Study of voids in ductile materials at microscopic level using Discrete Dislocations Plasticity simulations Author Muhammad Usman Registration Number 00000273912 Supervisor Dr. Sana Waheed DEPARTMENT OF

Abstract

Ductile fracture at the macroscopic scale is the collective result of different phenomena, including dislocation motion and voids coalescence, occurring at the nano and micro length scale, respectively. Coalescence of voids, at the microscopic length scale, results in the formation of cracks which ultimately leads to ductile fracture of the material. Many continuum level theories have been proposed to study void nucleation, coalescence and their formation of cracks. These theories are length independent and experimental results are used to incorporate the length scale in these theories. On the other hand, atomistic theories are computationally very expensive. Discrete Dislocation Plasticity (DDP) bridges the gap between the atomistic and continuum level plasticity theories. A number of DDP studies have analyzed the effect of crystal orientation and void size on the rate of void growth. However, the effect of void shapes on their growth, especially elliptical voids, is not studied. The aim of this study is to investigate the effect of void shape on the micro-mechanism of void growth by using Discrete Dislocation Plasticity simulations. For voided crystals, conventional DDP produces a continuous slip step throughout the material even if a dislocation escapes from a non-convex domain. To overcome this issue, the Extended Finite Element Method (XFEM) is used here to incorporate such strong embedded discontinuities. Different aspect ratios of the voids are considered under uniaxial and biaxial deformation boundary conditions. The results suggest that voids having the smaller surface area tend to have low growth rate i.e. “smaller is slower”. Also strain hardening and void growth rate are higher under biaxial loading as compared to uniaxial loading. Furthermore, orientation of slip system as well as that of the void, both strongly affect the plastic behavior of the material. The results of this study provide a deeper understanding of ductile fracture with applications in manufacturing industry, aerospace industry and micro-electronic devices i.e. NEMS/MEMS.