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“auxetic” = “negative Poisson ratio”
The following paragraph is from the paper:
“Four fundamental mechanical properties of materials in isotropic elasticity are Poisson’s ratio (ν), Young’s modulus (E), shear modulus (G), and bulk modulus (K). Note that these four parameters are interrelated, and the applicability of the theory of elasticity is limited to stress-strain conditions wherein the stress is below the yield point. Young’s modulus is the measure of stiffness in the linear elastic range. Poisson’s ratio is the least studied among these elastic constants. However, it governs the deformation feature of materials under various loading conditions [1,2]. This property is represented by the negative of the ratio between transverse and longitudinal strains [3]. The majorities of materials have a positive Poisson’s ratio that is about 0.5 for rubber and 0.3 for glass and steel [3]. The thermodynamic requirement in the theory of elasticity for a conservative system demonstrates that for homogeneous isotropic materials, the theoretical bound of Poisson’s ratio is from ́1 to 0.5. Therefore, the existence of materials with negative Poisson’ ratio (NPR) has long been accepted and they are known as “auxetic” materials [3,4].”
FROM:
Arash Ghaedizadeh, Jianhu Shen, Xin Ren and Yi Min Xie (Centre for Innovative Structures and Materials, School of Engineering, RMIT University, GPO Box 2476, Melbourne 3001, Australia),
“Tuning the performance of metallic auxetic metamaterials by using buckling and plasticity”, Materials, Vol. 9, No. 1, 54, doi:10.3390/ma9010054
ABSTRACT: Metallic auxetic metamaterials are of great potential to be used in many applications because of their superior mechanical performance to elastomer-based auxetic materials. Due to the limited knowledge on this new type of materials under large plastic deformation, the implementation of such materials in practical applications remains elusive. In contrast to the elastomer-based metamaterials, metallic ones possess new features as a result of the nonlinear deformation of their metallic microstructures under large deformation. The loss of auxetic behavior in metallic metamaterials led us to carry out a numerical and experimental study to investigate the mechanism of the observed phenomenon. A general approach was proposed to tune the performance of auxetic metallic metamaterials undergoing large plastic deformation using buckling behavior and the plasticity of base material. Both experiments and finite element simulations were used to verify the effectiveness of the developed approach. By employing this approach, a 2D auxetic metamaterial was derived from a regular square lattice. Then, by altering the initial geometry of microstructure with the desired buckling pattern, the metallic metamaterials exhibit auxetic behavior with tuneable mechanical properties. A systematic parametric study using the validated finite element models was conducted to reveal the novel features of metallic auxetic metamaterials undergoing large plastic deformation. The results of this study provide a useful guideline for the design of 2D metallic auxetic metamaterials for various applications.
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