Date of Award
Master of Science (MS)
Nacre, a natural nanocomposite with a brick-and-mortar structure existing in the inner layer of mollusk shells, has been shown to optimize strength and toughness along the laminae (in-plane) direction. However, such natural materials more often experience impact load in the direction perpendicular to the layers (i.e., out-of-plane direction) from predators. The dynamic responses and deformation mechanisms of layered structures under impact load in the out-of-plane direction have been much less analyzed. The optimal design of protective material systems by leveraging the bioinspired structure has not yet been achieved. The main objective of this thesis is to investigate the dynamic mechanical behaviors of nacre-inspired layered nanocomposite films under impact in the out-of-plane (i.e., thickness) direction by using a model system that comprises alternating multilayer graphene (MLG) and polymethyl methacrylate (PMMA) phases. With a validated coarse-grained (CG) molecular dynamics simulation approach, my thesis systematically studies the mechanical properties and impact resistance of the MLG-PMMA nanocomposite films with different internal nanostructures, which are characterized by the layer thickness and the number of repetitions while keeping the total volume constant. As the layer thickness decreases, the effective modulus of the polymer phase confined by the adjacent MLG phases increases. This observation demonstrates that the adopted CG models capture the nanoconfinement effect on the polymer phase. I then use ballistic impact simulations to explore the dynamic responses of nanocomposite films in the out-of-plane direction. I find that the impact resistance and dynamic failure mechanisms of the films depend on the internal nanostructures. Specifically, when each layer is relatively thick, the nanocomposite is more prone to spalling-like failure induced by compressive stress waves from the projectile impact. Whereas, when there are more repetitions and each layer becomes relatively thin, a high-velocity projectile sequentially penetrates the nanocomposite film. In the low projectile velocity regime, the film develops crazing-like deformation zones in PMMA phases. Such crazing-like deformation is believed to dissipate the energy and delocalize the concentrated impact loading effectively. Furthermore, I find that the position of the soft PMMA phase relative to the stiff graphene sheets plays a significant role in the ballistic impact performance of the investigated films. In summary, this thesis provides insights into the effect of nanostructures on the dynamic mechanical behaviors of layered nanocomposites under impact loading along the thickness direction. The revealed dependence and underlying deformation mechanisms can lead to effective design strategies for impact-resistant films.
Chiang, Cho Chun, "Dynamic Mechanical Behaviors of Nacre-Inspired Graphene-Polymer Nanocomposites Depending on Internal Nanostructures" (2021). All Theses. 3654.