Fiber pullout and toughness study in nanocomposites

In nanotube-reinforced polymer composites, nanotubes are typically curved, entangled, and randomly oriented. Such reinforcement morphology greatly impacts the thermo-mechanical properties of the material. Little work has been done to relate toughness and strength in nanocomposites to the nano-reinforcement in-situ morphology and connectivity. In this dissertation the effects of nanotube curvature and inclination on nanocomposite mechanical properties are studied. Knowledge of such relationships will provide useful information for nanocomposite design.;At first, a single curved fiber pull-out model for nanocomposites is proposed. This analytical model modifies a classic shear-lag analysis to account for fiber curvature. Constant and Coulomb friction models are applied at interface due to relative movement between fiber and matrix during fiber pull-out. Both interface friction models predict higher pull-out forces for more curved fibers. Fiber curvature effects are more pronounced, however, for the Coulomb friction model due to the consideration of radial compressive stresses at interface. Finite element models are analyzed and compared with analytical models. The results show reasonable agreement between the two.;Next, a matrix crack bridged by curved fibers is analyzed to study fiber curvature effects on nanocomposite toughness. Based on single fiber pull-out curves, a crack bridged by intact fibers is first modeled. Results indicate large fiber curvature and interfacial friction can effectively toughen nanocomposites. Later a crack bridged by both intact and failed fibers is studied considering fiber statistical strength following Weibull distribution. The probability density function for fiber failure, the average pull-out length, and the average bridging stress are determined. Results indicate with fiber failure, increasing fiber curvature and interface friction could decrease composite toughness.;The last model in the dissertation considers nanotube inclination. Nanocomposite fracture tests involve the pull-out and rupture of inclined nanotubes. Critical bending shear properties of nanotubes are obtained through finite element simulations. An analytical model for inclined fiber pull-out is applied to study fiber failure mechanisms and, along with experimental data, to determine the interfacial shear strengths (ISS) in nanocomposite samples. The calculated ISS indicates proper functionalization can significantly improve the stress transfer ability at nanotube/matrix interface, which is beneficial for nanocomposite overall performance.