Auxetic Materials Research Articles

Two-dimensional multifunctional metal-organic frameworks with large in-plane negative Poisson ratios and photocatalytic water splitting properties.

Auxetic materials with multifunctional properties are highly sought after for application in modern nano-devices. However, the majority of reported inorganic auxetic materials exhibit low negative Poisson's ratios (NPR), poor flexibility, and limited functionality. In this study, we employ density-functional-theory (DFT) first-principles simulations to design a series of two-dimensional (2D) metal-organic frameworks (MOFs) M2C4X4 (M = Cu, Ag, Au; X = O, S, NCN) that display intriguing auxetic behavior, superior flexibility and appropriate photocatalytic water-splitting properties. These M2C4X4 MOFs are assembled from carbon tetragon motifs and exist in both cis- and trans-isomer forms, with the NPR ranging from -0.17 to -0.90. Notably, trans-Cu2C4(NCN)4 exhibits a high NPR of -0.90, while cis-Cu2C4(NCN)4 achieves an NPR of -0.67. Both isomers demonstrate excellent flexibility, characterized by ultra-low Young's modulus and high fracture strengths. Furthermore, their direct band gaps, strong light-harvesting capabilities, and long excited-state lifetimes make them promising candidates for the photocatalytic oxygen evolution reaction in water. These results provide a viable strategy for the design and synthesis of novel optoelectronic multifunctional materials.

Predicting Poisson's Ratio: A Study of Semisupervised Anomaly Detection and Supervised Approaches.

Auxetics are a rare class of materials that exhibit a negative Poisson's ratio. The existence of these auxetic materials is rare but has a large number of applications in the design of exotic materials. We build a complete machine learning framework to detect Auxetic materials as well as Poisson's ratio of non-auxetic materials. A semisupervised anomaly detection model is presented, which is capable of separating out the auxetics materials (treated as an anomaly) from an unknown database with an average precision of 0.64. Another regression model (supervised) is also created to predict the Poisson's ratio of non-auxetic materials with an R2 of 0.82. Additionally, this regression model helps us to find the optimal features for the anomaly detection model. This methodology can be generalized and used to discover materials with rare physical properties.

Crashworthiness of Additively Manufactured Auxetic Lattices: Repeated Impacts and Penetration Resistance.

Auxetic materials have recently attracted interest in the field of crashworthiness thanks to their peculiar negative Poisson ratio, leading to densification under compression and potentially being the basis of superior behavior upon impact with respect to conventional cellular cores or standard solutions. However, the empirical demonstration of the applicability of auxeticity under impact is limited for most known geometries. As such, the present work strives to advance the investigation of the impact behavior of auxetic meta-materials: first by selecting and testing representative specimens, then by proceeding with an experimental and numerical study of repeated impact behavior and penetration resistance, and finally by proposing a new design of a metallic auxetic absorber optimized for additive manufacturing and targeted at high-performance crash applications.

Investigation of the Auxetic of a novel geometric structure and improvement of Poisson’s ratio at different inner thicknesses

Poisson’s ratio, one of the important mechanical properties of materials and structures, is positive for almost all of the known materials and structures. However, auxetic materials or structures has negative Poisson’s ratios. Characteristics of the auxetic structures are very important to be used in design of a new structure. Computational or experimental studies on auxetic structures have been increasing in literature. In this study, a new auxetic lattice structure with different Poisson’s ratios was designed and studied by finite element analysis. Mechanical properties of the newly designed auxetic lattice structures were analyzed with different lattice inner thickness. Results showed that change in inner thickness affects the Poisson’s ratio, mass, volume and surface area of the newly designed Auxetic lattice structures.

Evaluation of Mechanical Properties of PLA Auxetic Structures Produced by Additive Manufacturing

FDM (fused deposition modeling) is one of the most commonly used technologies in additive manufacturing. This technology is used to additively manufacture components from various polymer materials, mostly PLA (polylactic acid), etc. PLA filament is a widely used polymer for 3D printing due to its biodegradability, biocompatibility, and processability. In the study, PLA raw material and cellular auxetic structures were used in the design. Auxetic designs are called metamaterials, they are structures with advanced properties and can be obtained with various geometries. The auxetic designs used in the study are missing rib, re-entrant honeycomb and chiral. One of the biggest advantages of auxetic cellular materials is that it is not bulk material. Having a skeletal structure provides high strength at low density. Today, based on this mechanism, designs that can be used in engineering applications are being studied. It has an important place especially in the medical field, as well as in the areas where high precision and specific products are designed and produced. Considering its relationship with 3D printing technology, 3D printing enables the fabrication of auxetic structures for complex and personal designs. The novelty of auxetic structures comes from their topological features, which display counterintuitive response to the applied load. For the purpose of compare the properties of mechanical tensile, compression, surface roughness tests were applied. It is concluded that the presence of chiral structures improves mechanical performance. The chiral auxetic sample exhibited a maximum stress of 6.68 MPa, the missing-rib auxetic sample displayed a maximum stress of 2.26 MPa, and the re-entrant auxetic sample demonstrated a maximum stress of 3.68 MPa. These results obtained from the tests align well with the range reported in the literature, which falls between 1-12 MPa. The surface roughness of the all-auxtetic structure, perpendicular to the printing direction was higher than the measurements taken parallel to the printing direction.

Modeling and Simulation of Auxetic Materials for Balistic Protection

A type of structural metamaterials known as auxetics has a negative Poisson's ratio. Auxetic structurals have been found to possess a number of better qualities when compared to traditional ones, including: greater energy absorption, stronger indentation resistance, and enhanced mechanical properties. As a result, auxetic structures are becoming more known as a high-performance, lightweight defensive construction that can survive collisions and blasts.

Curve beam for strengthening the negative Poisson's ratio effect of rotating auxetic metamaterial: Experiments and simulations

This article described a three-dimensional lattice design method for customizing the negative Poisson’s ratio effect in a rotating rigid structure. By using bent beams as the basic building blocks of the lattice, a new class of curved beam rotating auxetic (CBRA) lattice was proposed. Through a combination of numerical simulations and experimental tests, the mechanical responses of representative three-dimensional auxetic lattice structures under compressive load were systematically studied, including Poisson’s ratio, energy absorption, and a multi-stage deformation mode that facilitated the tuning of mechanical properties. Additionally, the influence of structural geometric parameters on the elastic performance was discussed in detail. Unlike most rotating auxetic materials that could only achieve the negative Poisson’s ratio effect within a small strain range, the results demonstrated that the uniform Poisson’s ratio of the proposed three-dimensional lattice structure could achieve positive and negative adjustment within a strain range of 0–0.5. The design flexibility would contribute to making it a promising candidate for industrial and biomedical applications, such as scaffolds and skin grafts. The acceleration of the application of new three-dimensional expandable lattice structures in engineering applications made this system more versatile than similar non-graded systems.

Numerical Prediction of the Yield Surface of a Chiral Auxetic Structure

For today's requirements, the material itself is often not sufficient anymore. This leads to structural cellular materials and metamaterials, which allow for more degrees of freedom by a strong structure–property relationship. Auxetic structures investigated in this contribution belong to metamaterials. Auxetic metamaterials show huge advantages in their properties, e.g., an enhanced specific energy absorption capacity (SEAC) as well as a negative Poisson's ratio, combined with a very high stiffness‐to‐weight ratio. Experimental probing of these structures is very challenging, hence, up to now, auxetic materials have only been investigated in uniaxial, bending, or shear loading. But for an industrial application, the knowledge of the multiaxial yield behavior is inevitable. For the first time, the present study deals with the numerical probing of the yield surface for a chiral auxetic structure applying multiaxial loading. The changes in Poisson's ratio, SEAC, and the resulting nonconvex yield surface are studied numerically. It shows that the highest specific energy absorption capacities are reached under compression load cases while higher stiffnesses and yield stresses are achieved under tensile loading. There is a strong structure–property relationship and load case dependency for Poisson's ratio as well as for the SEAC.

A study on auxetic-inspired side structure for enhanced crashworthiness

A novel type of side structure of ship is proposed, inspired by auxetic materials with a negative Poisson's ratio, for enhancing the crashworthiness of double-hull vessels. Three different unit cells (VRE, HRE, and ARR) are designed and numerical simulation is conducted to demonstrate their resistance to collision. A comparative study on the crashworthiness of the proposed side structures and those of traditional side structures (double hull, X-core, and Y-core) is performed and a superior capability of energy absorption and collision resistance of the proposed side structures is observed. These findings demonstrated a potential for auxetic-based structures to be applied as a collision resistance structure implemented in various protective structures.

Nonlinearity induced negative Poisson’s ratio of two-dimensional nanomaterials

Materials exhibiting a negative Poisson’s ratio have garnered considerable attention due to the improved toughness, shear resistance, and vibration absorption properties commonly found in auxetic materials. In this work, the nonlinear effect on the Poisson’s ratio was derived theoretically and verified by first-principle calculations and molecular dynamics simulations of two-dimensional nanomaterials including graphene and hexagonal boron nitride. The analytic formula explicitly shows that the Poisson’s ratio depends on the applied strain and can be negative for large applied strains, owing to the nonlinear interaction. Both first-principle calculations and molecular dynamics simulations show that the nonlinear effect is highly anisotropic for graphene, where the nonlinearity-induced negative Poisson’s ratio is much stronger for the strain applied along the armchair direction. These findings provide valuable insights into the behavior of materials with negative Poisson’s ratios and emphasize the importance of considering nonlinear effects in the study of the Poisson’s ratio of two-dimensional materials.

Direct measurement of negative Poisson ratios of auxetic materials via the impulse excitation technique (IET)

The direct measurement of Poisson ratios via the impulse excitation technique (IET) is based on determining the frequency ratio between the resonant frequencies of flexural and torsional vibrations of circular discs. However, the direct measurement of negative Poisson ratios, which occur in auxetic materials, has been impeded until now by the absence of numerical values of Martinček’s solution for frequency ratios smaller than 1.35, because the relevant ASTM standards have not considered the possibility of negative Poisson ratios so far. Here we provide this missing link by reconsidering Martinček’s solution and presenting the necessary values in the forms of graphs and fit relations from which the Poisson ratio, including negative ones (down to −0.273), can be determined from frequency ratios down to 1.1 for a wide range of specimen aspect ratios (down to 4).

Machine learning-based stiffness optimization of digital composite metamaterials with desired positive or negative Poisson's ratio

Mechanical metamaterials such as auxetic materials have attracted great interest due to their unusual properties that are dictated by their architectures. However, these architected materials usually have low stiffness because of the bending or rotation deformation mechanisms in the microstructures. In this work, a convolutional neural network (CNN) based self-learning multi-objective optimization is performed to design digital composite materials. The CNN models have undergone rigorous training using randomly generated two-phase digital composite materials, along with their corresponding Poisson's ratios and stiffness values. Then the CNN models are used for designing composite material structures with the minimum Poisson's ratio at a given volume fraction constraint. Furthermore, we have designed composite materials with optimized stiffness while exhibiting a desired Poisson's ratio (negative, zero, or positive). The optimized designs have been successfully and efficiently obtained, and their validity has been confirmed through finite element analysis results. This self-learning multi-objective optimization model offers a promising approach for achieving comprehensive multi-objective optimization.

Negative Poisson’s Ratios of Layered Materials by First-Principles High-Throughput Calculations

Auxetic two-dimensional (2D) materials, known from their negative Poisson’s ratios (NPRs), exhibit the unique property of expanding (contracting) longitudinally while being laterally stretched (compressed), contrary to typical materials. These materials offer improved mechanical characteristics and hold great potential for applications in nanoscale devices such as sensors, electronic skins, and tissue engineering. Despite their promising attributes, the availability of 2D materials with NPRs is limited, as most 2D layered materials possess positive Poisson’s ratios. In this study, we employ first-principles high-throughput calculations to systematically explore Poisson’s ratios of 40 commonly used 2D monolayer materials, along with various bilayer structures. Our investigation reveals that BP, GeS and GeSe exhibit out-of-plane NPRs due to their hinge-like puckered structures. For 1T-type transition metal dichalcogenides such as MX 2 (M = Mo, W; X = S, Se, Te) and transition metal selenides/halides the auxetic behavior stems from a combination of geometric and electronic structural factors. Notably, our findings unveil V2O5 as a novel material with out-of-plane NPR. This behavior arises primarily from the outward movement of the outermost oxygen atoms triggered by the relaxation of strain energy under uniaxial tensile strain along one of the in-plane directions. Furthermore, our computations demonstrate that Poisson’s ratio can be tuned by varying the bilayer structure with distinct stacking modes attributed to interlayer coupling disparities. These results not only furnish valuable insights into designing 2D materials with a controllable NPR but also introduce V2O5 as an exciting addition to the realm of auxetic 2D materials, holding promise for diverse nanoscale applications.

Design, preparation and characterization of braiding auxetic yarns with a high negative Poisson’s ratio

In recent years, research work on auxetic textile materials has become more and more popular. Helical auxetic yarns realize a negative Poisson’s ratio effect at the yarn level, wherein the braided auxetic yarns are prepared to realize the structural stability of the auxetic yarns. However, the degree of the elongation auxetic effect has not been solved. In this paper, composite braided auxetic yarns and re-braided auxetic yarns were successfully fabricated by a ring spinning machine and a high-speed braiding machine, and the Poisson’s ratio curves of the two yarns were tested by image analysis. Through the measurement of the Poisson’s ratios of these two yarns, the experimental results show that they both have a good negative Poisson’s ratio effect, and the degree of auxetic effect increases correspondingly. The composite braid technology makes the auxetic yarns retain the original auxetic effect while the degree of auxetic effect was increased. The preparation of these two kinds of auxetic yarns provides a certain experimental basis for the development of auxetic textile materials in the future, and also provides more improvement ideas for scholars.

Hysteresis behavior of Auxetic Perforated Steel Plate Shear Walls with elliptical and peanut-shaped cutouts

Although auxetic (negative Poisson’s ratio) materials and structures have unique mechanical properties, their application in earthquake-resistant structures is rarely reported. This paper proposed a novel Auxetic Perforated Steel Plate Shear Wall (AP-SPSW) for seismic mitigation in building structures. The AP-SPSW incorporates the unique mechanical properties of auxetic materials and structures to reduce out-of-plane buckling and improve seismic performance of SPSWs. Both elliptical and peanut-shaped perforation configurations were proposed for the AP-SPSW. Through small-scale numerical simulations of the auxetic unit cells, the deformation mode of the auxetic perforated material was discussed and a preliminary design was presented to ensure the materials with negative Poisson’s ratio. In addition, ten AP-SPSW specimens were prepared for cyclic loading tests, which demonstrated that the AP-SPSW possesses stable hysteresis behavior and excellent energy dissipation capacity. Key parameters affecting the hysteresis performance of the AP-SPSW were identified as porosity of the perforated panel and thickness of the ligament between neighboring perforations. This study also focused on the global shear buckling behavior of the auxetic perforated panel plates. It was found that the auxetic deformation mechanism benefits shear buckling resistance in the SPSW panel. Finally, an analytical model was proposed for the shear strength of the auxetic panel. The limitations of this study were also discussed, along with the work required for further exploration. The results of this study provide valuable insights for the development of innovative seismic-resistant structures that incorporate auxetic materials and structures.

Metamaterials with Poisson's ratio discontinuity by means of fragmentation–reconstitution rotating units

This paper presents for the first time two types of metamaterials based on the fragmentation–reconstitution of rotating units in order to produce Poisson's ratio discontinuity at the original state. For both metamaterials, each rotating unit takes the form of a rhombus that comprises eight sub-units. During on-axis stretching, each rhombus fragments into eight rotating sub-units. When the prescribed strain is reversed, these eight sub-units reconstitute back into a single rotating rhombus such that they rotate as a rigid body. Using geometrical construction, the incremental Poisson's ratio was established at the original state. In the case of large deformation, the finite Poisson's ratio was formulated in conjunction with the maximum allowable rotations for full stretching along both axes and for full compression. The family of on-axes Poisson's ratio versus rotational angles for various shape descriptors displays a fork-shaped distribution with discontinuity at the original state. Two major distinguishing factors of these metamaterials—property discontinuity at the original state with constant and variable Poisson's ratio under compression and tension, respectively—allow them to function in ways that cannot be fully performed by conventional materials or even by auxetic materials and metamaterials.

A NOVEL YARN FOR PERSONEL PROTECTION IN KNITTED SPORTSWEAR

In the textile and clothing sector, a wide range of auxetic textiles have been made and shown great application potential in many areas. Creating auxetic effect at the yarn stage is relatively a simple approach since helical auxetic yarns (HAY) can be made only by winding or twisting different conventional filaments together with existing spinning machinery. Employing sports safety equipment is a cost-effective solution for avoiding injury and increasing the safety and protection. In the area of materials development for sports safety equipment, an important candidate is auxetic materials. In this study, polyester filament/elastane based helical auxetic yarns (HAY) and knitted fabrics from the yarns were developed that will offer dampening effect against injuries during sports activities. Effects of count of the elastane component and the presence of a third component in the HAY structure on the auxetic behaviour of the knitted fabrics were also studied.

A Novel Quasi-Planar Two-dimensional Carbon Sulfide with Negative Poisson's ratio and Dirac fermions.

In the present study, a novel and unconventional two-dimensional (2D) material with Dirac electronic features hasbeen designed using sulflower with the help of density functional theory methods and first principles calculations.This 2D material comprises of hetero atoms (C, S) and belongs to the tetragonal lattice with P4/nmm space group. Scrutiny of the results show that the 2D sheet exhibits a nanoporous wave-like geometrical structure. Quantummolecular dynamics simulations and phonon mode analysis emphasize the dynamical and thermal stability. Thenovel 2D sheet is an auxetic material with an anisotropy in the in-plane mechanical properties. Both compositionand geometrical features are completely different from the necessary conditions for the formation of Dirac conesin graphene. However, the presence of semi-metallic nature, linear band dispersion relation, massive fermions andmassless Dirac fermions are observed in the novel 2D sheet. The massless Dirac fermions exhibit highly isotropicFermi velocities (vf = 0.71 × 106 m/s) along all crystallographic directions. The zero-band gap semi metallic featuresof the novel 2D sheet are perturbative to the electric field and external strain.

Computational evaluation of absorption characteristics of ceramic-based auxetic materials in X-band frequency range

This research study investigates the absorption capabilities of ceramic-based auxetic metamaterials within the X-band frequency range, emphasising their potential application in stealth technology. Four distinct auxetic topologies have been chosen for this purpose: star, re-entrant, anti-tetrachiral, and missing-rib/cross-chiral while maintaining an equal cross-sectional area for comparison analysis. A computationally efficient homogenisation scheme based on the variational asymptotic method is used to evaluate the effective properties of these auxetics. The absorption spectra are then obtained by evaluating scattering matrices using these effective properties. The influence of auxetics out-of-plane thickness, incidence and polarisation angles on the proposed ceramic absorber’s absorption spectra is evaluated. One of the interesting observations is the identical absorption capabilities of star and missing-rib/cross-chiral geometries despite their distinct architectures. The star and missing-rib/cross-chiral based absorbers achieved a maximum absorption of 99.99% or a minimum reflection loss (RL) of −40 dB with a thickness of 3.50 mm. The RL is less than −10 dB (the standard for an electromagnetic (EM) absorber) for all incidence angles less than 70∘. The findings of this study hold significant potential for the advancement of ceramic-based auxetic metamaterials in EM absorption applications within the aerospace industry.

Bioinspired Integrated Auxetic Elastomers Constructed by a Dual Dynamic Interfacial Healing Strategy.

Auxetic materials are appealing due to their unique characteristics of transverse expansion while being axially stretched. Nevertheless, current auxetic materials are often produced by the introduction of diverse geometric structures through cutting or other pore-making processes, which heavily weaken their mechanical performance. Inspired by the skeleton-matrix structures in natural organisms, this study reports an integrated auxetic elastomer (IAE) composed of high-modulus cross-linked poly(urethane-urea) as a skeleton and low-modulus non-cross-linked poly(urethane-urea) as a complementary-shape matrix. Benefiting from disulfide bonds and hydrogen-bond-promoted dual dynamic interfacial healing, the resulting IAE is flat, void-free, and has no sharp soft-to-hard interface. Its fracture strength and elongation at the break are increased to 400% and 150%, respectively, of the values of corrugated re-entrant skeleton alone, while the negative Poisson's ratio (NPR) reserves within a strain range of 0%-104%. In addition, the advantageous mechanical and auxetic properties of this elastomer are further confirmed by finite element analysis. The concept of combining two dissimilar polymers into an integrated hybrid material solves the problem of the deterioration in mechanical performance of auxetic materials after subtractive manufacturing, while preserves the NPR effect in a large deformation, which provides a promising approach to robust auxetic materials for engineering applications.