Search

• This paper firstly reviews recent development of auxetic composite with fillings. • A key focus lies in the enhancement mechanism and parametric design of auxetic composite with fillings. • Auxetic composites and traditional composites were compared in terms of mechanisms and energy absorption. • It summarises current limitations and future research regarding the design, fabrication, and applications of auxetic composites. Auxetic materials have garnered significant attention due to their lightweight and excellent energy absorption capabilities. Nonetheless, they often display relatively lower stiffness when compared with conventional materials. To address this limitation and enhance their mechanical properties, researchers have explored various avenues, including designing hybrid auxetic structures by combining two or more auxetic unit cells and developing auxetic composites using multiple materials. While previous reviews extensively covered hybrid auxetic structures, discussing their classification, design methodologies, fabrication techniques, applications and mechanical behaviours, there has been a noticeable gap in the literature concerning auxetic composites with fillings. Therefore, this paper concentrates on auxetic composites with fillings, delving into their classifications, mechanical responses, and underlying mechanisms. This review article also critically examines different design factors that influence the performance of auxetic composites and compares them with conventional counterparts in terms of mechanisms and mechanical properties. Overall, auxetic composites exhibit superior mechanical characteristics compared to equivalent conventional materials. However, several challenges and limitations persist regarding the design, fabrication, and applications of auxetic composites. [ABSTRACT FROM AUTHOR]

• Auxetic origami honeycomb structure 3D-printed using stainless steel. • Impact tests on cushioning performance of auxetic origami structures conducted. • Auxetic origami structures exhibit higher plateau stress and specific energy absorption. • A smaller width-to-thickness ratio leads to higher energy absorption. • The folding angle significantly affects the stress and energy absorption. The honeycomb structure with a negative Poisson's ratio, inspired by the Miura origami unit, exhibits three-dimensional negative Poisson's ratio characteristics and effective energy dissipation performance. This unique property provides significant potential in the field of protection applications. This study aims to comprehensively understand the impact of auxetic and origami structures on the cushioning performance of honeycomb structures. For this purpose, 316 L stainless steel specimens were fabricated using 3D printing technology and subjected to drop hammer impact tests, and the results were verified by finite element simulation. This paper focuses on assessing the deformation mode and energy dissipation performance of origami auxetic honeycomb structures with varying width-to-thickness ratios and folding angles under different impact energy levels. The results indicate that the negative Poisson's ratio origami specimens exhibit higher plateau stress and specific energy absorption compared to their positive and zero Poisson's ratio origami counterparts with the same geometry. In addition, a smaller width-to-thickness ratio and higher input impact energy enhance the cushioning performance of honeycomb structures. [ABSTRACT FROM AUTHOR]

Bi-level tailoring of cellular metamaterials involving a dual design space of unit cell and elementary beam level architectures has recently gained traction for the ability to achieve extreme elastic constitutive properties along with modulating multi-functional mechanical behavior in an unprecedented way. This article proposes an efficient analytical approach for the accurate evaluation of all constitutive elastic constants of asymmetric multi-material variably-thickened hexagonal lattices by considering the combined effect of bending, stretching, and shearing deformations of cell walls along with their rigid rotation. A tri-member unit cell is conceptualized, wherein all nine constitutive constants are obtained through the mechanics under one cell wall direction and subsequent repetitive coordinate transformations. We enhance the design space of lattice metamaterials substantially here by introducing multiple exploitable dimensions such as asymmetric geometry, multi-material unit cells and variably-thickened cell walls, wherein the conventional monomaterial auxetic and non-auxetic hexagonal configurations can be analyzed as special cases along with other symmetric and asymmetric lattices such as a range of rectangular and rhombic geometries. The generic analytical approach along with extensive numerical results presented in this paper opens up new avenues for efficient optimized design of the next-generation multi-functional lattices and cellular metamaterials with highly tailored effective elastic properties. • Bi-level tailoring of cellular metamaterials involving a dual design space of unit cell and elementary beam-level architectures. • Accurate evaluation of all constitutive elastic constants of asymmetric multi-material variably-thickened hexagonal honeycomb lattices. • Expanded design space by introducing multiple exploitable dimensions such as: asymmetric geometry, composite multi-material unit cells, variably-thickened cell walls (derived through physics-based rationale) and their coupled effect. • New avenues for efficient optimized design of the next-generation multi-functional lattices and cellular metamaterials with highly tailored effective elastic properties. [ABSTRACT FROM AUTHOR]

• In order to improve the bonding strength between the structures, a new fiber-reinforced CorTube structure is designed and fabricated. • The auxetic characteristic and the effective young's modulus of the structure are analyzed theoretically, and the relationship between the configuration parameters and the theoretical solutions is discussed. • The deformation characteristics and failure forms of the structure under low-speed impact load are studied. • The influence of impact position and impact energy on the mechanical properties of the structure is revealed. Auxetic structures are gaining great attention due to their unique contraction deformation characteristics under compression and impact. In this paper, high performance carbon fiber-reinforced composites are used to fabricate the auxetic structure consist of corrugated sheets and tubes (CorTube). The quasi-static uniaxial compression and low-velocity impact properties of composite CorTube structure are explored. The response of the composite CorTube structures under quasi-static compression loads are analyze through a combination of theoretical analysis, simulations, and experimental tests. Additionally, drop-weight impact tests are conducted using a rigid impactor with a hemispherical head to examine the effects of impact energy levels, impact locations, and corrugated sheet thickness on the impact response of CorTube structure. Enhancing corrugated sheet-tube bonding via modified cross members and reducing tube crushing during quasi-static compression are notable findings. The results also highlight the remarkable auxetic properties of the composite CorTube under low-speed impact, and the impact resistance could be enhanced by increasing the corrugated sheet thickness and stiffness. Various failure modes were observed, including cracks, pits, tube crushing, and delamination. Significantly, peak-impacted specimens exhibited greater maximum displacement with lower peak impact forces. This study offers insights into the deformation and failure modes of auxetic CorTube structures under low-velocity impact. [ABSTRACT FROM AUTHOR]

Blast and ballistic impacts are high-strain rate events that can cause catastrophic damage to structures and endanger human lives. Shielding materials play a critical role in mitigating the risk of injury and damage caused by such events. However, these materials must withstand extreme conditions of pressure, strain, strain-rate, multi-mode stresses, and temperature during impact. The development of novel structures and arrangements, including monolithic, hybrid, and high-performing fiber-reinforced polymers, requires advanced testing techniques to ensure their effectiveness in the field. This state-of-the-art paper reviews the latest experimental techniques for characterizing the impact resistance, shock energy absorption, high strain-rate responses and endurance of materials used for shielding. The paper provides dedicated sections on blast and ballistic impact experimentation techniques, covering diverse testing methods for various material classes and configurations used in shielding. It also discusses complex material mechanics during impact response, projectile classifications and characterizations, and the development of strategies for high-performing shielding configurations. Overall, this review provides a comprehensive overview of the latest experimental techniques for materials subjected to blast/ballistic impact scenarios, offering valuable insights for researchers and practitioners working on the design and testing of shielding materials. [Display omitted] • Experimental techniques for high strain rate characterization - Flyer plate test, Taylor test, Split Hopkinson Pressure bar tests, Drop hammer tests. • For blast characterization, shock tube experiments, charge detonation, ballistic pendulum reviewed. • For ballistic characterization, standards, projectile classifications and characterizations, X-ray computed tomography, Photon Doppler Velocimetry, VISAR reviewed. • Materials - high strength steels, aluminum alloys, ceramics, polymers, fiber reinforced composites, cementitious and cellular (auxetic) metamaterials. [ABSTRACT FROM AUTHOR]

Although the auxetic structure exhibits excellent mechanical properties, the low stiffness, and strength still limit its development. Considering the high stiffness and strength of the sandwich structure, a novel sandwich beam with an enhanced auxetic core (SCH SWB) was proposed in this paper. The accuracy of the finite element (FE) modeling was first verified by quasi-static bending experiments. Then, based on FE simulation, the deformation modes and energy absorption were explored. The results showed that local indentation and global bending deformation coexisted under the mid-span loading, and the core can absorb the most energy. Furthermore, using the analogy method, and based on the yield criterion and indentation response of the foam sandwich beam, a theoretical model was established to predict the force response, which considered the influence of local indentation on the overall bending, and the double-plateau characteristic of the auxetic core. The theoretical results were basically consistent with the simulation. Subsequently, the influence of the key parameters of the core and faces on the mechanical responses was discussed in depth, and it was found that changing the key geometric parameters and the material combination of the faces and the core can significantly vary the energy absorption capacity. Finally, several sandwich beams with different auxetic cores were compared, and the results proved that SCH SWB exhibited superior bending resistance and higher energy absorption. This paper presents a new idea and theoretical basis for studying the mechanical response of auxetic sandwich beams. The results show that the auxetic sandwich beam has great potential in aerospace, automobile crash, impact protection of key engineering structures, etc. • A novel sandwich beam with an enhanced SCH auxetic core was proposed and investigated. • The global bending and local indentation were co-exist under quasi-static concentrated loading. • A theoretical analytical model was firstly established to predict the mechanical response of the beam under fixed boundary. • The contribution of the core on the energy absorption was higher than the top/bottom faces. • The SCH sandwich beam exhibited better bending resistance TEA and SEA capacity compared other common auxetic sandwich beam. [ABSTRACT FROM AUTHOR]

• A neural-network-based framework is proposed for performance prediction and inverse design of a porous auxetic metamaterial. • The real-time prediction towards the auxeticity and stiffness can improve the inverse design. • The framework can be extended to structural material design with complex characteristics. Auxetic metamaterials are widely applied in energy-absorbing systems and soft robots due to their superior mechanical properties. The properties of auxetic metamaterials are determined by their architectures, which means it is feasible to obtain metamaterials with target performance through structural design. This paper proposes a framework with neural networks to generate a nearly real-time prediction of the auxeticity and stiffness simultaneously of porous materials. Trained neural networks accurately provide computationally inexpensive predictions on response histories. Based on the prediction database given by the neural network, the parametric analysis can be conducted to provide the mutual influence and trend relationship of auxeticity and stiffness of the porous metamaterial. In addition, inverse design can also be conducted for seeking optimal architecture in terms of the auxeticity and stiffness quickly and accurately. With the method, the target nonlinear properties can be accurately and rapidly designed for porous metamaterials. The relative error of neural network predictions is less than 1.5% compared with experimental tests, and less than 8.0% compared with the numerical simulations obtained by the finite element method. The proposed framework can be extended to more structural materials to provide design guideline. [Display omitted] [ABSTRACT FROM AUTHOR]

• The development of cementitious structural/load-bearing elements from auxetic materials is scarce. • Achieving auxeticity while satisfying serviceability limit state for load-bearing cementitious composites remains a challenge. • The bond pull-out behaviour of auxetic materials from cementitious matrices remains largely unknown. • Helical auxetic yarn system can be developed for metallic fibres and used as fibre-reinforcements in cementitious matrices. • Further investigation into the shear behaviour of auxetic structures would provide insight into their shear resistance and use in shear walls. Auxetic materials expand in the lateral direction when stretched axially and contract laterally when compressed axially, thereby resulting in a negative Poisson's ratio. This counter-intuitive behaviour results in such materials having a very wide range of potential benefits such as lateral confinement and improved bonding with cementitious matrices. This phenomenon has resulted in a proliferation of research in the use of auxetic materials in cementitious construction. However, numerous studies have focused on laboratory-scale auxetic cementitious composite samples for non-structural applications, while only very few recent studies have attempted to achieve auxetic behaviour for full-scale structural elements. Studies on auxetic cementitious materials have continued to be exploratory, with a variety of reported findings together with differing recommendations. This paper, therefore, reviews the state of the art on the application of auxetics in cementitious construction, the challenges and opportunities associated with the development and use of these innovative materials, and recommendations for future research to encourage uptake of these materials by engineers. It examines more than 100 primary research articles on the mechanical properties, design, optimisation, and specific applications of auxetics in cementitious composites. An important finding from the review is that the benefits derived from auxetic reinforcements require deformations which far exceed serviceability limits specified for structural elements in static loading. Therefore, the application of a chosen auxetic geometry will require bespoke design procedures to satisfy both strength and stiffness requirements, especially for cementitious composites. Furthermore, the finite element modelling approach of concrete damage plasticity is also noted as an essential tool for analysing the significant deformation behaviour of the structures. The review concludes that there is great potential for auxetic materials and structures through the careful selection of application-specific materials, and the enhancement of bonding between the cement matrix and the auxetic phase. Moreover, hybridising the geometries of the auxetic reinforcement can maintain a balance between the stiffness essential for load-bearing members and the advantages derived from auxeticity. [Display omitted] [ABSTRACT FROM AUTHOR]

Auxetic materials and structures have been attracting attention due to their extraordinary mechanical properties that stand out due to their high capacity to absorb energy. Some types of auxetic tubular structures have been studied and designed in diverse engineering fields such as mechanical, aerospace and medical. This manuscript cites more than a hundred papers containing the definitions, designs, structural analyses and optimization, mechanical properties, and specific applications of tubular design of auxetic structures. It can be noted from the present paper that additive manufacturing has been one of the most common manufacturing techniques used in many cases to manufacture tubular structures, and numerical analysis was essential to analyzing the behavior of the structures. The purpose of this paper is to assist researchers and engineers in using the methodology step by step to develop and apply auxetic tubular structures. • A review of auxetic tubes: definitions, optimization, properties, and applications. • Discussion about the potential applications of auxetic tubular structures. • Design and mechanical behavior of tubular auxetic structures. • Manufacturing trends of general auxetic structures and auxetic tubes. • Challenges and future directions on the auxetic tubes. [ABSTRACT FROM AUTHOR]

• The star-shaped tubular structures of STL structures were designed, manufactured and studied. • The proposed structures exhibited two distinct deformation modes. • The STL structure has a better auxetic behavior with different materials under quasi-static uniaxial compression. • The auxetic effect of STL structures were investigated under different load direction. A majority of tubular structures have been widely studied due to their superior mechanical performance. Existing works on star-shaped tubular structures have focused on improving their stability and energy absorption performance. This paper proposes a novel star-shaped tubular lattice structure (STL), which not only possesses excellent mechanical properties but also exhibits exceptional auxetic effect. In addition, such structures exhibited two distinct deformation modes under different loading directions. Numerical and experimental studies of the mechanical behavior of the star-shaped tubular structure demonstrated low peak stresses under lateral loading and superior bearing capacity and stability under axial compression. Most importantly, the structure embodied a significant auxetic effect under the direction of both loads. The mechanical performance of the star-shaped tubular structures was investigated by changing wall thickness and angles, which can realize the optimal design. This study enriches the research on star-shaped tubular structures and provides a new perspective and reference for the design of auxetic tubular metamaterials in the future. Furthermore, the star-shaped tubular structure with auxetic behavior has considerable potential for applications in civil engineering and protective fields. [Display omitted] [ABSTRACT FROM AUTHOR]

Re-entrant anti-trichiral honeycomb is a typical auxetic material. The collapse process can be divided into three stages while its negative Poisson's ratio (NPR) effect under large deformation is still unclear. In the present paper, the NPR effects of re-entrant anti-trichiral honeycombs under large deformation are studied by numerical and theoretical method. Numerical results are verified by the experiments with respect to their deformation mechanism and stress-strain behavior. Theoretical formulas are derived to predict the NPR effect of the honeycomb under large deformation. The analytical predictions are in good agreement with the FEM results. It is revealed that the NPR effect of the honeycomb is magnified with the global strain in the second stage while is weaken in the third stage of the deformation process. The decrease of the ligament-ratio and the mean radius will result in more obvious NPR effect. Variation of cell-wall thickness doesn't have much impact on the Poisson's ratio in the second stage, while smaller cell-wall thickness will induce larger NPR in the third stage. The present work is supposed to shed light on the design and fabrication of re-entrant anti-trichiral honeycombs with desired Poisson's ratio. Image 1 • Analytical predictions of the NPR effect under large deformation exhibits a good agreement with the numerical results. • A special deformation mechanism and stress-strain behavior are illustrated by the experiments and numerical simulations. • The negative Poisson's ratio (NPR) of the honeycombs varies differently with strain at different stages. • Variation of cell-geometry parameters greatly influence the NPR effect of the honeycombs. • More evident NPR effect is exhibited when the honeycomb gradually develops into the conventional re-entrant honeycomb. [ABSTRACT FROM AUTHOR]

Sandwich structures are widely used in engineering as lightweight and high-strength materials. In recent years, there has been increasing attention toward new design ideas aimed at improving the adaptability and variability of sandwich structures for broader applications. This paper proposes a new stretchable sandwich structure. Experiments and finite element methods are conducted to provide the influence of stretchable design on the mechanical properties of traditional sandwich structures. Then, three novel stretchable sandwich structures with 3D lattice core layers are also designed to explore the applicability of this method. The results demonstrate that the stress and energy absorption capacity of stretchable lattice sandwich structures remains essentially constant. In addition, stretchable sandwich structures possess the capability of in-plane deformation and active bending. By combining the functional characteristics of sandwich structures and rotating polygons, stretchable sandwich structures can adjust their surface area, porosity, and in-plane negative Poisson's ratio effect while maintaining bearing capacity. This study suggests improvements in the utilization and adaptive capacity of sandwich structures in defense engineering and other applications by overcoming the problem of limited deformation faced by traditional sandwich structures. [Display omitted] • A new stretchable sandwich structure and its design method is proposed. • Three lattice sandwich structures (LSSs) are also designed to explore the applicability of this method. • The surface area of the structure can be adjusted. • The LSSs do not reduce energy absorption during in-plane deformation. • The sandwich structures have in-plane auxetic behavior. [ABSTRACT FROM AUTHOR]

Auxetic structures have theoretically superior bending resistance that was previously inaccessible. Compared with the planar two-dimensional (2D) auxetic structures, the three-dimensional (3D) counterparts may exhibit auxetic behavior in multiple directions. Recently, increasing studies have been devoted to studying the mechanical properties of 3D auxetic structures. In this paper, two 3D honeycomb structures were proposed, namely 3D re-entrant honeycomb and 3D hexagonal honeycomb. Subsequently, the mechanical properties, deformation modes, and deformation mechanisms of these two honeycomb structures under three-point bending were investigated experimentally and numerically. Compared with the 3D non-auxetic hexagonal honeycomb, the 3D auxetic re-entrant honeycomb exhibits higher ductility and fracture resistance. In addition, unlike the conventional honeycomb structure, the mid-span section of the 3D auxetic honeycomb exhibits a trapezoidal deformation mode under bending. Finally, the influence of the number of unit cells and geometric parameters on the mechanical properties of the 3D honeycomb structure were systematically studied by parametric analysis. The excellent bending performance provides a basis for the application of auxetic honeycomb in the fields of biomedicine, soft robots, and buffer devices. [Display omitted] • Two 3D honeycomb structures, namely 3D auxetic honeycomb and 3D non-auxetic honeycomb, were designed and fabricated. • The mechanical properties, failure modes, and deformation mechanism of two specimens are investigated under three-point bending. • Compared with the 3D non-auxetic honeycomb, the 3D auxetic honeycomb exhibits better ductility and a trapezoidal deformation pattern at midspan. [ABSTRACT FROM AUTHOR]

The aim of this paper is to develop two novel in-plane auxetic structures with remarkable stiffness and negative Poisson's ratio (NPR) appropriate for compression and tensile load-bearing structures. The combination of re-entrant and arrow-head (RAH) unit-cells as well as star and arrow-head (SAH) unit-cells are hybridized and configured as new auxetic structures. The conceptual design is based on the stretch-dominated-mechanism (SDM) deformation of the proposed structures. To investigate the mechanical behavior of the designed structures under compressive and tensile loading, the samples are manufactured by 3D fused deposition modeling printing method and subjected to mechanical testing. The results show that the designed structures present tunable NPRs and approximately equal stiffnesses for both compressive and tensile loadings. In addition, an analytical analysis is carried out to determine the elastic constants of the structures, and a finite element (FE) simulation is conducted to verify the results with experimental and theoretical models. Good agreements are found for the elastic constants calculated from the experimental, theoretical, and FE methods. Meanwhile, the effects of the geometry parameters on the mechanical properties of the designed structures are also investigated, and the stiffness, stiffness to relative density ratio, and NPR are compared with those of existing studies. The comparison results suggest that our developed structures can be considered as strong candidates for the next generation of SDM auxetic structures with high stiffness capability and noticeable NPR values. [Display omitted] • Novel auxetic structures are designed based on the stretch-dominated-mechanism. • The designed structures demonstrate the close stiffness under compressive and tensile loadings. • The designed structures have enhanced stiffness and negative Poisson's ratio values, simultaneously. [ABSTRACT FROM AUTHOR]

Traditional auxetic metamaterials are characterized by negative Poisson's ratio but weak stiffness. In this paper, a new auxetic metamaterial, re-entrant star-shape anti-chiral auxetic metamaterial (Re-S-AC), is proposed and the in-plane mechanical properties of the Re-S-AC are investigated by experimental test and numerical analysis. The crushing test of the Re-S-AC fabricated by the selective laser melting (SLM) method is performed to investigate the deformation mechanism and validate the accuracy of the numerical model. Compared with the anti-tetrachiral and re-entrant star-shape, the Re-S-AC simultaneously exhibits concave and rotational deformation under the in-plane compression, and mean compression stress is highest at the same thickness and same mass. In addition, parametric analysis is conducted to comprehensively reveal the influence of geometric characteristics on the mechanical properties of the Re-S-AC. The results found that the node center distance, node diameter and wall thickness have significant effect on the mechanical properties and deformation mode of the Re-S-AC. Compared with other parameters, wall thickness has the greater effect on the mechanical properties. When wall thickness is from 0.4 mm to 0.8 mm, the Young's modulus, yield strength, and mean compression stress increase by 447.27%, 530.62%, and 370.87%, respectively. This study provides a reference method for improving the mechanical properties of the auxetic metamaterial. • A new auxetic metamaterial (Re-S-AC) is proposed by coupling concave-rotation deformation. • The deformation mechanism of the Re-S-AC is explored by quasi-static crushing experiment. • The Re-S-AC has better mean compression stress than the anti-tetrachiral and re-entrant starshape metamaterial. • Parametric analysis is conducted to reveal the influence of geometric parameters on the mechanical properties. [ABSTRACT FROM AUTHOR]

This paper designs two chiral compression–twist coupling lattice structures with positive Poisson's ratio (PPR) and negative Poisson's ratio (NPR), and studies their compressive mechanical behaviors. Experiments and finite element (FE) simulations under the quasi-static load are firstly done. Compared to the PPR case, the maximum rotation of the lattice structure with the NPR effect is increased over 40%. Then dynamic compressive mechanical behaviors of the PPR and NPR lattice structures are investigated with different velocities of loads. Impact resistance performances of present structures are further researched, and gradient designs are introduced to improve the impact resistance performance. It is found that different Poisson's ratios show great effects on quasi-static and dynamic mechanical behaviors. As the velocity of load increases, both the initial deformation region and initial compact region are closer to the side of the applied load. Under the middle velocity load, the NPR lattice structure shows the smaller initial peak stress but less energy absorption. With the further increase of load velocity, the PPR lattice structure presents both the better energy absorption and smaller stress peak. The positive gradient (PG) rises the energy absorption, while the negative gradient (NG) abates the peak stress. As the NG design is used, the peak stress of the NPR lattice structure decreases from 23.3 MPa to 16.5 MPa. Besides, gradient designs diminish discrepancies of mechanical behaviors caused by different Poisson's ratios. • Two new chiral compression–twist coupling structures are designed and fabricated. • Quasi-static tests are done and gained results can be used as benchmark solution. • Dynamic compressive behaviors of the structures are studied for the first time. • Compression processes show new phenomena and meaningful conclusions are gained. • Gradient designs are introduced and the impact resistance performances are improved. [ABSTRACT FROM AUTHOR]

This paper aims to investigate the crushing behaviour and protective performance of an innovative auxetic structure, named the hourglass structure (HGS), under in-plane compressive loads. The HGS was fabricated with a 3D printing method through fused filament fabrication technique. The 3D printed HGS specimens were subjected to a displacement-controlled in-plane quasi-static compressive load. A 3D comprehensive numerical model was developed and calibrated with the experimental results. The calibrated numerical model was used for a parametric study to explore the effects of relative density, functional grading, and crushing velocity on the protective performance of the HGS. Furthermore, a theoretical model was proposed to correlate plateau stress of the HGS with geometric design parameters, effective Poisson's ratio, and matrix material properties. Verified with a series of numerical analyses of different HGS designs, the proposed theoretical model predicted plateau stress with less than 10% relative error. In addition, experimental, numerical, and theoretical analyses uncovered load–deformation relationship, Poisson's ratio, crushing mechanism, and energy absorption capacity of the novel auxetic. Overall, the HGS exhibited excellent features for protective engineering applications such as shock mitigation and blast energy absorption. [Display omitted] • Design, fabrication, and examination of the mechanical properties of an innovative auxetic structure. • Development of a theoretical model to predict energy absorbing characteristics of the innovative auxetic. • A series of parametric analyses to investigate the effects of geometric, structural, and loading parameters. • Evaluation of protective performance of the innovative auxetic structure. [ABSTRACT FROM AUTHOR]

Auxetic metamaterials with negative Poisson's ratio (NPR) are attracting tremendous attention due to their unusual and intriguing mechanical properties. This paper proposes a novel functionally graded (FG) beam made of graphene origami (GOri)-enabled auxetic metamaterials (GOEAMs) and investigates its nonlinear free vibration characteristics tuned by GOri. The beam consists of multilayer GOEAMs with GOri content changed across the beam thickness in a layer-wise mode such that the auxetic property and other material properties are varied in a graded form and can be effectively estimated by genetic programming (GP)-assisted micromechanical models. The Timoshenko beam theory and von Kármán type nonlinearity are adopted herein to derive the nonlinear kinematic equations that are numerically solved by the differential quadrature (DQ) approach. Detailed parametric studies are performed to discuss the influences of GOri content, distribution pattern, GOri folding degree, and temperature on the nonlinear frequencies of FG-GOEAM beams. Numerical results indicate that the nonlinear free vibration behaviors of the beam can be effectively tuned via GOri parameter and distribution. • Nonlinear free vibration characteristics of shear deformation metamaterial beams are investigated. • Tunability of the nonlinear vibration performance is achieved via graphene origami material parameters. • Influences of functionally graded and homogeneous metamaterial distribution patterns are discussed. • Graded metamaterial distributions offer superior beam stiffness with higher nonlinear natural frequency. [ABSTRACT FROM AUTHOR]

The development of smart structures, aimed at optimizing the response of engineering systems, is a modern and very promising field. Such advanced structural element can be obtained, in some cases, by using the so-called auxetic materials, i.e. materials exhibiting negative Poisson׳s ratio. In the present paper a brief introduction on auxetic materials and on their use and potentiality is presented. In particular their application to obtain smart layered plates is discussed; it is shown as these materials allow, in certain conditions, to get a desired stiffness and a nearly linear behaviour (even under large displacement conditions) of bended plates, without changing the plate׳s geometry and materials. Such potentialities are presented and discussed based on numerical analyses, and a mechanical interpretation of the observed structural behaviour is proposed. [ABSTRACT FROM AUTHOR]

In this paper, modeling and analysis of postbuckling of sandwich cylindrical shells surrounded by an elastic medium are presented. The sandwich cylindrical shells consist of two metal face sheets and auxetic graphene-reinforced metal matrix composite (GRMMC) core with in-plane negative Poisson's ratio (NPR). Two cases of the compressive postbuckling of sandwich cylindrical shells under axial compression in thermal environments and the thermal postbuckling of sandwich cylindrical shells caused by a uniform temperature rise are considered. Each ply of auxetic GRMMC core possesses in-plane NPR and can have different graphene volume fraction to achieve a piece-wise functionally graded (FG) distribution through the thickness domain of the core. The thermo-mechanical properties of both metal face sheets and the GRMMC core are temperature dependent. The governing equations are formulated based on the Reddy's third order shear deformation shell theory coupled with von Kármán kinematic nonlinearity. The shell-foundation interaction and the thermal effects are considered in the modeling. Applying a singular perturbation technique in conjunction with a two-step perturbation approach, we obtain the postbuckling solutions for perfect and imperfect sandwich cylindrical shells. The effects of the FG pattern, the face sheet-to-core-to-face sheet thickness ratio, and the foundation stiffness on the postbuckling behaviors of sandwich cylindrical shells with auxetic GRMMC core are studied in detail. • Postbuckling of sandwich cylindrical shells with auxetic GRMMC core is presented for the first time. • Both in-plane negative Poisson's ratios and functionally graded patterns are considered. • The material properties of GRMMC core and face sheets are temperature dependent. [ABSTRACT FROM AUTHOR]

In this paper, rigid polyurethane (PU) foam was filled into hollow auxetic tube for enhancing the energy absorption. The rigid PU foams and stainless-steel auxetic tube with different geometrical parameters were manufactured, respectively. They were then assembled to form composite foam-filled auxetic tube (FFAT). Four auxetic tubes and four FFATs were respectively tested to examine the enhancement of energy absorption. Tubular types and the effects of parameters including wall thickness, and ellipticity, on FFATs were analyzed numerically by using the validated models. The results show that the overall absorbed energy of FFAT is larger than the sum of single foams and hollow auxetic tube under compression. The geometrical parameters of wall thickness, ellipticity, have a considerable effect on the structural deformation mode and energy absorption. These findings could promote the applications of auxetic composite structures in protective engineering. • The first polyurethane foam-filled auxetic tubes (FFAT) are designed, fabricated and investigated. • The proposed FFATs could exhibit superior mechanical properties under axial compression. • The findings are useful to guide the design of advanced tubular structures for energy absorption. [ABSTRACT FROM AUTHOR]

Auxetic composite laminates are a new type of engineering materials that have unique features for important potential applications. This paper examines the effect of in-plane negative Poisson's ratio (NPR) on the thermal postbuckling behaviors of graphene-reinforced metal matrix composite (GRMMC) plates. The plates rest on an elastic foundation and are subjected to a uniform temperature rise. The GRMMC layers with different volume fractions of graphene reinforcement can be arranged to achieve piece-wise functionally graded (FG) patterns across the plate thickness and the material properties of the GRMMC layers are temperature-dependent. The Reddy's third order shear deformation plate theory and the geometric nonlinearity of von Kármán-type are applied to formulate the thermal postbuckling equations for GRMMC laminated plates. The nonlinear problem can be solved by a two-step perturbation approach. Parametric study is performed for (± 10) 5T and (± 10) 3T GRMMC laminated plates possessing in-plane NPR. The results reveal that the buckling temperatures for (± 10) 5T and (± 10) 3T plates are significantly enhanced with an FG-X pattern for the plates. We found that due to the combined effect of FG and in-plane NPR, the thermal postbuckling strength of FG-X (± 10) 3T plate is higher than that of FG-X (± 10) 5T plate. • Thermal buckling of auxetic GRMMC laminated plates is presented for the first time. • Both in-plane negative Poisson's ratios and functionally graded patterns are considered. • The material properties of GRMMC layer are anisotropic and temperature dependent. • The new findings for the thermal postbuckling behaviors of GRMMC laminated plates are presented. [ABSTRACT FROM AUTHOR]

Auxetic materials and structures have attracted increasing attention because of their extraordinary mechanical properties. Various types of auxetic tubular structures have been designed and studied in diverse fields, including mechanical and medical engineering. In this paper, design methods and advanced manufacturing technologies of auxetic tubular structures are extensively reviewed, including various types of cellular auxetic tubes, nonporous and porous auxetic tubes, macro and micro auxetic tubes. Furthermore, auxetic behaviour, mechanical properties and potential applications of auxetic tubular structures are elaborated. Finally, the challenges and opportunities on the auxetic tubes are discussed to inspire future research work. • Design methods and advanced manufacturing technologies of auxetic tubular structures are extensively reviewed for the first time. • Auxetic behaviour, mechanical properties and potential applications of auxetic tubular structures are introduced. • Challenges and opportunities on the auxetic tubes are discussed to inspire future work. [ABSTRACT FROM AUTHOR]

Recently, auxetic cellular solids in the forms of honeycombs and carbon nanotube reinforced composite (CNTRC) have great potential in a diverse range of applications. This paper studies the nonlinear free and forced vibration of sandwich cylindrical panel on visco-Pasternak foundations in thermal environment subjected to blast load The sandwich cylindrical panel consists of auxetic honeycombs core layer and two CNTRC face sheets. The Poisson' ratio of the auxetic core is negative and the material properties of CNTRC face sheets are assumed to continuously vary in the thickness direction according to four different types of linear functions. The blast load is determined from the distance of the center of blast to center of the structure and the mass of explosive materials. The fundamental equations are established based on Reddy's higher order shear deformation shell theory taking into account the effect of initial imperfection and visco-Pasternak foundations. The Galerkin and Runge–Kutta methods are used to obtain the nonlinear dynamic response and the natural frequency of the sandwich cylindrical panel. The numerical results show the effect of geometrical parameters, visco foundations, initial imperfection, temperature increment, nanotube volume fraction and blast load on the nonlinear vibration characteristics of the sandwich cylindrical panel. The accuracy of present approach and theoretical results is verified by some comparisons with the known data in the literature. • Nonlinear forced vibration. • Sandwich cylindrical panel with negative Poisson's ratio auxetic honeycombs core and CNTRC face sheets. • Using Redyy's higher-order shear deformation shell theory. • Galerkin's method and Runge–Kutta method are applied. • Taking into account the effect of initial imperfection and visco-Pasternak foundations. [ABSTRACT FROM AUTHOR]

We present in this paper two novel concepts of hybrid metamaterials that combine a core unit cell of re-entrant or cross-chiral shape and lateral missing ribs. The first topology is a hybrid between an anti-tetrachiral and a missing rib (cross-chiral) configuration; the second one has a variable cross-chiral layout compared to the classical missing rib square structure. Their in-plane mechanical properties have been investigated from a parametric point of view using finite element (FE) simulations. The two classes of metamaterials have been benchmarked to obtain optimized designs and specific effective properties. Nonlinear simulations and experimental tests of the new re-entrant missing rib metamaterials featuring optimized geometry parameters have been performed to understand the behavior of these architectures under large deformations. • Two novel hybrid metamaterials combining a core unit cell of re-entrant or cross-shape and lateral ligaments are proposed. • In-plane mechanical properties of the two metamaterials are parametrically investigated using finite element method. • Nonlinear simulations and experiments of the new re-entrant missing rib metamaterial with optimized feature are performed. [ABSTRACT FROM AUTHOR]

Auxetic materials are one of the metamaterials that have potential applications in many scientific and engineering fields. In this paper, an investigation is presented on the postbuckling responses of axially loaded graphenereinforced metal matrix composite (GRMMC) laminated cylindrical panels with in-plane negative Poisson's ratio (NPR). The panels are made of GRMMC laminates and rest on an elastic foundation in thermal environments. The graphene volume fraction in GRMMC layer may vary across the panel thickness in order to achieve a piece-wise functionally graded (FG) pattern. The GRMMC layers have in-plane NPR and temperature dependent material properties. The governing differential equations for the GRMMC laminated cylindrical panels are formulated based on the Reddy's third order shear deformation shell theory and are solved by using a singular perturbation technique along with a two-step perturbation approach. Numerical investigation is carried out to study the influence of the in-plane NPR, the FG patterns, the thermal environmental conditions and the foundation stiffness on the postbuckling responses of the GRMMC laminated cylindrical panels under axial compressive load. The results reveal that the in-plane NPR can lead to a substantial change of the postbuckling behaviors of the GRMMC laminated cylindrical panels. • A novel auxetic material structure is modeled. • Both in-plane negative Poisson's ratios and functionally graded patterns are considered. • The material properties of GRMMC layer are anisotropic and temperature dependent. • The new findings for the postbuckling behaviors of GRMMC laminated panels are presented. [ABSTRACT FROM AUTHOR]

Mechanical behaviors of a new foam filled 2D re-entrant hexagonal honeycomb with negative Poisson's ratio under planar compression have been investigated in this paper. Re-entrant hexagonal unit cell specimens were fabricated by aluminum alloy and polyurethane foam. The deformation modes and force-displacement curves of the unit cell specimens under compression were carried out experimentally and numerically, and a good agreement was observed between experimental results and numerical simulation results. Subsequently, mechanical behaviors of foam filled and void re-entrant hexagonal honeycomb under planar compression were observed by numerical simulation. Compared to the void honeycomb, the foam filled honeycomb has a higher specific energy absorption capability due to a higher plateau stress. According to the mechanism of auxetic effect, the re-entrant honeycomb would move inward when subjected to compression along the orthogonal direction, which cause a biaxial compression of the foam and an increasing of stiffness/strength/energy absorption capability. The present investigations provide thorough insight into the strengthened re-entrant hexagonal cellular honeycomb, and could be used in the development of novel light weight smart functional structures. Image 1 • A new foam filled 2D re-entrant hexagonal honeycomb with negative Poisson's ratio topology is proposed. • Auxetic behavior of foam filled re-entrant honeycomb is presented. • Excellent specific energy absorption of the foam filled honeycomb due to higher plateau stress is presented. [ABSTRACT FROM AUTHOR]

This paper treats energy absorption and crushing characteristics of auxetic foam-filled square-section tubes under axial loading condition. The influence of auxeticity effect on the structural collapse was experimentally investigated to quantify the interaction effect between Poisson's ratios of foam (ranging from −0.31– 0.03) and square tube walls under quasi-static and dynamic loadings. The validated computer simulation technique was employed to conduct a series of parametric study in order to evaluate the influence of tube parameters on the energy absorption responses. It is evident that negative Poisson ratio effectively influences the crushing response and energy absorption capability of auxetic foam-filled square tubes. Moreover, it was found that increasing the auxeticity level of foam filler enhances crashworthiness performance of foam-filled structures under both quasi-static and dynamic loading conditions. The primary outcome of this study is a design guideline for the use of auxetic foam in different ranges of Poisson's ratio as a core for an energy absorber device where impact loading is expected. Image 1 • An increase in the auxeticity level of foam core enhances the energy absorption of auxetic foam-filled square tubes. • Interaction effect between auxetic foam core and square tube was directly influenced by varying foam Poisson's ratio. • The auxeticity level affects the strain rate sensitivity of auxetic foam materials. • Increasing the width and height of filled tube makes the structure heavier without affecting the absorbed energy. [ABSTRACT FROM AUTHOR]