Auxetic Composites Research Articles

Application of machine learning algorithm and Carrera unified formulation in thermal buckling analysis of a functionally graded graphene origami enabled auxetic metamaterial sandwich plate with an auxetic concrete foundation

This study presents a comprehensive thermal buckling analysis of sandwich plates composed of functionally graded graphene origami-enabled auxetic metamaterial (FG-GOEAM) face sheets on an auxetic concrete foundation, using Carrera’s unified formulation (CUF) as the theoretical framework. FG-GOEAM materials are emerging as advanced composites, combining exceptional mechanical resilience, tunable auxetic behavior, and high thermal stability, making them suitable for extreme environments. By employing CUF, a powerful and adaptable modeling approach, this work accurately captures the complex mechanical interactions within the FG-GOEAM sandwich structure under thermal loads, incorporating both material gradation and auxetic properties. To further enhance the precision and efficiency of this thermal buckling analysis, a deep neural network (DNN) is developed as a machine learning algorithm to predict critical temperature differences, based on a dataset generated through mathematics simulation. The DNN model demonstrates excellent predictive capability, validated by close alignment between its estimates and CUF results, thus reducing computational costs while maintaining high accuracy. Parametric studies are conducted to assess the effects of material gradation, aspect ratios, and foundation properties on thermal buckling performance. The results highlight the superior thermal stability of FG-GOEAM structures and the potential of DNNs to serve as reliable, computationally efficient tools for advanced structural analysis. This study provides a novel, integrated framework for high-fidelity thermal buckling prediction in complex auxetic composites, paving the way for broader applications in engineering fields requiring lightweight, thermally stable structures.

Unraveling the reinforcing mechanisms for cementitious composites with 3D printed multidirectional auxetic lattices using X-ray computed tomography

This study investigates the mechanical properties of cementitious composites with 3D-printed auxetic lattices, featuring negative Poisson’s ratios (auxetic behavior) in multiple directions. These lattices were fabricated using vat photopolymerization 3D printing, and three base materials with varying stiffness and deformation capacities were analyzed to determine their impact on the composites’ mechanical behavior. To unravel the reinforcing mechanisms of multidirectional auxetic lattices, which exhibit auxetic behavior in both planar and out-of-plane directions, X-ray computed tomography (X-ray CT) was utilized to analyze composite damage evolutions under different strain levels. The micro-CT characterization reveals that auxetic lattices more effectively constrain crack growth and dissipate energy by distributing stress evenly within the cement matrix. In contrast, due to lack of lateral confinement, the non-auxetic lattice reinforced composites primarily dissipate energy through extensive crack propagation and interfacial damage, leading to lower peak strength. When strain exceeding 5%, although the confinement from the auxetic behavior diminished with crack propagation, the lattice can still maintain the composite’s structural integrity, resulting in 1.7 times higher densification energy than conventional cement-based materials. These findings provide valuable insights for designing auxetic lattice-reinforced cementitious composites with enhanced load-bearing capacity and improved dissipation capabilities.

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.

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.

Mechanically Ultra-Robust Fluorescent Elastomer for Elaborating Auxetic Composite.

Fluorescent elastomers are predominantly fabricated through doping fluorescent components or conjugating chromophores into polymer networks, which often involves detrimental effects on mechanical performance and also makes large-scale production difficult. Inspired by the heteroatom-rich microphase separation structures assisted by intensive hydrogen bonds in natural organisms, an ultra-robust fluorescent polyurethane elastomer is reported, which features a remarkable fracture strength of 87.2MPa with an elongation of 1797%, exceptional toughness of 678.4MJm-3 and intrinsic cyan fluorescence at 445nm. Moreover, the reversible fluorescence variation with temperature could in situ reveal the microphase separation of the elastomer in real time. By taking advantage of mechanical properties, intrinsic fluorescence and hydrogen bonds-promoted interfacial bonding ability, this fluorescent elastomer can be utilized as an auxetic skeleton for the elaboration of an integrated auxetic composite. Compared with the auxetic skeleton alone, the integrated composite shows an improved mechanical performance while maintaining auxetic deformation in a large strain below 185%, and its auxetic process can be visually detected under ultraviolet light by the fluorescence of the auxetic skeleton. The concept of introducing hydrogen-bonded heteroatom-rich microphase separation structures into polymer networks in this work provides a promising approach to developing fluorescent elastomers with exceptional mechanical properties.

Peanut shaped auxetic cementitious cellular composite (ACCC)

Auxetic cementitious cellular composites (ACCCs) exhibit desirable mechanical properties (e.g., high fracture resistance and energy dissipation), due to their unique deformation characteristics. In this study, a new type of cementitious auxetic material, referred to as peanut shaped ACCC, has been designed and subsequently architected using additive manufacturing techniques. Two peanut shaped ACCCs specimens with different pseudo-minor axes have been tested under uniaxial compression with Digital Image Correlation (DIC) to assess their compressive behavior, peak strength, Poisson’s ratio, and energy dissipation capacity. Additionally, cyclic tests were conducted to investigate their compressive resilience properties, further elucidated through microstructural analysis using a digital optical microscope. The mechanical test results were also compared with those of previously developed elliptical-shaped ACCCs. Furthermore, a numerical model was used to simulate the mechanical behavior of peanut shaped ACCCs under uniaxial compression, and showed a good agreement with the experimental data. The auxetic behavior observed in peanut shaped ACCCs arises from the rotation of sections facilitated by fiber bridging at the ligament of adjacent holes within the cementitious unit cell. In comparison to elliptical-shaped ACCCs, peanut shaped ACCCs can exhibit a slightly more negative Poisson's ratio and mitigate stress concentration. The reduction of stress concentration enables peanut shaped ACCCs to dissipate substantial energy, showcasing enhanced ductility and toughness. In cyclic tests, peanut shaped ACCCs exhibit superior recoverable deformation elasticity, attributed to robust fiber bridging capacity. The exceptional mechanical properties exhibited by peanut shaped ACCCs offer a scalable solution for developing energy-absorbent and multifunctional cementitious materials for smart infrastructure.

Flexural behaviour of cementitious composites embedded with 3D printed re-entrant chiral auxetic meshes

3D printed auxetic metamaterials can be used to make high performing cementitious composites to strengthen existing structures and elements due to their negative Poisson’s ratio behaviour and high energy absorbing characteristics. In this paper, three different re-entrant chiral auxetic (RCA) meshes of various cell geometries and orientations were developed by 3D printing them using poly-lactic acid (PLA) and thermoplastic polyurethane (TPU) filament. The developed meshes were tested under out-of-plane flexure to study their load carrying capacity, ductility and energy absorption characteristics, especially to characterise the best cell orientation. The horizontal cells provided enhanced load carrying and energy absorption characteristics for all three cell geometries for both materials. These RCA meshes were then embedded into low and high strength premix cement mortar matrices to develop auxetic cementitious composites (ACCs). In total, 42 ACC specimens were casted and tested under flexural loading. The results were studied in terms of their failure patterns, load-displacement responses, flexural capacities, ductility and energy absorption. The RCA meshes made of PLA filament showed limited capacity and energy absorption as compared to RCA meshes made of TPU filament due to extended flexibility and resilience provided by TPU meshes. The RCA meshes with a denser cell structure exhibited highest flexural capacity and effective energy absorption of 14 700 kJ m−2 for TPU-RCA mesh embedded into high strength cement mortar matrix. The results obtained in this study have enabled to understand the flexural behaviour of cementitious composites embedded with 3D printed auxetic lattices and to strengthen the existing structures.

Design and Development of 2D Woven Auxetic Fabric and Composites Based on Wave Form Geometry

Design and Development of 2D Woven Auxetic Fabric and Composites Based on Wave Form Geometry

Auxetic cementitious composites (ACCs) with excellent compressive ductility: Experiments and modeling

Auxetic cementitious composites (ACCs) with improved mechanical properties are created, by casting 3D printed polymeric auxetic reinforcement structures inside cementitious mortar. Four types of ACCs incorporating reinforcement with different auxetic mechanisms were prepared: “re-entrant” (RE), “rotating-square” (RS), “chiral” (CR) and “missing-rib” (MR). Experiments and finite element models were used to study the compressive behavior of the ACCs. The results indicate that all ACCs have high compressive ductility. Specifically, the RE shows the highest ductility, manifested by 853% and 708% higher energy absorption than the reference mortar and the auxetic structure itself, respectively. In addition, the RE and RS are found to exhibit stronger crack-arresting effect under compression. Therefore, they achieved comparable compressive strength to the reference mortar, which is considerably higher than CR and MR. Furthermore, decreasing the volumetric ratio of the auxetic structure by half, the ductility of the RS reduces by 32.2%, while decreasing the water-to-binder ratio of the cementitious matrix from 0.4 to 0.3 only increases the compressive strength by 18.5%. Moreover, the two-dimensional finite element analyses used herein show a good match with experiments but become less accurate at high strain levels, due to their inability to capture the out-of-plane failure of the ACCs.

Nonlinear vibrations of auxetic honeycomb thin plates based on the modified Gibson functions

Auxetic honeycombs possess many excellent properties, making them ideal cores for sandwich structures that can absorb energy effectively. This paper investigates the nonlinear vibrations of auxetic honeycomb composite plates, focusing on the primary resonance, super-/sub-harmonic resonances of the composite plates. Two pure panels sandwich an auxetic honeycomb core to form the honeycomb composite plate. The proposed modified Gibson function enables the derivation of effective material properties. By considering geometric nonlinearity, the nonlinear motion equations are derived through the implementation of the Hamilton principle, and then nonlinear ordinary differential equations are given by introducing stress functions and the Galerkin method. The nonlinear response curves are then determined by the multiple scale method. The impact of important parameters on the nonlinear vibration of auxetic honeycomb composite plates is evaluated in this study.

Auxetic cementitious cellular composite (ACCC) PVDF-based energy harvester

The high deformation capacity of auxetic cementitious cellular composites (ACCCs) makes them promising for strain-based energy harvesting applications in infrastructure. In this study, a novel piezoelectric energy harvester (PEH) with ACCCs and surface-mounted PVDF film based on strain-induced piezoelectric mechanisms has been designed, fabricated, and experimentally tested. Furthermore, a numerical model for simulating the energy harvesting of ACCC-PVDF system undergoing repeated mechanical loading has been established and validated against the experimental data. The mechanical behavior of ACCCs was simulated by the concrete damage plasticity model during the preloading stage, which was converted to the second-elasticity model during cyclic loading stage. Based on the mechanical responses, analytical formulas for piezoelectric effects were developed to calculate the output voltage of the PVDF film. The output voltages of the ACCCs-PVDF system under different loading amplitudes and loading frequencies were assessed. The experimental results and models of the ACCCs-PVDF energy harvester lay a solid foundation for utilizing architected cementitious composites in energy harvesting applications to supply self-power electronics in infrastructure.

Auxetic frame with programmable strength and stiffness: Design, investigation and perspective

To broaden the family of available matrices for auxetic composites, an auxetic TPU frame with cubic matrix cases is proposed, tested, and optimized. Quasi-static compression was applied to the new structure to investigate its mechanical performance. Verified finite element models were established for demonstrating the programmable strength and stiffness. Except for traditional materials that can be portably filled into auxetic frames, aluminum cubic tubes, as an instance of multiple matrixes, were filled into the frame. The newly fabricated auxetic composite structure was also tested experimentally. Besides, a section on multi-functional perspective and potential applications were given at the end to better illustrate the proposed structure in practice. The optimized auxetic frame can be used alone or with multiple strengthening matrixes in many aspects including energy absorbers and smart sensors.

Auxetic materials and structures for potential defense applications: An overview and recent developments

Auxetic behavior is a promising new area for use in defense applications. In comparison to a conventional material, an auxetic material has superior properties because of having a negative Poisson’s ratio; it gets broadened when stretched or becomes smaller when compressed. Furthermore, auxetic materials have enhanced properties such as shear resistance, indentation resistance, fracture toughness, energy absorption, and so on. These improved properties make auxetic materials very appealing and have the potential to revolutionize their applications in aerospace, sports, automotive, construction, biomedical engineering, smart sensors, packaging, cushioning, air filtration, shock absorption and sound insulation, and defense personal protective equipment. This article examines the most recent scientific advances in auxetic materials and structures, such as auxetic textiles (fibers, yarns, and fabrics), auxetic textile-reinforced composites, and auxetic foams, as well as their exceptional auxetic behavior and various approaches to achieving them. Although many potential applications have been proposed, actual applications of auxetic materials in defense are still limited. This is an in-depth review article, and its main goal is to serve as a foundation for future studies concerning the topic.

Enhanced energy absorption of auxetic cementitious composites with polyurethane foam layers for building protection application

Cementitious composites with impact energy absorption capabilities can have wide applications in protective structures. This paper presents an experimental investigation on the flexural response and energy absorption of auxetic cementitious composites reinforced with polyurethane foam. In total, ninety (90) composite specimens were prepared from polymer cement mortar overlayed with either carbon fibre fabric or auxetic fabric and polyurethane foam insertions in the upper and lower regions. These specimens were tested under flexural loads using a four-point loading setup at varying loading speeds from 1 mm/min to 300 mm/min. Digital image correlation method was employed to study the failure modes, displacement profiles, load-displacement curves, and energy absorption characteristics. The auxetic fabric overlayed composites exhibited increased energy absorption and ductility without any signs of debonding compared to the commonly used carbon fibre overlayed composite specimens. Polyurethane foam insertions in the upper part of the composites prevented splitting of the specimen by reducing the crack widening in the compression zone of the cementitious matrix. Additionally, an increase in loading rate improved the energy absorption capacity of the auxetic fabric cementitious composites compared to carbon fibre cementitious composites. Findings from this study will have applications for protecting structures that are vulnerable to impacts.

Dynamic Compressive and Flexural Behaviour of Re-Entrant Auxetics: A Numerical Study.

Re-entrant auxetics offer the potential to address lightweight challenges while exhibiting superior impact resistance, energy absorption capacity, and a synclastic curvature deformation mechanism for a wide range of engineering applications. This paper presents a systematic numerical study on the compressive and flexural behaviour of re-entrant honeycomb and 3D re-entrant lattice using the finite element method implemented with ABAQUS/Explicit, in comparison with that of regular hexagonal honeycomb. The finite element model was validated with experimental data obtained from the literature, followed by a mesh size sensitivity analysis performed to determine the optimal element size. A series of simulations was then conducted to investigate the failure mechanisms and effects of different factors including strain rate, relative density, unit cell number, and material property on the dynamic response of re-entrant auxetics subjected to axial and flexural loading. The simulation results indicate that 3D re-entrant lattice is superior to hexagonal honeycomb and re-entrant honeycomb in energy dissipation, which is insensitive to unit cell number. Replacing re-entrant honeycomb with 3D re-entrant lattice leads to an 884% increase in plastic energy dissipation and a 694% rise in initial peak stress. Under flexural loading, the re-entrant honeycomb shows a small flexural modulus, but maintains the elastic deformation regime over a large range of strain. In all cases, the compressive and flexural dynamic response of re-entrant auxetics exhibits a strong dependence on strain rate, relative density, and material property. This study provides intuitive insight into the compressive and flexural performance of re-entrant auxetics, which can facilitate the optimal design of auxetic composites.

A Nonlinear Repeated Impact Model of Auxetic Honeycomb Structures Considering Geometric Nonlinearity and Tensile/Compressive Deformation

Abstract This study aims to improve the impact protection performance of composite structures by combining a honeycomb core with negative Poisson’s ratio and graphene platelets reinforced (GPR) face sheets. The paper investigates the nonlinear repeated low-velocity impact responses of auxetic honeycomb composite plates, taking into account loading-unloading-reloading processes. Effective material properties of the auxetic honeycomb core and GPR face sheets are obtained by using the proposed modified Gibson function and Halpin–Tsai model. Then, taking into account geometric nonlinearity, the nonlinear equations of motion for the system were derived by Hamilton's principle. Afterward, the time-varying contact force between the composite plate and a spherical impactor is defined by the modified nonlinear Hertz contact theory. The Galerkin method and variable-step Runge–Kutta algorithm are selected to obtain nonlinear impact responses. The proposed methods are verified by finite element simulation and experiment. Finally, the study evaluates the effects of key parameters on the nonlinear repeated low-velocity impact responses.

Low velocity impact performance of 3D auxetic composites embedded with re‐entrant triangle inclusions

AbstractThis paper reports the low‐velocity impact performance of 3D auxetic composite embedded with re‐entrant triangle inclusions by both experiments and finite element (FE) simulations. Both auxetic and non‐auxetic samples were manufactured by embedding inclusions made from carbon fiber/epoxy laminates into silicone rubber matrix. The pure silicone rubber was also manufactured for comparison. All samples were subjected to low‐velocity impact with an impact energy of 147 J and an impact speed of 5.42 m/s and their impact load‐time and energy‐time curves were compared between the experimental results and FE simulation. The impact performances under different impact energies from 60 to 120 J were further investigated based on the validated FE models. The results show that the auxetic composites show better energy absorption capacity and impact resistance than non‐auxetic composites with the same number of inclusions at all energy levels. More inclusions can result in better impact resistance of auxetic composites when the impact energy is under 120 J. The auxetic composites possess 10% higher crash efficiency than non‐auxetic samples. The embedding of inclusions leads to easier damage of the composite under impact energy of 147 J. The research provides a better understanding on the impact performance of auxetic composites made from random inclusions.

Low velocity impact behavior of auxetic CFRP composite laminates with in-plane negative Poisson’s ratio

Introducing auxeticity or negative Poisson’s ratio is one potential solution to mitigate the low velocity impact damage of fiber reinforced polymer matrix composites, which can be achieved by tailoring the layup of an anisotropic composite laminate. This study aims to investigate the effect of laminate-level in-plane negative Poisson’s ratio on the low velocity impact behavior of carbon fiber reinforced polymer (CFRP) matrix composites using numerical simulations. The layups of the auxetic composites that allow them to produce negative Poisson’s ratios are identified based on the Classical Lamination Theory and verified through fundamental coupon-level experimental tests. To ensure meaningful comparisons, the non-auxetic counterpart composites are designed by allowing them to produce positive in-plane Poisson’s ratio while closely matching the longitudinal effective modulus of the auxetic laminate. The simulation results indicate that the auxetic laminates suffer smaller (12.6% on average) delamination area in top and bottom interfaces, much smaller (38% on average) matrix compressive damage in the top and bottom plies, and smaller (14.6% on average) fiber tensile damage area in each ply of the laminate at relatively higher impact energies (5 and 8 J).

Heterogeneous geometric designs in auxetic composites toward enhanced mechanical properties under various loading scenarios

Cellular architected materials have demonstrated exceptional energy absorbing and dissipating capability mostly under compression, but integrating such geometric design strategy into a practical scenario requires consideration of a more complex loading state. In this study, we evaluate geometric patterns for 3D-printed core lattices embedded in silicone rubber composite units that are subjected to uniaxial compression, a combined compression-shear load, and loading-unloading cycles. Mechanical properties of composite units with regular cellular patterns were characterized under different loading scenarios through experiments and simulations. Our results show that various geometric patterns exhibit different advantages under certain loading conditions. Then, we further develop heterogeneous geometric patterns to achieve tailorable and enhanced stiffness under compression and shear load. Finally, the developed composite units demonstrate excellent energy absorption under compressive loading-unloading cycles although slow stiffness degradations were observed. Our findings pave a way for integrating 3D printing into advanced composite designs for various energy-absorbing applications.

Multifunctional auxetic and honeycomb composites made of 3D woven carbon fibre preforms

Three dimensional (3D) woven composites started to find applications in various industrial sectors, mainly in aerospace and with a potential in automotive. 3D-woven fabrics can be architected to form complex and near-net-shape preforms ready for automated composites manufacturing. The 3D-woven honeycomb fabric is designed to include additional functionality into finished composites, such as positive and negative Poisson's ratios. In this study, complex honeycomb architectures were created using various weave designs to demonstrate the effects of auxetic behaviours when manufactured into a composite structure. A Staubli 3D-weaving system equipped with Jacquard UNIVAL 100 and creel of 3072 6 k carbon fibre tows were used to weave the designed honeycomb architecture. With the aid of hard polyester foam inserts, the 3D-woven fabrics were converted to honeycomb and auxetic preforms. These preforms were infused using epoxy resin to manufacture a set of honeycomb and auxetic composite structures. In comparison with the baseline honeycomb structure, it is proven that the developed auxetic composites exhibited negative Poisson’s ratio of − 2.86 and − 0.12 in the case of tensile and compression tests respectively.