Poisson's ratio: physical significance, distribution in isotropic materials, and deformation illustration. (a) Schematic diagram of Poisson's ratio definition [28]. (b) Distribution of Poisson's ratio νacross various isotropic materials plotted as a function of the bulk-to-shear modulus ratio B/G. The theoretical range for homogeneous isotropic materials is −1 ≤ ν ≤ 0.5 [28]. (c) Schematic illustration of the deformation behavior of conventional materials and auxetic metamaterials under compression and tension, presented in both 2D and 3D forms [4].