Date of Award

2020

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

Gregory Odegard

Committee Member 1

Ranjit Pati

Committee Member 2

Trisha Sain

Committee Member 3

Gowtham S

Abstract

Significant research effort has been dedicated for decades to improve the mechanical properties of aerospace polymer-based composite materials. Lightweight epoxy-based composite materials have increasingly replaced the comparatively heavy and expensive metal alloys used in aeronautical and aerospace structural components. In particular, carbon fibers (CF)/graphene nanoplatelets (GNP)/epoxy hybrid composites can be used for this purpose owing to their high specific stiffness and strength. Therefore, this work has been completed to design, predict, and optimize the effective mechanical properties of CF/GNP/epoxy composite materials at different length scales using a multiscale modeling approach. The work-flow of modeling involves a first step of using molecular dynamics (MD) with a reactive force field (ReaxFF) to predict the structure and mechanical behavior of the GNP/epoxy materials at the molecular level. A micromechanics approach is then used to model and predict the mechanical properties of the CF/GNP/epoxy hybrid composite at the bulk level. One of the major findings of this study refers to an alignment behavior of phenyl rings in epoxy with the planar GNP surface at the interphase region. This alignment plays an important role to drive the molecular density of epoxy at the interphase and promote the GNP-epoxy interfacial adhesion. The results also validate the use of ReaxFF in MD modeling of such nanocomposites as the predicted properties compare well with experiment.

The impact on the mechanical properties of aerospace epoxy materials reinforced with pristine GNP, highly concentrated Graphene Oxide (GO), and Functionalized Graphene Oxide (FGO) has also been investigated in this study. A systematic computational approach to simulate the reinforcing nanoplatelets and probe their influence on the mechanical response of the epoxy matrix at both nanoscale and bulk levels. The nanoscale outcomes indicate a significant degradation in the in-plane elastic and shear moduli of the nanocomposite when introducing large amounts of oxygen and functional groups to the robust sp2 structure of the GNP. However, the wrinkled and rough topology of GO and FGO promotes the nanoplatelet-matrix interlocking mechanism which produces a significant improvement in the out-of-plane shear modulus. In addition, surface functionalization of GNP promotes the nanoplatelet-epoxy interfacial interaction/adhesion significantly which is important for the material toughness. Using micromechanics analysis, the influence of the nanoplatelets content and aspect ratio on the mechanical response of the proposed nanocomposites has also been predicted and validated with experimental data available from the literature. Generally, there is an improvement in the predicted mechanical response of the bulk nanocomposite materials with increasing nanoplatelets content and aspect ratio.

The predicted mechanical properties of the nanoplatelet/epoxy nanocomposites are then used to generate hybrid composite models reinforced with unidirectional CF. The micromechanics predictions are used to analyze the reinforcing effect of the proposed nanoplatelets on the unidirectional CF/nanoplatelet/epoxy hybrid composites. Three laminated hybrid composite panels are also modeled and analyzed to address the reinforcing effect of the proposed nanoplatelets on the laminated hybrid composite panels. The predicted mechanical properties of the laminated hybrid composite panels are important in assessing the mechanical performance of in-service structural components.

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