Auxetic Materials Research Articles

Negative longitudinal piezoelectric effect and electric auxetic effect in ferroelectric HfO2 and related fluorite-structure ferroelectrics

Unusual negative longitudinal piezoelectric effect (NLPE) and electric auxetic effect (EAE) have essential implications for designs of piezoelectric sensors and actuators. The emerging ferroelectric HfO2 is recently discovered to have both effects, while the underlying physical mechanisms remain elusive. To understand and regulate these intriguing effects, it is crucial to investigate the piezoelectricity in ferroelectric HfO2 and related fluorite-structure ferroelectrics. Here, we corroborate using first-principles calculations that all twelve fluorite-structure ferroelectrics covered in this study possess the NLPE. A chemical tendency of piezoelectricity is demonstrated, i.e., the larger the “iconicity,” the stronger the NLPE. The structural origin is attributed to the predominant influence of the triple-coordinated anion displacement, namely, the more “ionic” fluorite-structure ferroelectrics exhibit larger anion displacement under a pressure or strain, which gives rise to a more negative internal-strain contribution dominating over the positive clamped-ion contribution and hence a stronger NLPE. Moreover, we confirm several electric auxetic materials in fluorite-structure ferroelectrics with finite electric field calculations. We find that the piezoelectricity of electric auxetic materials is suppressed by the external electric field along the polar direction, since it weakens the bonding heterogeneity. The unraveled fundamental understanding of the NLPE and EAE in this study may profoundly benefit the design and application of fluorite-structure ferroelectrics.

A novel three-component helical auxetic yarn with staple fibers prepared by ring spinning

Auxetic materials with negative Poisson’s ratio have excellent physical and mechanical properties, such as shear resistance, fracture resistance and energy absorption. To expand the application of the helical auxetic yarn, a novel three-component helical auxetic yarn was developed by utilizing modified ring spinning technology. The three-component helical auxetic yarn incorporated staple fiber components into the traditional two-component helical auxetic yarn, using cotton (staple wrap fiber)/polyester (stiffer wrap filaments)/spandex (elastomeric core filaments) as the raw material for spinning. Compared with two-component helical auxetic yarn, the addition of cotton components did not significantly impact Poisson’s ratio, and the three-component helical auxetic yarn maintained a relatively stable negative Poisson’s ratio under prolonged strain. The spinning parameters of helical auxetic yarns have an impact on the yarn’s structure, which subsequently impacts the yarn’s negative Poisson’s ratio. It was found that there were significant differences in Poisson’s ratio of helical auxetic yarn between different twist factors and draft ratio of the filaments. The test results revealed that, the maximum negative Poisson’s ratio of the three-component helical auxetic yarn could reach −4.66 under a twist factor [Formula: see text] of 3.40, an elastomeric core filament draft ratio of 1.02 and a stiffer wrap filament draft ratio of 1.03.

Blast-Resistant Design of Reinforced Concrete Slabs with Auxetic-Shaped Reinforcement Layout

This paper presents a numerical study of a blast-resistant design of reinforced concrete panels with a novel auxetic reinforcement layout inspired by auxetic materials, which have a negative Poisson’s ratio, i.e., shrink under compression and expand under tension. A series of two-way supported panels reinforced with re-entrant auxetic-shaped rebars were numerically tested under a TNT explosion. The high-fidelity multi-physics explicit solver of LS-DYNA was utilized to analyze the efficiency of the proposed design. Firstly, the incident pressure of a TNT explosion data and the structural response of a conventional reinforced concrete panel under a TNT explosion were successfully validated by comparing with the experimental and empirical results. Secondly, the blast-resistant capacity of the proposed model was evaluated in comparison to two different conventional designs. Moreover, a parametric study was carried out to reveal the driving parameters of the newly proposed auxetic-shaped reinforcement design. It has been proved that the proposed auxetic reinforcement layout significantly reduces the spalling radius and increases the energy absorption capacity of panels. As a result of the parametric study, the increased reinforcement volume ratio was ineffective on the spalling radius, although the cell size of auxetic reinforcement was found to be quite effective for the blast-resistant design of concrete panels. Overall, the proposed re-entrant auxetic reinforced panel performed far better than conventional designs under blast load. With the recent developments in 3D printing technology, the proposed auxetic reinforcement layout is a strong candidate to deal with blast-resistant designs of concrete panels.

Effect of structural parameters on formability of auxetic weft-knitted fabrics based on zigzag structure

Auxetic materials are characterized by a negative Poisson’s ratio (NPR), which expands when stretched and shrinks when compressed. Due to the synclastic curvature of auxetics, they have good formability. These properties make them useful in applications requiring formability, such as composites, fashion and clothing, medical, protective, and sports fields. In this research, auxetic weft-knitted fabrics based on foldable structures were knitted. The effect of structural parameters including fabric pattern, yarn count, and loop length on the formability of auxetic weft-knitted fabrics was investigated and compared to plain weft-knitted fabric (Jersey). Auxetic fabrics with three distinct patterns and three different loop lengths were knitted using polyester yarns of varying three counts. Then, the formability experiment was conducted using the hemisphere mold, and the forming energy of fabric samples was obtained. The results demonstrated that auxetic fabrics exhibit superior formability to conventional (non-auxetic) fabrics, attributed to their synclastic curvature. Among the auxetic fabrics, those with larger zigzag angles, finer yarn counts, and larger loop lengths showed higher formability. These findings enhance the potential applications of auxetic fabrics in various industries such as medical and protective, emphasizing their ability to conform to complex shapes with minimal wrinkling.

A novel engineered sandwich composite with ceramic-coated graphite fibre-reinforced face sheets and an auxetic core for enhanced broadband electromagnetic absorption: design using multiscale computational analysis

Abstract A novel sandwich composite is designed for enhanced broadband absorption using a multiscale computational analysis. The sandwich panel is configured with BaTiO3 coated graphite fiber reinforced polymer (C-GFRP) composite, while BaTiO3 based auxetic material is used as the core material. A computationally efficient in-house tool based on the variational asymptotic method is used to homogenise the electromagnetic properties and evaluate the reflection loss characteristics of the proposed sandwich structure using the scattering matrices. A detailed parametric study of 300 analyses is conducted to identify the optimal sandwich panel configurations for achieving broadband reflection loss under the normal incidence of transverse electric (TE) and transverse magnetic (TM) polarised waves. Two different configurations have been obtained, satisfying the requirements of broadband reflection loss in the X-band frequency range. Configuration (a) Vf = 15% without any ceramic coating, and configuration (b) Vf = 20% with ceramic coating volume fraction (Cf) of 70%. Configuration (a) and (b) maintained the desired reflection loss of greater than -10 dB up to an incidence angle of 40◦ and 60◦, respectively. With the demonstrated capability of covering the entire broadband frequency range, the proposed configurations can serve as baselines to explore novel ceramic-based engineered composite architectures for broadband absorption applications.

Unlocking Auxetic Behavior in Recyclable Thermosetting Foams Enabled by Dynamic Disulfide Cross‐linking Strategy

AbstractAuxetic foams with a negative Poisson's ratio (NPR) have attracted considerable attention in material engineering due to their outstanding performance in seismic and energy absorption. Nevertheless, thermoplastic auxetic foams are compromised by weak non‐covalent crosslinking that diminishes the mechanical strength and durability of foams. Conversely, thermosetting foams with chemical crosslinking, although mechanically robust, face challenges in elaborating auxetic structure and in achieving recyclability. Herein, an alternative approach is proposed to tackle this dilemma by incorporating dynamic disulfide bonds into the polymer network for preparing a thermosetting polyurethane foam with covalent adaptable network. By leveraging the unidirectional multi‐effect compression technique, the topological network reorganization of foam is induced, transforming the initial circular open‐cell structure into a re‐entrant cell structure. This structural transformation endows the foam with stable NPR capability, achieving a minimum Poisson's ratio value of −0.4 within 30% compressive strain. Benefiting from its reinforced network structure, the foam also demonstrates high compressive strength (6.47 MPa) and tensile strength (1.67 MPa). Furthermore, it is recyclable and can be recompressed into thermosetting films. This work offers a straightforward approach to making auxetic thermosetting foams with good mechanical and recyclable properties, which is interesting for the development of high‐performance auxetic materials.

Dynamic damage behavior of auxetic textile reinforced concrete under impact loading

To investigate the dynamic compression behavior of Textile-reinforced concrete (TRC) under impact load, low speed and high speed impact tests were performed using a ball drop test and a split Hopkinson pressure bar (SHPB) with ϕ50mm, respectively. In view of the advantages of auxetic materials in impact resistance and energy absorption, the auxetic fiber preforms with negative Poisson's ratio effect were woven into meshes and added to the cement-based materials to obtain the auxetic textile-reinforced concrete. Thus, the auxetic textile reinforced cement-based material is obtained. In this paper, TRC with different textile types, laying layers and surface treatments were prepared, and impact tests with different impact speeds were carried out to evaluate the impact resistance of TRC. The impact damage behavior of TRC was analyzed through the method of digital scattering analysis, and then the impact damage mechanism of TRC was investigated. The results show that the auxetic effect of auxetic textiles can still be stabilized under impact loads of different velocities, which provides better control of deformation and energy consumption in the matrix material. Under different impact heights of falling balls, the impact fracture energy of cementitious materials reinforced with auxetic textiles was increased by up to 22.5 and 20 times, respectively, compared with that of blank samples.

Numerical and experimental investigation of 3D printed tunable stiffness metamaterial with real-time response using digital light processing technology

A tunable mechanical metamaterial is a type that can be altered or manipulated to change its physical properties in various situations. This study introduces a novel type of metamaterial, hydro-tunable metamaterials (HTMs), which can be dynamically adjusted in real-time by filling or draining it with water. The development of HTMs has significant implications for various fields, including mechanical engineering, biomedical applications, and energy absorption. The study involves designing and manufacturing HTMs using a Digital Light Processing (DLP) 3D printing process. The design process involves modifying an initial basic auxetic structure to create a closed and sealed structure accommodating fluid. The printed samples are then characterized using mechanical testing and finite element analysis (FEA). Experiments and simulations have found that a sample containing water behaves differently from a sample without water, resulting in an increase in stiffness. This difference can be leveraged to modify the stiffness and strength of the structure. This phenomenon is attributed to the incompressibility of water within the structure. Water exerts a hydrostatic pressure on the auxetic material, resulting in increased stiffness and resistance to compression. This technique highlights the potential of HTMs to be dynamically adjusted in real-time, leading to enhanced energy absorption and improved performance. An additional FEA was conducted to examine the impact of water pressure on the mechanical behavior of the structure. The results indicate that applying pressure or temperature to the water can significantly enhance the mechanical properties of the water-filled sample.

Recent Advances in Wearable Electromechanical Sensors Based on Auxetic Textiles

AbstractTextile‐based electromechanical sensors are increasingly used as wearable sensors for various applications, such as health monitoring and human‐machine interfaces. These sensors are becoming increasingly popular as they offer a comfortable and conformable sensing platform and possess properties that can be tuned by selecting different fiber materials, yarn‐spinning techniques, or fabric fabrication methods. Although it is still in its early stages, recent attempts have been made to introduce auxeticity to textile sensors to enhance their sensitivity. Having a negative Poisson's ratio, i.e., undergoing expansion laterally when subjected to tensile forces and contraction laterally under compressive forces, makes them distinct from conventional sensors with positive Poisson's ratio. This unique feature has demonstrated great potential in enhancing the performance of electromechanical sensors. This review presents an overview of electromechanical sensors based on auxetic textiles (textiles made from auxetic materials and/or non‐auxetic materials but with auxetic structures), specifically focusing on how the unique auxetic deformation impacts sensing performance. Sensors based on different working mechanisms, including piezoelectric, triboelectric, piezoresistive, and piezocapacitive, are covered. It is envisioned that incorporating auxeticity and electromechanical sensing capabilities into textiles will significantly advance wearable technology, leading to new sensors for health monitoring, fitness tracking, and smart clothing.

Transient thermal fracture analysis of a honeycomb layer with a central crack

Auxetic honeycomb materials with negative Poisson’s ratio received rapidly growing attention in recent years owing to their exceptional thermomechanical properties in constructing sandwich composites. The objective of this article is to investigate the transient thermal behavior and fracture risk of a honeycomb layer with a central crack subject to a sudden thermal shock by theoretical modeling. Two different material configurations along both orthogonal directions, as well as the conventional and auxetic hexagonal alumina honeycomb cells, are examined to illustrate their effects on the thermal stress intensity factors. To solve the thermoelastic governing equations subject to complex boundary conditions, the methodology of integral transform with singular integral equation is employed. The numerical results demonstrate that the auxetic honeycombs can reduce the thermal stress intensity factors significantly compared to their conventional counterparts. The ratio of stress intensity factors in auxetic to non-auxetic honeycombs under the same absolute value of the internal cell angle θ decreases monotonically with increasing |θ|. In addition, the effects of the relative density, crack length, and crack position are investigated. Our findings would provide a more comprehensive understanding of the honeycomb’s fracture behaviors and contribute to the material design of the sandwich composites with honeycomb cores.

Coupling Chiral Cuboids with Wholly Auxetic Response.

Auxetic materials have been extensively studied for their design, fabrication and mechanical properties. These material systems exhibit unique mechanical characteristics such as high impact resistance, shear strength, and energy absorption capacity. Most existing auxetic materials are two-dimensional (2D) and demonstrate half-auxetic behavior, characterized by a negative Poisson's ratio when subjected to either tensile or compressive forces. Here, we present novel three-dimensional (3D) auxetic mechanical metamaterials, termed coupling chiral cuboids, capable of achieving negative Poisson's ratio under both tension and compression. We perform experiments, theoretical analysis, and numerical simulations to validate the wholly auxetic response of the proposed coupling chiral cuboids. Parametric studies are carried out to investigate the effects of structural parameters on the elastic modulus and Poisson's ratio of the coupling chiral cuboids. The results imply that the Poisson's ratio sign-switching from negative to positive can be implemented by manipulating the thickness of Z-shaped ligaments. Finally, the potential application of the coupling chiral cuboids as inner cores for impact-resistant sandwich panels is envisioned and validated. Test results demonstrate a remarkable 49.3% enhancement in energy absorption compared to conventional solid materials.

Auxetic Biomedical Metamaterials for Orthopedic Surgery Applications: A Comprehensive Review.

Poisson's ratio in auxetic materials shifts from typically positive to negative, causing lateral expansion during axial tension. This scale-independent characteristic, originating from tailored architectures, exhibits specific physical properties, including energy adsorption, shear resistance, and fracture resistance. These metamaterials demonstrate exotic mechanical properties with potential applications in several engineering fields, but biomedical applications seem to be one of the most relevant, with an increasing number of articles published in recent years, which present opportunities ranging from cellular repair to organ reconstruction with outstanding mechanical performance, mechanical conduction, and biological activity compared with traditional biomedical metamaterials. Therefore, focusing on understanding the potential of these structures and promoting theoretical and experimental investigations into the benefits of their unique mechanical properties is necessary for achieving high-performance biomedical applications. Considering the demand for advanced biomaterial implants in surgical technology and the profound advancement of additive manufacturing technology that are particularly relevant to fabricating complex and customizable auxetic mechanical metamaterials, this review focuses on the fundamental geometric configuration and unique physical properties of negative Poisson's ratio materials, then categorizes and summarizes auxetic material applications across some surgical departments, revealing efficacy in joint surgery, spinal surgery, trauma surgery, and sports medicine contexts. Additionally, it emphasizes the substantial potential of auxetic materials as innovative biomedical solutions in orthopedics and demonstrates the significant potential for comprehensive surgical application in the future.

Revealing the mechanism of the electron-transfer induced and enhanced intrinsic auxeticity in 2D penta-materials

The intrinsic auxeticity of materials has long been attributed to their unique geometric configurations. Triggering and enhancing auxeticity presents significant challenges, as many nanomaterials display subtle auxetic effects with a negative Poisson's ratio (NPR) typically above -0.4. This paper introduces three materials with identical symmetry—two-dimensional pentagonal structures Penta-B2×2Y2 (i.e., Penta-B2C4, Penta-B2N4, and JP-B2C2N2)—to investigate a novel mechanism influencing material auxeticity: electron transfer. Specifically, variations in electron transfer cause Penta-B2N4 and JP-B2C2N2 to show axial auxetic effects, whereas Penta-B2C4 demonstrates opposite behavior. The axial NPR of Penta-B2N4 is ∼ -0.05, while for JP-B2C2N2, it reaches ∼-0.46, significantly surpassing that of typical intrinsic auxetic materials. This represents an increase of approximately 920 % compared to Penta-B2N4. Additionally, JP-B2C2N2 features semi-metallic properties, where the conduction band minimum (CBM) and the valence band maximum (VBM) are tangent to the Fermi level. Consequently, minor alterations in external conditions can induce a transition between semiconductor and metallic states in JP-B2C2N2. Thus, the 2D material JP-B2C2N2 emerges as a promising candidate for nanoelectronic and electromechanical applications. Furthermore, this study also enhances the academic understanding of auxetic properties in nanomaterials by linking their mechanical and electronic characteristics and laying a theoretical foundation for further experimental exploration of auxeticity.

Handles with Reentrant Cells for Use as Oar Handles: Design Considerations, Physical Characteristics, and End‐Users’ Perceptions

Rowing is a sport that requires athletes to perform the action of pulling and rotating an oar by hand, applying forces of considerable magnitude. Herein, inspired by the notion that auxetic materials and metamaterials behave differently compared to their conventional counterparts, it is examined how a handle prototype, specifically designed to incorporate the classic re‐entrant motif, behaves and how end‐users perceive it. Physical experiments conducted on such prototype, which measured the contact pressures, suggest that on average, higher pressures are measured when pulling with this reentrant grip compared to its non reentrant counterpart, indicating that this re‐entrant prototype should feel firmer. More importantly, respondents of a survey are asked to give their feedback, and different views on which handle they would prefer to use are provided. The ones who preferred the prototype with the reentrant features report that they preferred it because it felt firmer and allowed for a better grip. This suggests that there is potential for further investigation into whether handles, oar handles in particular, made from auxetic components, re‐entrant cells, or other motifs which are well known for their negative Poisson's ratio characteristics, could provide a better and more secure grip and be used in sports applications.

A Split-Plot Experimentation Strategy for Making Causal Inferences in Advanced Materials: Auxetic Polyurethane Foam Manufacturing and Processing Analysis.

Development of advanced materials is often time consuming and expensive because of the large number of variables involved and experiments needed. An effective experimentation strategy would accelerate development by reducing the required amount of experiments without sacrificing the obtainable information. In this paper, the development of auxetic polyurethane (PU) foams was discussed as a case study. Auxetic materials are materials with a negative Poisson's ratio and have potential in many structural and functional applications. Auxetic PU foams are the most studied auxetic materials, and their manufacturing and properties are affected by many processing and environmental factors. This paper introduces a sophisticated design of experimental methodology to help reduce the experimental effort while effectively screening these factors. This methodology is then applied in an experiment to illustrate its utility and distinct advantages that greatly facilitate material development.

Topology Optimization of Auxetic Hyperelastic Biomaterials for Enhanced Tailored Properties and Ultra‐High Expansion

AbstractTopology optimization using finite element analysis offers a promising approach for designing new biomaterials with mechanical properties similar to human skin and superior auxetic properties compared to conventional materials. This innovative technique addresses the challenges associated with trial‐and‐error‐based material design and experimental iterations. In this study, finite element‐based topology optimization is employed to achieve optimized material structural patterns that maximize the lateral expansion of previously established auxetic designs and replicate specific directional properties observed in human skin. The topology‐optimized models, including slit, I‐shape, anisotropic I‐shape, triangular pattern, and re‐entrant shapes, demonstrate material structures with lower maximum stress, reducing the likelihood of material rupture failure and enhancing lateral expansion by more than 50% compared to the initial patterns. These novel designs and their unique behaviors are verified using the theoretical model of hyperelastic auxetic materials. This study represents the first analysis of topology optimization applied to soft hyperelastic biomaterials, focusing on both maximizing auxetic properties and replicating auxetic properties of human skin. The auxetic designs and topology optimization technique developed in this research hold potential for integration into biomedical and personal protective applications.

Auxetic textiles, composites and applications

Auxetic textiles are intriguing materials with unusual capabilities. These materials exhibit a negative Poisson’s ratio with extraordinary performance in toughness, resilience, shear resistance, and acoustic properties mainly due to their special structure and associated deformation mechanics. Auxetic materials can also have applications around energy absorption and can effectively be used for vibration damping and shock absorbency. The exceptional behaviour of auxetic textiles can be obtained by utilising specific fibrous materials and introducing auxetic geometry and structures differently during weaving, knitting, and nonwoven manufacturing. This issue of Textile Progress highlights the fundamental aspects of various auxetic structures and their properties in general and a detailed analysis of auxetic textile structures and composites, their manufacturing processes, modelling, characterisation, and applications. These materials can potentially revolutionise their applications in sports, automotive, construction industry, biomedical engineering, aerospace, marine, and defence personal protective equipment. Fundamental understanding of auxetic geometry, followed by developing and analysing these geometries by analytical and computational modelling, translating the geometry into appropriate textile structures for actual fabric production, characterisation of auxetic fabrics and their composites, and finally, some innovative applications in technical textiles are some of the fascinating issues addressed in this review.

Auxetic Two‐Phase Chevron Mechanical Metamaterial

This investigation explores novel two‐phase chevron mechanical metamaterials that exhibit auxetic properties. Unlike traditional foam‐like cellular or porous auxetic materials, these designs are composed of chevron patterned layers embedded in anisotropic matrix. This innovation design allows for auxeticity in two orthogonal in‐plane directions (bi‐auxeticity) or in all in‐plane directions (complete auxeticity), providing not only a general strategy but also detailed guidelines for designing non‐porous auxetic mechanical metamaterials with tunable auxetic behaviors. One goal of this work is to explore the mechanical behavior, specifically effective stiffness and Poisson's ratio, of these new designs and to identify the design space for auxetic behavior using numerical and experimental methods. Systematic finite element (FE) simulations are conducted using ABAQUS and Python scripts to quantify effective stiffness and Poisson's ratio within a small strain range. To validate the numerical predictions, three representative designs are selected and fabricated via multi‐material polymer jetting. Uniaxial tension experiments are conducted on these specimens. Design spaces for non‐auxeticity, partial‐auxeticity, and complete auxeticity are identified through an integrated numerical approach. Theoretical criteria for determining the completeness of auxeticity are proposed and verified via FE simulations.

High strain rate effect and dynamic compressive behaviour of auxetic cementitious composites

Masonry walls are known for their limited impact resistance, even when retrofitted with CFRP (carbon fibre-reinforced polymer) and nanomaterials. This paper presents the findings of an experimental study using split Hopkinson pressure bar (SHPB) tests aimed at exploring the potential application of auxetics textile reinforced mortar (TRM) composites in impact protection. This study will provide the initial understanding of the composite as a structural reinforcement solution for masonry walls. Key findings reveal that the peak strength increases with rising strain rates, highlighting significant strain rate sensitivity in TRMs with auxetic (AX) and carbon fabric (CF) reinforcements. AX samples exhibit better energy absorption as compared to the reference plain mortar (PM) samples, particularly at higher strain rates, surpassing CF samples beyond 150 s−1. Moreover, the insertion of auxetic and carbon fabrics eliminates crack development and mitigates the severity of sample failure. The negative Poison ratio effect of auxetic fabrics significantly enhances the lateral confinement, ultimately improving the dynamic performance of AX samples compared to CF samples. These findings underscore the potential of auxetic materials in enhancing dynamic performance, particularly under high strain rates, with clear implications for engineering applications, including in masonry buildings.

On the behavior of auxetic inserts: a numerical analysis to derive design guidelines

Auxetics are a class of materials and metamaterials with a negative Poisson’s ratio (ν) and have gained tremendous popularity over the last three decades. Many studies have focused on characterizing designs that allow obtaining a negative ν. However, some open issues remain concerning understanding the auxetic behavior in operational conditions. Studies have been centered on analyzing the response of specific auxetic topologies instead of treating auxeticity as a property to be analyzed in a well-defined structural context. This study aims to contribute to the investigation of auxetic materials with a structural application, focusing on maximizing performance. The field of application of auxetics for designing inserts was selected and a model of a nail-cavity system was created to determine the effects of different design choices on the system behavior by exploring relationships between selected parameters and the auxetic insert behavior. The exploration combines finite element modeling analyses with their surrogate models generated by supervised learning algorithms. This approach allows for exploring the system’s behavior in detail, thus demonstrating the potential effectiveness of auxetics when used for such applications. A list of design guidelines is elaborated to support the exploitation of auxetics in nail-cavity systems.