PhD Defence at DTU Mechanical Engineering

PhD Defence Friday 11th November: Micro-Structural Evolution and Size-Effects in Plastically Deformed Single Crystals

Friday 28 Oct 16

Salim A. El-Naaman from DTU Mechanical Engineering defends his PhD, "Micro-Structural Evolution and Size-Effects in Plastically Deformed Single Crystals" Friday 11 November 2016 at 13:00. The defence takes place in Building 421, Auditorium 074, at the Technical University of Denmark. Professor Christian F. Niordson is supervisor, and Associate Professor Kim Lau Nielsen is co. supervisor.

Abstract
Extensive research has gone into the development of micro-mechanics based gradient plasticity continuum theories, which are necessary for modeling micron-scale plasticity when large spatial gradients of plastic strain appear. While many models are successful in capturing the macroscopic effects related to strain gradients, most predict smooth microstructures. The evolution of dislocation micro-structures, during plastic straining of ductile crystalline materials, is highly complex and nonuniform.

Published experimental measurements on deformed metal crystals show distinct pattern formation, in which dislocations, of the geometrically necessary kind, are arranged in wall and cell structures. This particular subset of signed dislocations, which have a net Burgers vector, are the main source for the observed size-effects and are directly linked to the gradients in plastic strain. It is clear that many challenges are associated with modeling dislocation structures, within a framework based on continuum fields, however, since the strain gradient effects are attributed to the dislocation micro-structure, it is a natural step, in the further development of gradient theories, to focus on their ability to capture realistic micro-structural evolution.

This challenge is the main focus of the present thesis, which takes as starting point a non-work conjugate type back stress based higher order crystal plasticity theory. Within this framework, several possibilities for the back stress relation are formulated, based on a postulate of a gradient energy as well as by engaging in a phenomenological approach. Through an extensive numerical investigation, the proposed back stress formulations are shown to o er novel modeling capabilities both in terms of micro-structural predictions but also in terms of capturing complex macroscopic behavior tied to the presence of long range internal stresses.

A formulation based on a near linear gradient energy reveals striking similarities to formulations based on discrete dislocation theory, and shows promising capabilities within the adopted higher order theory. Moreover, the present work offers new insight into plane strain modeling of face centered cubic crystals.