ABSTRACT
Chapter 2 discusses mechanical metamaterials and metastructures with extremal mechanical properties, focussing on definitions, a literature review, and description of unusual mechanical properties of the systems. First, a concept of metamaterials is introduced. They are defined as human-designed and human-made, not observed in nature, composite structures with unusual or non-typical properties, which are determined mainly by morphology of the structure and, to a lesser degree, by chemical or phase composition. Afterwards, a background on extremal materials is provided, with several examples of metamaterials that exhibit extremal mechanical properties. Then, unusual properties of mechanical metamaterials and lattices are discussed. They include: negative Poisson's ratio, vanishing shear modulus, negative compressibility and negative stiffness. Finally, three groups of adaptive systems with extraordinary properties are presented: programmable metamaterials, origami- and kirigami-inspired systems, and tensegrity-based metamaterials and structures. Adaptive systems are not the only metamaterials or metastructures with extremal mechanical properties, however, they have attracted much attention of the scientists in recent years, as they have a wide range of potential applications in engineering.
