11. Mechanical Metamaterials and Metadevices

Building upon the success of electromagnetic and acoustic metamaterials, mechanical metamaterials have been developed for obtaining extraordinary or extreme elasticity tensors and mass-density tensors to thereby mold static stress fields or the flow of longitudinal/transverse elastic vibrations in unprecedented ways. With the advances in additive manufacturing techniques that have enabled fabricating materials with arbitrarily complex micro-/nano-architectures, the rationally designed micro-/nano-architecture of mechanical metamaterials gives rise to unprecedented or rare mechanical properties that could be exploited to create advanced materials with novel functionalities. For instance, extremal metamaterials are extremely stiff in certain modes of deformation, while they are extremely soft in other modes of deformation; proper micro- and nano-architectural control can allow for unique material performance such as ultra-lightweight, high stiffness and high strength materials, negative Poisson’s ratio, negative stiffness, and negative thermal expansion coefficient. This chapter will give a brief review focusing on recent advances and remaining challenges in this emerging field. Examples are auxetic, ultra-lightweight, negative mass density, negative modulus, penta-mode, dilational, anisotropic mass density, origami, nonlinear, bistable, reprogrammable, and seismic shielding mechanical metamaterials.