Hierarchical auxetic and isotropic porous medium with extremely negative Poisson’s ratio

Abstract

We propose a novel two-dimensional hierarchical auxetic structure, consisting of a porous medium in which a homogeneous matrix includes a rank-two set of cuts characterised by different scales. The six-fold symmetry of the perforations makes the medium isotropic in the plane. Remarkably, the mesoscale interaction between the first-and second-level cuts enables the attainment of a very small value of the Poisson's ratio, close to the minimum reachable limit of -1. The effective properties of the hierarchical auxetic structure are determined numerically, considering both a unit cell with periodic boundary conditions and a finite structure containing a large number of repeating cells. Further, results of the numerical study are validated experimentally on a polymeric specimen with appropriately arranged rank-two cuts, tested under uniaxial tension. We envisage that the proposed hierarchical design can be useful in numerous engineering applications exploiting an extreme auxetic effect.

Experimental realization of tunable Poisson’s ratio in deployable origami metamaterials

“…The actual realization of origami-based devices required a deep understanding on the factors that can induce discrepancies from the mechanical properties predicted from the theory and those found experimentally, such as the effect of boundary conditions [29], manufacturing process [30][31][32], panels rigidity and thickness accommodation [33,34]. Although several studies have dealt with the experimental validation of Poisson's ratio effect of architected metamaterials and kirigami-based structures [35][36][37][38][39], only few provide an equivalent investigation in the case of origami metamaterials [23,[40][41][42]. Here, we propose a new experimental setup to determine the mechanical properties of deployable origami metamaterials during the entire folding/unfolding process.…”

“…The gripping system, called Saint-Venant fixture, has been carefully designed to allow the complete deployment of the tested metamaterials in the transverse direction, preventing sample frustration at the constrained regions. Usually, uniaxial testing experiments on metamaterials require very long samples to ensure a sufficiently large area in the centre of the sample without boundary effects [37]. In fact, standard gripping devices induce stress concentrations in the bonded areas, giving the tested metamaterial a dog bone shape that is symptom of a non-uniform transverse deformation.…”

An Optimized, Easy-to-use, Open-source GPU Solver for Large-scale Inverse Homogenization Problems

“…Solving IHPs (Sigmund, 1994) to distribute materials is a powerful method to obtain superior mechanical properties under given load and boundary conditions. Pioneering researchers use topology optimization to solve IHPs with different objectives, such as extreme shear or bulk moduli (Gibiansky and Sigmund, 2000), negative Poisson's ratios (Theocaris et al, 1997;Shan et al, 2015;Morvaridi et al, 2021), and extreme thermal expansion coefficients (Sigmund and Torquato, 1997). Although there are several open-source codes for microstructure design (Xia and Breitkopf, 2015;Gao et al, 2021), most of them focus on 2D microstructure design.…”

Recent advances in auxetics: Applications in cementitious composites

“…With the existence of strain boundaries in auxetic material, the advantages of NPR materials can be enhanced by inclining isotropy or attaining amplified anisotropy to optimise auxetic behaviour. Much effort has been put into creating isotropic auxetic structure to generate asymmetric auxetic patterns in multi-direction, which may enhance the integrated energy absorption capacity (Morvaridi et al, 2021; Schwerdtfeger et al, 2011; Shan et al, 2015). However, with a high degree of isotropy, cell ribs of cellular structure may not buckle and ensure auxetic deformations in all directions (Wang et al, 2018).…”