Molecular dynamics simulations are performed to compute thermomechanical properties of cured epoxy resins reinforced with pristine and covalently functionalized carbon nanotubes. A DGEBA-DDS epoxy network was built using the 'dendrimer' growth approach where 75% of available epoxy sites were cross-linked. The epoxy model is verified through comparisons to experiments, and simulations are performed on nanotube reinforced cross-linked epoxy matrix using the CVFF force field in LAMMPS. Full stiffness matrices and linear coefficient of thermal expansion vectors are obtained for the nanocomposite. Large increases in stiffness and large decreases in thermal expansion were seen along the direction of the nanotube for both nanocomposite systems when compared to neat epoxy. The direction transverse to nanotube saw a 40% increase in stiffness due to covalent functionalization over neat epoxy at 1 K whereas the pristine nanotube system only saw a 7% increase due to van der Waals effects. The functionalized SWNT/epoxy nanocomposite showed an additional 42% decrease in thermal expansion along the nanotube direction when compared to the pristine SWNT/epoxy nanocomposite. The stiffness matrices and are rotated over every possible orientation to simulate the effects of an isotropic system of randomly oriented nanotubes in the epoxy. The randomly oriented covalently functionalized SWNT/Epoxy nanocomposites showed substantial improvements over the plain epoxy in terms of higher stiffness (200% increase) and lower thermal expansion (32% reduction). Through MD simulations, we develop means to build simulation cells, perform annealing to reach correct densities, compute thermomechanical properties and compare with experiments.
Optimization of square composite laminates, with variable stiffness properties, for thermal buckling is presented. Spatially varying fiber paths produce material properties that are functions of position. In this work, a predefined fiber configuration is controlled by varying two global parameters; the angle at the edge and center of the plate. The critical buckling temperatures for such laminates are obtained numerically based on classical lamination theory and finite element method. The discretized domain simplifies analysis by transforming nonlinear fiber path functions to linear piecewise functions. Using this method, thermal responses for symmetric balanced laminates under constant thermal load is investigated. Optimal fiber angle configurations that maximize the critical buckling temperatures are obtained. Spatially varying fiber path configurations are found that increase the resistance of thermal buckling in comparison to straight fiber configurations.
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