Date of Award

Spring 2014

Degree Type

Thesis

Degree Name

Master of Science (MS)

Department

Mechanical Engineering

First Advisor

Marisol Koslowski

Committee Member 1

Byron Pipes

Committee Member 2

Alejandro Strachan

Abstract

Glassy polymers are very important in industrial and commercial components manufacturing. For instance, it is used to manufacture polymer-matrix composites, the type of composite used in the Dreamliner 787. But this kind of material is also limited by its tendency to fail in brittle manner. In this study, we use phase field model to characterize the yielding condition of glassy amorphous polymer under a wide range of loading conditions. The main failure mechanisms in amorphous glassy polymers, such as poly-methyl-methacrylate (PMMA), are crazing and shear yielding. Crazing is formed because of the nucleation of micro-voids in the location of stress concentrations and shear yielding plastic deformation in the form of shear band.

A Phase field model based on Griffith's theory is used to model the response of material under different loading condition ranging from pure deviatoric to isotropic volume expansion. The benefits of phase field modeling is that it can reduce the implementation complexity as it does not need to track the discontinuities in the displacement field. The model predictions for the macroscopic volumetric and deviatoric stress-strain response and phase field contour of PMMA are then compared with experimental data reported in the literature and atomistic simulation's results whose yielding criterion is energy-based and does not make assumption on whether the deformation is dominated by deviatoric strain or volumetric strain but the rate of mechanical work per unit volume. Our results shows that phase field model can the response of crazing behavior accurately in three dimensions. The outcome indicate that a 27% reduction of phase field damage parameter will achieved before the crazing emerges and the material yielding for any loading condition.

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