Doctor of Philosophy (PhD)
Department of Civil and Environmental Engineering
In this work, a thermodynamically consistent coupled thermo-mechanical gradient enhanced continuum plasticity theory is developed for small and finite deformations. The proposed model is conceptually based on the dislocations interaction mechanisms and thermal activation energy. The thermodynamic conjugate microstresses are decomposed into energetic and dissipative components. This work incorporates the thermal and mechanical responses of microsystems. It addresses phenomena such as size and boundary effects and in particular microscale heat transfer in fast-transient processes. Not only the partial heat dissipation caused by the fast transient time, but also the distribution of temperature caused by the transition from the plastic work to the heat, are included into the coupled thermo-mechanical model by deriving a generalized heat equation. One- and two-dimensional finite element implementation for the proposed gradient plasticity theory is carried out to examine the characteristics of the proposed strain gradient plasticity model. The derived constitutive framework and the corresponding finite element models are validated through the comparison with the experimental observations conducted on micro-scale thin films. This work is largely composed of three subparts. In the first part, the proposed model is applied to the stretch-surface passivation problem for investigating the material behaviour under the non-proportional loading condition in terms of the stress jump phenomenon, which causes a controversial dispute in the field of strain gradient plasticity theory with respect to whether it is physically acceptable or not. In the second part, two-dimensional finite element implementation for small deformation is performed to investigate the size effects and the grain boundary effect of small-scale metallic materials. In the last part, two-dimensional finite element implementation for finite deformation is carried out to study the size effects during hardening as well as the mesh-sensitivity during softening of the proposed model by solving the shear band problem.
Song, Yooseob, "A Coupled Thermo-mechanical Theory of Strain Gradient Plasticity for Small and Finite Deformations" (2018). LSU Doctoral Dissertations. 4633.