2, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Mechanical metamaterials composed of periodic space-filling plate assemblies are designed such that they exhibit an isotropic elastic response at the macro-scale. Moreover, specific configurations are considered with elastic moduli close to the theoretical Hashin-Shtrikman upper bound for porous solids. The fabrication of such structures with standard additive techniques is particularly challenging due to the closed-cell nature of the meso-structures. In this work, a strategy is developed to make closed cell meso-structures through 2-photon polymerization in a direct laser writing system. Prototype metamaterials with relative densities ranging from 10 to 40% are produced with minimal feature sizes of a few microns. Cubic metamaterial specimens are subjected to uniaxial compression loading in a custom-made displacement-controlled in-situ testing device. Aside from confirming the numerical estimates of the elastic moduli, the large strain response of plate-based metamaterials is determined and the density scaling of their specific energy absorption under uniaxial compression is discussed.