Mechanical properties of nanocomposites reinforced by graphene containing defects

Demand for high strength and lightweight structures in the field of aerospace engineering has led to the development of new advanced composites. Graphene displays remarkable electrical, mechanical, and thermal properties in various applications since its discovery. These exceptional characteristics have inspired great scientific endeavours into applying graphene in polymer composites as a promising nanofiller. Replacement of heavy structural components with graphene reinforced composites can offer significant potential for weight reduction. However, the issue of structural defects in graphene during the manufacturing process is a growing concern. These defects have been proven to have a significant impact on the mechanical properties of graphene, resulting in reduced properties of graphene/polymer composites. The basis for this research is to understand how these defects affect the performance of graphene/polymer composites. The primary aim of this research is to investigate the mechanical properties of defective graphene/polymer nanocomposites using the molecular dynamics (MD) simulating method.<br><br> In the first study of this research, the effect of structural defects on the tensile behaviors of graphene/epoxy nanocomposites are investigated in both the armchair and zigzag directions at room temperature 300K by the molecular dynamics simulations. Two types of vacancy defects, i.e., single-vacancy and double-vacancy, in single-layer graphene are considered. The results show that the overall material properties including Young's modulus, ultimate strength and strain decrease with an increasing number of missing atoms but are insensitive to defect distribution. Compared with double-vacancy, single-vacancy defects have a more significant influence on the material properties due to more dangling bonds in its structure. Also, it is demonstrated that the defect type, size, and location also significantly affect the mechanical properties of the nanocomposites. This work is essential as it explores the mechanism of defect type, size, distribution, and location underlying the tensile behaviors of graphene/epoxy composite at the nanoscale, suggesting that all the defect configurations are available for engineering graphene/epoxy matrix nanocomposites.<br><br> The second study investigated the temperature-dependent mechanical behaviors of graphene/polymer nanocomposites with a particular focus on the influences of vacancy defects. Numerical results reveal that with an increase in temperature, the tensile modulus and ultimate tensile strength are decreased but the ultimate strain is improved, irrespective of the defect configuration. The compressive properties, however, exhibit different behavior with compressive modulus, yield strength and yield strain all decreasing as temperature increases. Moreover, the effects of defect type, size, and location tend to be more significant at high temperatures. Also, a modified Halpin-Tsai model which considers both the effects of temperature and vacancy defects is introduced to estimate the material properties of graphene/epoxy composites, where the efficiency parameters are determined from the molecular dynamic simulations results. For the first time, the mechanisms of temperature and structural defects underlying mechanical properties of graphene/epoxy nanocomposites are provided at the atomic scale, which is of great importance in engineering applications of such materials.<br><br> Compared with the traditional composites, the interface of nanocomposites is much larger due to the enormous surface area per unit volume of the graphene. The interfacial load transfer from the graphene sheet to the polymer matrix plays a vital role in determining the performance of graphene/polymer nanocomposites. Therefore, the third study further explored the hydrogen functionalization enhancement on the tensile properties of graphene/epoxy nanocomposites. The pristine graphene sheet is functionalized by adding hydrogen atoms either along its edges or over its surface. The results show that the nanocomposites with functionalized graphene exhibit better material properties, including the tensile modulus, ultimate strength, and strain, compared with those reinforced by the pristine graphene. The superiority of hydrogenated graphene over the pristine graphene in terms of tensile properties enhancement suggests the functionalization may be an effective way to improve the interfacial bonding as well as the efficient load transfer between graphene and the matrix.<br><br> For the fourth study of this research, the pull-out simulations were carried out to investigate the interfacial behaviors of functionalized graphene/polymer composites. It is observed that the surface hydrogenated graphene enhances the interfacial mechanical properties more effectively compared with an edge functionalized graphene sheet in terms of the shear strength over a wide temperature range from 50K to 400K. Moreover, the nanocomposites reinforced by graphene modified with hydrogen atoms exhibits superior properties compared with the pristine graphene under the same defects of graphene. In other words, the functionalization of graphene lowers the defect sensitivity of the nanocomposites compared to the pristine graphene.<br><br> In the final study of this research, the data from previous chapters was applied to the Timoshenko beam theory to calculate the bending and vibration response of the graphene/composites beam. <br><br> Overall, this research demonstrates the mechanisms of the performance of graphene/epoxy composites from the atomistic perspective and provide valuable insights for the design and optimization of nanodevices reinforced by graphene.<br><br>