Extremely Non-Auxetic Behavior of a Typical Auxetic Microstructure Due to Its Material Properties

The re-entrant honeycomb microstructure is one of the most famous, typical examples of an auxetic structure. The re-entrant geometries also include other members as, among others, the star re-entrant geometries with various symmetries. In this paper, we focus on one of them, having a 6-fold symmetry axis. The investigated systems consist of binary hard discs (two-dimensional particles with two slightly different sizes, interacting through infinitely repulsive pairwise potential), from which different structures, based on the mentioned geometry, were formed. To study the elastic properties of the systems, computer simulations using the Monte Carlo method in isobaric-isothermal ensemble with varying shape of the periodic box were performed. The results show that all the considered systems are isotropic and not auxetic—their Poisson’s ratio is positive in each case. Moreover, Poisson’s ratios of the majority of examined structures tend to +1 with increasing pressure, which is the upper limit for two-dimensional isotropic media, thus they can be recognized as the ideal non-auxetics in appropriate thermodynamic conditions. The results obtained contradict the common belief that the unique properties of metamaterials result solely from their microstructure and indicate that the material itself can be crucial.

Modeling the Mechanical Response of Auxetic Metamaterials to Dynamic Effects

The paper investigates the mechanical response of a 3D auxetic structure created on the basis of a unit cell with pre-buckled structural elements to dynamic loading. The aim of the work is to study deformations of the auxetic structure made of an alpha titanium alloy during uniaxial compression at 100 m/s, to evaluate dissipative properties of the structure during high-speed deformation, and to estimate the characteristic time of the metamaterial’s compaction with a relative density of 0.0115. The numerical simulation of the metamaterial at effective strain rate of 2000 1/s has been performed using LS DYNA solver. To describe the mechanical behavior of the titanium alloy in frame elements, we use a model of an elastic-plastic damaged medium, which takes into account the strain rate sensitivity of the plastic flow, temperature changes due to dissipative effects, and the effect of the stress state triaxiality parameter on nucleation and growth of structural damages. The numerical studies have shown that the auxetic effect in the studied metamaterial is retained under high-rate elastoplastic deformation. At a speed of the uniaxial compression of 100 m/s, deformation in the volume of the metamaterial proceeds nonuniformly. Under dynamic loading of the considered auxetic metamaterial, the deformation and fracture modes depend not only on the parameters of the cell geometry, but also on the mechanical behavior of the framework material, as well as on the relative density. This makes it possible to control the deformations of the cells under mechanical stress. Layers of compacted cells are formed near the dynamic loading surface. The instability of the cells of the auxetic metamaterials increases the absorbed energy. The calculated value of the specific absorbed energy under dynamic uniaxial compression reaches 3.4 kJ/kg, and is comparable with the values for frame structures made of Ti-6Al-4V with an equivalent specific mass density. The results indicate the possibility of creating protective structures using auxetic cellular structures on the base of the pre-buckled elements of the rolled metal.

Auxetic Structures for Tissue Engineering Scaffolds and Biomedical Devices

An auxetic structure utilizing a negative Poisson’s ratio, which can expand transversally when axially expanded under tensional force, has not yet been studied in the tissue engineering and biomedical area. However, the recent advent of new technologies, such as additive manufacturing or 3D printing, has showed prospective results aimed at producing three-dimensional structures. Auxetic structures are fabricated by additive manufacturing, soft lithography, machining technology, compressed foaming, and textile fabrication using various biomaterials, including poly(ethylene glycol diacrylate), polyurethane, poly(lactic-glycolic acid), chitosan, hydroxyapatite, and using a hard material such as a silicon wafer. After fabricating the scaffold with an auxetic effect, researchers have cultured fibroblasts, osteoblasts, chondrocytes, myoblasts, and various stem cells, including mesenchymal stem cells, bone marrow stem cells, and embryonic stem cells. Additionally, they have shown new possibilities as scaffolds through tissue engineering by cell proliferation, migration, alignment, differentiation, and target tissue regeneration. In addition, auxetic structures and their unique deformation characteristics have been explored in several biomedical devices, including implants, stents, and surgical screws. Although still in the early stages, the auxetic structure, which can create mechanical properties tailored to natural tissue by changing the internal architecture of the structure, is expected to show an improved tissue reconstruction ability. In addition, continuous research at the cellular level using the auxetic micro and nano-environment could provide a breakthrough for tissue reconstruction.

Application of negative Poisson’s ratio honeycomb structure in force measurement

Purpose This paper aims to clarify the principle of force measurement using the auxetic structure by studying the relationship among the force, the change of transmittance and the change of output current of the solar cell. Design/methodology/approach This paper opted for an exploratory study using combining theory with experiment. This paper analysized the theoretical model and deformation of the auxetic structure. It used a hexagon honeycomb structure as a comparison in the experiment. The experiment was conducted on a universal testing machine, and the data was obtained by a digital acquisition card. The data was analyzed without using any signal processing means. Findings This paper provides the linearity and the sensitivity of the proposed force measurement method. It shows a good linear relationship between the input and output of this method and good sensitivity, stability and repeatability without using any signal processing means. Originality/value This paper provides new structural insights for force sensors and presents future research directions.

Impact Analysis of Auxetic Structures for Military Applications

Auxetic structures are special structures which tend to become wider when subjected to longitudinal tension instead of getting compressed, which implies structures having a negative poisson’s ratio. These structures are used in impact pads due to this unique property. In this comparative study were done on different types of materials and structures which are recognized for 3D printing the auxetic structures. The three stages of explicit dynamic analysis involves firstly selecting the most appropriate structures from chiral truss, re-entrant hexagon, arrow head and one non- auxetic structure which is hexagon structure. From this the structure having the least deformation at the impact point is selected which is re-entrant hexagon. Following this, keeping re-entrant hexagon as the structure, the next set of analysis is performed by varying the structure materials. Polycarbonate, polystyrene, polyvinyl chloride and high density polyethylene were studied and the analysis results showed, polyvinyl chloride as the suitable material. Lastly the limiting velocity for the impact is calculated by varying the impact velocity from 800m/s, 1000m/s and 1200m/s beyond which the structure experienced fracture. This study proposes the selection of suitable auxetic structure and material for manufacturing impact pads. Keywords: Auxetic structures, impact pads, indentation resistance, explicit dynamics, 3D printing, FDM, Poisson’s ratio