Octagonal auxetic metamaterials with hyperelastic properties for large compressive deformation

In this study, some new 2D auxetic metamaterials were investigated and called octagonal auxetic metamaterials. The structures were examined by numerical and experimental approaches and 3D printing additive manufacturing technique was employed to fabricate the structures from thermoplastic polyurethane elastomer (TPU) which is considered as a hyperelastic material. Auxetic behavior was caused by adding small triangles and squares to the vertices of the octagonal structure and applying high compressive strain. Although the octagonal structure showed negative Poisson's ratio at high compressive strain, the results suggested that adding triangles to the vertices of the octagonal structure as well as increasing the triangle edge length led to higher negative Poisson's ratio. In contrast, with small squares adding to the vertices of the octagonal structure, Poisson's ratio became slightly more negative. In addition, by increasing the square edge length, Poisson's ratio measured near zero or even positive values. Moreover, a parametric study was carried out by Python 3.6.5 programming language to find the best triangle and square edge length which caused the most auxeticity.