Title

Localization of plastic shear events in glassy materials

Date of Completion

January 2005

Keywords

Physics, Condensed Matter

Degree

Ph.D.

Abstract

An algorithm is introduced for the molecular simulation of constant-pressure plastic deformation in glassy materials at zero temperature. This allows for the direct study of volume changes associated with plastic deformation (dilatancy) in glassy materials. In particular, the dilatancy of polymer glasses is an important aspect of their mechanical behavior. The new method is closely related to Berendsen's barostat, which is widely used for molecular dynamics simulations at constant pressure. The new algorithm is applied to plane strain compression of a binary Lennard-Jones glass. Conditions of constant volume lead to buildup of system pressure with strain, and to a concommitant increase in shear stress. At constant (zero) pressure, by contrast, the shear stress remains constant up to the largest strains investigated (&egr; = 1), while the system density decreases linearly with strain. The linearity of this decrease suggests that each elementary shear relaxation event brings about an increase in volume which is proportional to the amount of shear. In contrast to the stress-strain behavior, the strain-induced structural relaxation, as measured by the self-part of the intermediate structure factor, was found to be the same in both cases. This suggests that in order to overcome the energy barriers, nucleation must continually grow in the case of constant volume deformation, but remain the same if the deformation is carried out at constant pressure. ^ The length scale of the elementary processes of plastic relaxation of amorphous polymers is still an open question. The computer simulation of plastic deformation gives the details of the plastic relaxation events. To study the localization of these events, a novel approach of the correlation of relative atomic strain is invented, in which Delaunay tesselation and Fast Fourier Transforms techniques are applied. Using this novel approach we have studied the localization of atomic strain in discrete relaxation events during plastic deformation of glassy materials. The strain in such relaxation events is highly localized in regions of atomic dimensions. The implications of the novel approach and our simulation results for a universal theory of plasticity of amorphous polymers will be discussed. ^