While the exploitation of size effects on the mechanical performance of nanoarchitected materials is a well-established design paradigm, to date practical implementations have been limited to very selective topologies, material systems, processing techniques and size regimes. Widespread adoption of this approach will require substantial progress in topology optimization, multi-scale modeling and scalable nanomanufacturing. This thesis advances the field of optimal design of size-effect strengthened materials by developing three novel capabilities.
Firstly, two-photon polymerization direct laser writing (TPP-DLW) and pyrolysis were used to fabricate plate-nanolattices for the first time. While architected materials are typically fabricated as polymeric structures via TPP-DLW, we found the viscoelastic properties of the polymer interfered with accurate comparisons against the upper bounds on stiffness and strength for an isotropic voided material. Thus, the polymeric structures were converted to pyrolytic carbon with strong linear elastic behavior for which the plate-lattice topology was confirmed to achieve the bounds in both stiffness and strength. Moreover, the combination of an optimal plate-lattice topology with size-effect strengthened pyrolytic carbon resulted in the highest specific strength architected material ever reported (at the time of publishing).
Although the plate-nanolattices possessed exceptional mechanical properties, there is no suitable additive manufacturing process capable of fabricating larger material volumes for practical use. By contrast, syntactic foams, i.e., closed-cell foams composed of hollow particles that form spherical nonoverlapping voids, are amenable to self-assembly and may have comparable specific strengths and stiffnesses to state-of-the-art plate-nanolattices if fabricated with a ceramic constituent material and hollow microparticles. Unfortunately, there is no current scalable process to produce highly controlled hollow particles in the quantity, dispersity and sizes required to fabricate these foams. Therefore, a process to fabricate hollow template-free ceramic particles is introduced here to enable future self-assembled size-effect strengthened syntactic ceramic foams, wherein the hollow ceramic particles become the porogen allowing exceptional control over the final pore size distribution. In addition to demonstrating tunability of the particle size and wall thickness, the process also included special considerations for scalability and extensibility to other material systems.
Lastly, a multi-scale computational procedure is developed to model the mechanical response and progressive failure mechanisms of size-effect strengthened ceramic syntactic foams, which may be produced in a scalable fashion using the particle porogens introduced previously. The computational approach includes a novel flaw field which may be generated for arbitrary porosity, and an accompanying material model which accounts for Weibull variability in the strength of brittle ceramics as well as increases in strength arising from reductions in the characteristic dimensions of geometric features. This model is developed in the framework of the material point method, constituting the first demonstration of a simulation incorporating large deformation, contact and fracture mechanics to investigate the strength and failure mechanisms of syntactic foams under uniaxial tension and compression.
This thesis lays the groundwork for the design and scalable fabrication of size-effect strengthened materials for real-world applications requiring extreme strength and minimum weight. While the focus of this work is on ceramics, many of the results are general and applicable to nanoarchitected materials made of any constituent in which size-effects and topology influence strength.
