Metacomposite and meta-sandwich structures using mechanical metamaterials
Lightweight sandwich structures are commonly used in aircraft that contain upper and lower skins made up of glass or carbon fabric composite with a honeycomb or foam core. Replacing them with mechanical metamaterials such as auxetic re-entrant honeycombs can show better performance. The mechanical metamaterials research field is still in its early stages and mostly their linear elastic behavior has been studied. The consideration of geometric nonlinearity opens up immense future possibilities of metamaterials for multidisciplinary applications. In the defense sector, body armor is mostly made of ceramic composite whereas the backing face comprises fiber or fabric-reinforced composites. From the available literature, a typical mechanical metamaterial is generally associated with four elastic properties: modulus of elasticity, Poisson’s ratio, shear modulus, and bulk modulus. These properties further correspond to the stretchability or compressibility, stiffness, and rigidity of the material. The geometrical design of the microstructures enables the tunability of metamaterial exhibiting unusual characteristics.
Typically, these mechanical metamaterials exhibit negative values of mechanical properties that are positive in conventional materials. They exhibit significantly enhanced engineering properties, such as zero or negative Poisson’s ratios, negative stiffness, negative compressibility, vanishing shear modulus, negative thermal expansion, and singularly nonlinear behavior with customized topological microstructures. Also, the term ‘metacomposite’ had been defined primarily in electromagnetics as a metamaterial piece characterized by double-negative electromagnetic features, i.e., a negative permittivity and negative permeability. However, recent studies show that various researchers use the term metacomposite for mechanical metamaterials which are composites and meta-sandwich structures. Indian defense systems face unique challenges in developing protective armor systems for mitigating both low and high-frequency ballistic loads and shocks, due to conventional designs and require tremendous advances in technology and transition to new metamaterials.