Ultralight Microlattice Materials with Unique Combination of Stiffness and Damping
Ultralight hollow microlattice materials offer tremendous potential for energy dissipation, thanks to a unique form of structural damping associated with local buckling of the hollow bars. This dissertation provides a comprehensive study of this damping mechanism and exploits it for the design of hollow microlattices with superior combinations of stiffness and damping at low mass. To encompass a wide design space, both metallic and hybrid (metal/elastomer) hollow microlattices are investigated. This structural damping mechanism is studied in detail and a simple mechanical model is developed and validated by experimental characterization. The model is adopted to optimize the microlattice geometry for maximum values of a damping figure of merit, expressing optimal combinations of high stiffness, low density and high damping coefficient. We find that hollow metallic microlattices exhibit exceptionally large values of this figure of merit; however, this level of performance requires extremely low relative densities (<0.1%), thus limiting the actual amount of energy dissipated.
In order to increase the damping figure of merit at higher densities, hollow microlattices with metal/elastomer/metal sandwich walls are investigated. The sandwich construction provides increased local buckling strength, thus increasing the amount of energy dissipated by the lattice in a loading cycle. At the same time, the elastomer provides additional energy dissipation through the classic intrinsic constrained-layer damping mechanism, which is active even at relatively high densities and low deformation amplitudes. An analytical model for stiffness and damping (both intrinsic and structural) of hybrid hollow microlattices is derived, and verified via Finite Elements analyses and experimental characterization. Finally, the model is adopted in optimal design studies to identify hybrid microlattices with ideal combinations of the same figure of merit used for metallic lattices. The results indicate that hybrid lattices are clearly superior.
Over the course of this work, significant discrepancies between predicted and measured values of the mechanical properties (e.g., stiffness, strength) of ultralight hollow microlattices were consistently observed. Such discrepancies are attributed to a complex stress state around the hollow nodes and the existence of a variety of manufacturing-induced geometric imperfections (e.g. cracks, non-circularity of the bars). The ultralight nature of the lattices investigated in this study makes them particularly sensitive to these defects. Here, a detailed study of such imperfections is performed with the aim of quantifying their effects on the mechanical performance of the lattices. The results confirm that the major discrepancy between analytical and experimental results can indeed be attributed to manufacturing-induced imperfections.