Carbon-based materials, such as graphene and carbon fibers, have good prospects for applications in reinforcements of polymers as advanced structural composites. Hence, the study of the mechanical properties of the constituents and the composites is becoming increasingly popular in both academia and industry.', 'Graphene has been shown to have a comparative advantage over traditional materials as composite reinforcements. There is a rapid rise of interest in harnessing the mechanical properties of graphene and its derivatives, like graphene oxide (GO), at larger length scales and higher material hierarchies, i.e. multi-layer assemblies. Despite the great progress in developing synthesis and characterization methods for these materials, the physical mechanisms governing the mechanical performance of multi-layer graphene (MLG) and GO sheets remain poorly understood. Experimental techniques offer limited insights into atomic-scale deformation processes, while atomistic simulations have proved prohibitively expensive in investigating the large-deformation and failure mechanisms of the multi-layer assemblies. Similar bottlenecks also exist for the characterization of matrix materials, such as epoxy resins. Previous atomistic investigations of epoxy resins fall short in linking nanoscopic observations to macroscopic properties, and thus, it limits the use of simulations in the computational design of new materials.', 'To overcome these critical issues, we first develop coarse-grained (CG) models of MLG and GO sheets, which are incorporated into molecular dynamics (MD) simulations that are capable of quantitatively reproducing their mechanical responses in both elastic and fracture regimes. Additionally, the CG model of GO accurately captures the heterogeneous mechanical properties of GO with different chemistries. With two to three orders of magnitude increase in computational efficiency, CG MD simulations are then used to elucidate the mechanisms of deformation and failure of MLG and GO sheets by collaborating with experimentalists. Specifically, we discover for the first time an atomic-level interlayer slippage process in MLG, which leads to repeatable energy dissipation of MLG sheets under film deflection tests. We also successfully elucidate the experimentally measured thickness-dependent strength of MLG sheets. In addition, differences in the failure modes of GO membranes under nanoindentation are shown to be due to differences in the chemistries of the contact area. We also show the potential of these CG models to study the fracture toughness of mesoscale graphene and GO sheets, as well as to investigate the mechanical properties and failure mechanisms of nacre-inspired nanocomposites. We also utilize our CG MD approach to study the ballistic impact behavior of thin MLG sheets. The simulation results reveal distinctive failure mechanisms that deteriorate the ballistic resistance of MLG sheets. Specifically, we have observed the in-plane cone wave formed upon impact can reflect from clamped boundaries and induce early perforation. In addition, the compressive wave in the thickness direction could result in spalling-like failure, similarly to the failure mechanism in the impact on macroscopic concrete specimen. To relate the observed failure mechanisms to microballistic experiments, we also develop analytical relationships, based on continuum mechanics theories, to bridge the size scales and provide a full picture of the deformation processes.', 'Among all the matrix materials, epoxy resin is promising in graphene-based nanocomposites as well as in carbon fiber reinforced composites with its excellent mechanical properties and temperature resistance. However, developing new and better ones always take a long time as we have limited understanding of the physics underlying their properties. To characterize its structure-property relationship from molecular scale, we first develop a robust atomistic model for its crosslinked structure and investigate the effect of resin functionality, crosslink degree, and component ratio on resin thermomechanical properties. More importantly, we link atomistic tensile simulations of the resin with its macroscopic fracture energy, using a theoretical fracture mechanics model developed for the polymeric system. With this theoretical model, we also provide physical insight into the molecular mechanisms that govern the fracture characteristics of epoxy resins. Then, to establish the structure-property relationship of carbon fiber composites from a multiscale perspective, we utilize MD simulation results and an analytical gradient model to characterize the properties of the interphase region between fiber and matrix, which are then integrated into finite element analysis to investigate the failure behavior of composites. Using this multiscale framework, we explicitly elucidate that the nanoscale interphase region is crucial to capture the realistic mechanical properties of composites, especially the non-linear response at large deformation.', 'The studies presented here illustrate important mesoscale physical mechanisms governing the mechanical performance of composite constituents such as MLG and GO sheets and epoxy resins as well as the entire composites. The insights obtained from the studies would lay the foundation for developing future impact and failure resistant composites.

Last modified

• 10/08/2019

Creator

• Zhaoxu Meng

DOI

• https://doi.org/10.21985/n2-9jqn-fr36

Subject

• Civil and Environmental Engineering

Keyword

• Multi-scale Modeling
• Computational Mechanics
• Nanocomposites
• Graphene
• Molecular Dyanmics
• Epoxy

Date created

• 2018-01-01

Resource type

• Dissertation

Rights statement

• In Copyright