU Doctoral Dissertations. 6067.
Voyiadjis, George Z.
Available for download on Wednesday, March 27, 2024