Torsional Buckling Instability in Hollow Nanolattices
Lucas Meza (Caltech), Julia Greer (Caltech)
Instability in Solids and Structures
Tue 9:00 - 10:30
Barus-Holley 190
Advancements in nanofabrication techniques have allowed for the construction of three-dimensional architectures with the lowest characteristic length scale on the order of tens of nanometers. Efforts have been made to mimic the hierarchy in length scale of naturally occurring cellular biomaterials, which exhibit remarkable mechanical strength and damage tolerance despite containing a significant fraction of weak and brittle constituent materials. Existing biomimetic structures have demonstrated noteworthy mechanical properties, but have yet to match the overall structural response of biomaterials. To effectively design biomimetic structures that are capable of replicating the enhanced performance of biomaterials, it is first necessary to develop a thorough understanding of the structural response of each constituent sub-structure. We report lateral torsional buckling instability in the compression of hollow 3D nanolattices observed both experimentally and computationally. The structures were comprised of hollow tubes with 4:1 aspect ratio elliptical cross-sections that were organized in octahedral geometries. The fabrication was done in a multi-step negative pattern process involving two-photon lithography, direct laser writing, atomic layer deposition, and O2 plasma etching. Characteristic dimensions ranged from tens of nanometers (wall thickness) to microns (tube diameter) to tens of microns (unit cell) to over 100 µm in total. In-situ nano-mechanical experiments and finite element modeling of the compressive deformation of a single unit cell revealed a structural bifurcation that gave rise to a hyperelastic load-displacement response. FEA results revealed that the constituent TiN sustained tensile stresses of 1.75GPa without failure, yet the structure failed catastrophically at a low applied stress of 0.87MPa. We discuss the mechanical response of the nanolattices in the framework of size-induced material property enhancement and structural buckling instability.