Date of Award

2022

Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy in Mechanical Engineering-Engineering Mechanics (PhD)

Administrative Home Department

Department of Mechanical Engineering-Engineering Mechanics

Advisor 1

Trisha Sain

Committee Member 1

Gregory Odegard

Committee Member 2

Ibrahim Miskioglu

Committee Member 3

Shawn Chester

Abstract

Polymers and their composites (PMC) have emerged as effective alternative materials in structural, aerospace, and automotive industries due to their lightweight and tunable properties compared to metals. However, these materials tend to degrade during their operations in extreme environments. In this work, two extreme conditions are considered: - i) high-temperature oxidative degradation of polymers and polymer-based composites ii) Fracture and damage of polymer-based composites under thermo-mechanical loading.

Polymer oxidation starts when oxygen from the ambient diffuses into the bulk material and initiates chemical reactions to develop a coarse, brittle oxide layer on the exposed surface. The oxidative degradation process is inherently complex in nature, as it involves a coupling between diffusion, reaction, and mechanics. As oxygen diffuses into the polymer, a series of chain reactions occur, resulting in residual shrinkage strain on the oxidized layer of the material due to escaping of the volatiles. Consequently, residual stress develops within the material, causing spontaneous cracking even without the application of external loading. Thus, the oxidative aging can cause premature cracking in the material and requires a better understanding of the interaction between the chemistry and mechanics at different length scales and timescales to comprehend the effect of thermo-oxidative aging of polymeric materials. In this work, a fully coupled thermodynamically consistent chemo-mechanical phase-field fracture model is developed that attempts to bridge the gap between the experimental observations and a constitutive theory for thermo-oxidative aging in polymeric materials. To accomplish this, a novel approach has been adopted considering the chemical reactions at the polymer macromolecular level, a reaction-driven transient network evolution theory at the microscale, and a constitutive model at the macroscale. Finally, a phase-field fracture theory is added to the chemo-mechanical model to predict the oxidation-induced fracture in the polymer under mechanical loading. The model has been further extended to a homogenized continuum theory to capture the anisotropic oxidation characteristic of the fiber-reinforced polymer matrix composites. Specialized forms of the constitutive equations and the governing partial differential equations have also been developed for the polymers and the composite systems and numerically implemented in finite elements by writing ABAQUS user-defined element (UEL) subroutine.

Lastly, a unified phase-field fracture model is developed to create an experimentally validated, physically motivated, and computationally tractable model to predict the fracture response of the unidirectional fiber reinforced polymer matrix composites. A homogenized, coupled thermo-mechanical model is developed considering a thermo-viscoelastic polymer matrix. The model is numerically implemented by writing a ABAQUS user-element subroutine (UEL). The model can predict the constitutive response and direction-dependent damage propagation and final fracture in commercially acquired unidirectional glass-fiber-reinforced epoxy composite both at different fiber orientations and at different temperatures in substantial agreement with the experiments.

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