This dissertation outlines investigations aimed atadvancing structure-based mechanical energy absorption in metamaterials. The goal of these investigations are to devise a new class of cellular metamaterials characterized by a multi-stable internal architecture and analyze the impact of those attributes on the mechanical energy absorption performance. In pursuit of this goal, three main outcomes are achieved.

The first outcome is multi-modal energy absorption in a metamaterial enabled by a rotationally multi-stable node embedded within a dual-chiral layer. Numerical simulation of several two- dimensional lattices demonstrates energy-absorbing hysteresis in their quasistatic loading curves under tensile, compressive, and shear deformations. Furthermore, this energy absorbing capacity is demonstrated in many loading directions, with directional absorption dictated by the underlying lattice's rotational symmetry.

The second outcome is enhanced structural properties and absorption performance achieved by tuning key kinematic parameters and microstructure in order to direct theload-displacement hysteresis toward that of an ideal absorber. Theoretical analysis of the unit cell characterizes the stiffness and peak load as functions of fundamental design parameters. It is shown numerically how these may be utilized to manipulate the loading curve prior to the onset of energy absorption, allowing for implementation of the design in structural applications. The onset of energy absorption leads to a plateau in the load-displacement curve, the length of which may be tailored in a similar fashion, with the mean height of the plateau heavily influenced by the microstructure.

The third outcome is control overthe directionality of the metamaterial absorption performance by mimicking the poly-crystalline microstructure of metals/alloys in metamaterials through “meta- grains” with spatially prescribed lattice orientation. Energy absorption in specific directions is optimized via the Non-dominated Sorting Genetic Algorithm II, treating target absorption values as objective functions and grain orientations as variables to be optimized. A simple bi-directional case is demonstrated experimentally to validate numerical results. The parameters dictating the polycrystalline structure are then examined by optimizing the directional stiffness of a lattice modeled with FEM beam elements In addition, we take a detour to observe other applications and phenomena in multi-stable metamaterials, namely a mechanical memory device and acoustic supratransmission.