Mechanical and Civil Engineering Seminar: PhD Thesis Defense

Abstract:

Transportation, infrastructure, personnel protection, and all other applications requiring dynamic impact resistance drive the demand to develop advanced manufacturing for structural materials that are simultaneously lightweight and superior at energy absorption. Polymer matrix composites (PMCs) are widely used in mechanical applications due to their high strength-to-weight ratios, high stiffness-to-weight ratios, corrosion resistance, and design flexibility. However, these composites often suffer from matrix and interfacially driven failure mechanisms under dynamic compression. Nano-architected materials are an emergent class of metamaterials capable of achieving high-stiffnesses and strengths while also exhibiting high specific energy absorptions under micro particle impacts. Most studies of nano-architected materials focus on periodic lattices geometries or other cellular solids while their use as reinforcements in composites, especially at large-scales, remains limited. Combining the works of nano-architected metamaterials and PMCs, we create nano-architected composites possessing high mechanical energy absorptions under dynamic compression without the need for dense constituent reinforcement materials.

We first explore how nano-architected materials are fabricated at large-scales, and we demonstrate how they can be incorporated with epoxy polymer matrices to create nano-architected composites using molding methods. Subsequently, we characterize these nano-architected composites, showing how changing fabrication parameters can produce various configurations of nano-architecture within samples, and how fabrication limitations can result in defects at multiple scales. Through various quasi-static and dynamic testing methods, we study how nano-architectures deform, fail, and contribute to composite performance - decoupling their effects from defects. Our study shows that while composite performance can be mitigated by defects, increasing the amount of nano-architectures present in composites leads to higher strengths and delayed catastrophic failure. In-situ observations of these tests allow us to directly connect deformation and failure mechanisms to enhanced stress-strain performance. We then use phenomenological modeling to relate mechanical performances across loading rates showing high nano-architected rate sensitivities and verifying the role of nano-architectures in delaying catastrophic failure. Nano-architected performance is compared against classical PMCs reported in literature demonstrating their high energy absorptions and unique capability to address shortcomings of fiber- and particle-reinforced composites.

Our nano-architected composites offer a new way to toughen polymer matrix composites, utilizing small scale architectures rather than changes to constituent material composition to delay catastrophic failures. These nano-architected composites are a unique demonstration of the capabilities of nano-architectures to perform under high rates, loads, and energies, paving the route to new possibilities for composite design. Our modeling work of nano-architected composite performance shows the potential of these materials to absorb energies through the introduction and control of nano-architected structures.

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Also available to attend over Zoom: https://caltech.zoom.us/j/84099990742