Auxetic Composites Research Articles

Highly stretchable, stability, flexible yarn-fabric-based multi-scale negative Poisson’s ratio composites

In this paper, we report a new multi-scale NPR composite based on auxetic fabric/elastomer matrix (AF/EM) composite structure. The multi-scale AF was obtained via using helical auxetic yarn (HAY) as weft yarns and constructing re-entrant hexagonal (REH) geometry structure on the macro scale, then the AF was combined with EM to fabricate the auxetic composite (AC). The AF and AC exhibit the NPR of −1.492 and −0.379. Simultaneously, high tensile strain over 108% (AF) and 82% (AC) can be subjected, indicating good mechanical performance. The superior performance of the AC is its ability to maintain NPR and resilience, with elastic recovery ratio over 94% and work recovery coefficient with a stable value over 80% after 10 cycles. Also, the damping properties results further evidence the good elasticity of our AC. The paper shows that our AF and AC are promising candidates in protective composites.

Compressive Property of an Auxetic-Knitted Composite Tube Under Quasi-Static Loading

Abstract This research investigates the compressive property of a novel composite based on a weft-knitted auxetic tube subjected to a quasi-static compression test. In order to maximize the influence of the fiber content on the compression test, a Kevlar yarn was used in knitting the tubular samples using three different auxetic arrow-head structures (i.e. 4 × 4, 6 × 6 and 8 × 8 structure). A quasi-static compression test was conducted under two different impact loading speeds (i.e. 5 mm/min and 15 mm/min loading speed). The results indicate that the energy absorption (EA) property of the auxetic composite is highly influenced by the auxeticity of the knitted tubular fabric.

Low-velocity impact properties of composite reinforced by auxetic warp-knitted spacer fabric

Auxetic warp-knitted spacer fabric composite is famous for its excellent mechanical properties and has been applied in many potential fields, such as automobile industry, aerospace field, and so on. Four kinds of composites with different negative Poisson’s ratios were used for low-velocity impact tests. The influence of different negative Poisson’s ratio effect and initial impact energy on the impact resistance and energy absorption ability of composites are studied. Result shows that the peak impact load, energy absorption, and impact resistance of composites increase with an increase in the auxetic effect of reinforcement. The composite with better auxetic effect shows lower damage level after low-velocity impact. And the higher initial impact energy can easily weaken the impact resistance ability of composites.

Isogeometric topology optimization for computational design of re-entrant and chiral auxetic composites

Auxetic composites, a kind of rationally artificial materials, possess superior multifunctional properties due to a mixture of materials. In this paper, an Isogeometric Topology Optimization (ITO) method is proposed for computational design of both the re-entrant and chiral auxetic composites in both 2D and 3D. The homogenization is numerically implemented using isogeometric analysis (IGA) to predict macroscopic effective properties of microstructures, where the periodic boundary formulation is imposed. An effective Non-Uniform Rational B-splines (NURBS)-based Multi-Material Interpolation (N-MMI) model is applied to compute material properties of all points in composite microstructures, mainly including the Fields of Design Variables (DVFs), Fields of Topology Variables (TVFs), and multi-material interpolation. A unified ITO formulation is developed for 2D and 3D auxetic composites, where an appropriate objective function with a weight parameter is defined to control the generation of different deformation mechanisms. Finally, several numerical examples are performed to demonstrate the effectiveness of the proposed ITO method, and a series of 2D and 3D auxetic composites with the re-entrant and chiral deformation mechanisms are found. The optimized composite structures are simulated using ANSYS to show the auxetic behavior.

Numerical technique for the 3D microarchitecture design of elastic composites inspired by crystal symmetries

A numerical methodology developed for the microarchitecture design of 3D elastic two-phase periodic composites with effective isotropic properties close to the theoretical bounds is here presented and analyzed. This methodology is formulated as a topology optimization problem and is implemented using a level-set approach jointly with topological derivative.The most salient characteristic of this methodology is the imposition of preestablished crystal symmetries to the designed topologies; we integrate a topological optimization formulation with crystal symmetries to design mechanical metamaterials.The computational homogenization of the composite elastic properties is determined using a Fast Fourier Transform (FFT) technique. Due to the design domains are the primitive cells of Bravais lattices compatible with the space group imposed to the material layout, we have adapted the FFT technique to compute the effective properties in 3D parallelepiped domains.In this work, to find the topologies satisfying the proposed targets, we test four space groups of the cubic crystal system. Thus, the achievement of composites with effective elasticity tensor having cubic symmetry is guaranteed, and the isotropic response is then enforced by adding only one scalar constraint to the topology optimization problem.To assess the methodology, the following microarchitectures are designed and reported: two auxetic composites, three pentamode materials, and one maximum stiffness composite. With only one exception, all the remaining topologies display effective elastic properties with Zener coefficients approximating to 1.

Characterisation of polymer cement mortar composites containing carbon fibre or auxetic fabric overlays and inserts under flexure

An experimental investigation on the flexural response of polymer cement mortar matrix overlayed or inserted with carbon fibre or auxetic fabric layers subject to four levels of rates of loading (1–150 mm/min) is reported. Seventy-two specimens made from plain polymer cement mortar, composites with auxetic and carbon fibre fabric layers overlayed at the bottom and inserted at the mid-depth of the specimens were prepared and tested under four point bending. The failure mode including debonding, peak load, load-deflection behaviour, longitudinal strain at peak load, ultimate stage and energy dissipation are reported. All composite specimens exhibited higher peak load, longitudinal strain and energy dissipation in comparison to the plain polymer mortar specimens. Auxetic fabric composites exhibited increased energy dissipation and longitudinal strain at peak load without any sign of debonding. Carbon fibre composites, on the other hand, failed due to debonding at a lower longitudinal strain. A finite element model is presented to demonstrate the debonding tendencies observed in the experiments. Increase in rate of loading improved the performance of the auxetic fabric composites but reduced the energy dissipation of carbon fibre composites.

Enhanced compressive mechanical properties of aluminum based auxetic lattice structures filled with polymers

Auxetic lattice structures are prospective candidates in many engineering applications because of their unique negative Poisson's ratio effect as well as excellent mechanical properties. An aluminum based auxetic lattice structure reinforced by polymer fillers was fabricated through pressure infiltration technology, and the compressive mechanical properties of the resultant auxetic composites were then investigated through experimental tests combined with finite element analyses in the present study. The results show that the composites exhibit higher elastic modulus, compressive strength and energy absorption capacity compared with original aluminum based auxetic lattice structures. This enhancement is ascribed to intensified interaction between the struts and the polymer filler resulted from the special deformation mode of auxetic lattice structures. Furthermore, the deformation mechanism mode of the auxetic structures and corresponding auxetic composites were also discussed. It is therefore suggested that there will be pronounced enhancement in auxetic lattice structures if the negative Poisson's ratio effect is restrained.

Review of Elastomeric Materials for Application to Composites Reinforced by Auxetics Fabrics

The auxetic composites are materials characterized by a negative Poisson’s ratio value. Such composites can be made of conventional materials with a positive value of Poisson’s ratio, but the specific arrangement of the composite reinforcement structures provides this composition with a negative Poisson’s ratio. It is also possible to create such structures from already existing auxetic subassemblies. In comparison to conventional composites, the auxetic reinforced ones have a lot of positive features, i.e.: higher shear resistance, better crack resistance and better damping properties. The aim of this article was to present the review of elastomeric materials for the possible application as the matrix for composite reinforced by auxetic fabrics, especially fabrics made of Helical Auxetic Yarn (HAY).

Mechanical properties of sandwich composites with 3d-printed auxetic and non-auxetic lattice cores under low velocity impact

This work presents a comparative study on the reliability of auxetic (re-entrant honeycomb) and non-auxetic (diamond lattice and conventional honeycomb) lattice composites. The analyzed specimen consists of two unidirectional carbon fiber reinforced composite (CFRP) face sheets and a 3D-printed polymeric core. Low velocity impact tests are conducted first to characterize the unit cell deformation pattern, and we further explore its influence on core structure behavior as well as sandwich panel performance. It is found that the re-entrant topology exhibits lower energy absorption capacity but superior robustness and durability. Consequently, the re-entrant panel performs best in both force mitigation and energy dissipation, provided that the impact energy is appropriate. Furthermore, employing re-entrant core not only stabilizes the occurrences of the face sheet penetration as the impact energy increases, but also grants the sandwich panel consistent behaviors under multi-cycle impacts. These unique performances are due to the global instability of the auxetic structure, which yields more compliant deformation and less stress concentration. Resultant discrepancies shall be interpreted with the sandwich core deformation for validation. These findings pave the way for developing new class of auxetic lattice composites, especially under cyclic loading conditions, through a combination of rational design and 3D printing.

Axisymmetric auxetics

Auxetic materials expand/contract transversely while tensioned/compressed in the longitudinal direction, thus they possess a negative Poisson’s ratio. According to the theory of elasticity, their existence in isotropic forms is theoretically possible, however, they are found in nature only in anisotropic forms. Auxetics are expected to possess enhanced relative static and dynamic properties, thus, several auxetic composites have been proposed and manufactured to mimic such behavior. This study shows the development of axisymmetric auxetics, a novel class of negative Poisson’s ratio composites obtained by the revolution of reentrant lattices in turn of an axis. It is shown by experimental testing and FEA that an auxetic circumferential deformation mechanism increases the overall Poisson’s ratio and promotes an elevation of the Specific Young’s modulus relative to regular honeycombs. Furthermore, the developed composites are compared in terms of Specific Young’s modulus and densities to commercial foams and other non-stochastic auxetics. It is concluded that they present enhanced specific properties for relative densities in the range of 0.28–0.35 and may be a promising route for future applications.

Dynamic constitutive response of novel auxetic Kevlar®/epoxy composites

A comprehensive experimental investigation was performed to study the dynamic compressive constitutive response of novel auxetic Kevlar®/epoxy laminated composites. Strain rate response was investigated using the split Hopkinson pressure bar (SHPB) test setup. Laminated composites were fabricated using the vacuum infusion process. Short Nylon fibers were flocked between the laminates with different flock densities and flock length. For obtaining dynamic force equilibrium in SHPB experiments, a copper pulse shaper was used to increase the rising time of incident pulse. To have a comparison, woven Kevlar®/epoxy composites were also characterized at similar strain rates. In addition, quasi-static tests were also performed on both woven and auxetic laminated composites for completeness of the study. For quasi-static loading conditions, auxetic composites showed higher peak strain and lower peak stress compared to woven composites. For non-flocked composites, both auxetic and woven composites showed rate dependency. Woven composites provided 353% increase in peak stress when the strain rate increased from 1200 s−1 (low) to 3300 s−1 (high). However, in the same conditions, auxetic composites showed only 155% increase in peak stress. For different flocking conditions, woven composites showed rate dependency for all strain rates, but auxetic composites demonstrated rate dependency only from low to medium strain rates. Both auxetic and woven composites experienced shear failure under quasi-static compression, where auxetic composites failed at higher shear angle of 37°, but woven composites had a failure angle of 30°. For impact loads, under no flocking condition, woven composites did undergo severe edge failure at all strain rates, but auxetic composites showed a sign of edge failure only at high strain rates. With the flocking condition, auxetic composites had through thickness shear failure and woven composites experienced splitting and fibrillation of Kevlar® fibers.

Finite Element Analysis of Three-Dimensional (3D) Auxetic Textile Composite under Compression.

This paper reports a finite element (FE) analysis of three-dimensional (3D) auxetic textile composite by using commercial software ANSYS 15 under compression. The two-dimensional (2D) FE model was first developed and validated by experiment. Then, the validated model was used to evaluate effects of structural parameters and constituent material properties. For the comparison, 3D non-auxetic composite that was made with the same constituent materials and structural parameters, but with different yarn arrangement in the textile structure was also analyzed at the same time. The analysis results showed that the auxetic and non-auxetic composites have different compression behaviors and the auxetic composite has better the energy absorption capacity than the non-auxetic composite under the same compression stress. The study has provided us a guidance to design and fabricate auxetic composites with the required mechanical behavior by appropriately selecting structural parameters and constituent materials.

Exploiting negative Poisson's ratio to design 3D-printed composites with enhanced mechanical properties

Auxetic materials exhibiting a negative Poisson's ratio are shown to have better indentation resistance, impact shielding capability, and enhanced toughness. Here, we report a class of high-performance composites in which auxetic lattice structures are used as the reinforcements and the nearly incompressible soft material is employed as the matrix. This coupled geometry and material design concept is enabled by the state-of-the-art additive manufacturing technique. Guided by experimental tests and finite element analyses, we systematically study the compressive behavior of the 3D printed auxetics reinforced composites and achieve a significant enhancement of their stiffness and energy absorption. This improved mechanical performance is due to the negative Poisson's ratio effect of the auxetic reinforcements, which makes the matrix in a state of biaxial compression and hence provides additional support. This mechanism is further supported by the investigation of the effect of auxetic degree on the stiffness and energy absorption capability. The findings reported here pave the way for developing a new class of auxetic composites that significantly expand their design space and possible applications through a combination of rational design and 3D printing.

Low-velocity impact properties of 3D auxetic textile composite

3D auxetic textile composite is a special type of auxetic materials which has attracted more attention in recent years. Its deformation behaviour and mechanical properties under quasi-static compression have been investigated in the previous studies. In this paper, a further study on its mechanical properties under low-velocity impact is presented. Both single and repeating impact tests were conducted at different impact energy levels ranging from 12.7 to 25.5 J. Results show that the 3D auxetic textile composite has better impact protective performance than the 3D non-auxetic textile composite due to better transmitted force reduction and higher energy absorption capacity under single-time impact and higher structural stability under repeating impacts. The results also show that the difference in protective performance between the auxetic and non-auxetic textile composites get increased with the increase in impact energy.

Topology optimization for functionally graded cellular composites with metamaterials by level sets

The application of auxetic composites in practice often relies on a compromise between properties as auxetics are mostly too porous (not dense enough or not stiff enough) to bear structural loads. Hence, the focus of this paper is topological design optimization of new functionally graded cellular composites with auxetics using a level set method. Firstly, a new hierarchical multi-scale formulation is developed to account for both the auxetic behavior of the microstructure and the stiffness of the macrostructure. The composite, comprising multiple layers of periodic microstructures, is tailored to have functionally graded properties for stiffness and auxetic behaviors, subject to volumetric gradient constraints. Secondly, the microstructures underpinning composite layers are topologically designed under the consideration of boundary and loading conditions of the macrostructure. Finally, a level set method is applied to evolve the shape and topology of the microstructure for each layer, with the numerical homogenization method to evaluate the effective properties of the microstructures. Several numerical examples are used to demonstrate the effectiveness of the proposed method. It can be seen that such composites systematically gear together the features of the functionally graded materials, cellular composites, and metamaterials towards a new kind of man-made composites.

Finite Element Modeling of Multilayer Orthogonal Auxetic Composites under Low-Velocity Impact.

The multilayer orthogonal auxetic composites have been previously developed and tested to prove that they own excellent energy absorption and impact protection characteristics in a specific strain range under low-velocity impact. In this study, a three dimensional finite element (FE) model in ANSYS LS-DYNA was established to simulate the mechanical behavior of auxetic composites under low-velocity drop-weight impact. The simulation results including the Poisson’s ratio versus compressive strain curves and the contact stress versus compressive strain curves were compared with those in the experiments. The clear deformation pictures of the FE models have provided a simple and effective way for investigating the damage mechanism and optimizing the material, as well as structure design.

A study on negative Poisson’s ratio effect of 3D auxetic orthogonal textile composites under compression

More and more researches have been focused on auxetic composite materials and a number of composite structures have been fabricated, synthesized or theoretically predicted. Since their structures are complex, their mechanical behavior is very difficult to be characterized. The purpose of the present paper is to systematically investigate the negative Poisson’s ratio effect of a novel three-dimensional auxetic orthogonal textile composite under compression. Firstly, a set of equations are derived for the theoretical calculation of the Poisson’s ratio of the composite under uniaxial compression via an analytical analysis. Secondly, a finite element model (FEM) is created by ANSYS Parameter Design Language and is verified by experiment. The deviation between the simulation and experimental results are carefully discussed. Thirdly, the effects of geometry parameters and material properties on the negative Poisson’s ratio behavior of the composite are discussed based on the FEM simulated results. At last, a general basis is concluded. It is expected that the outcomes of this study could be useful to guide the design and fabrication of auxetic textile composites with required negative Poisson’s ratio behavior.

Characterisation of cementitious polymer mortar – Auxetic foam composites

Application of auxetic materials in automobile and biomedical engineering is widely reported and their usage in civil engineering is emerging. This paper reports the manufacturing and characterising of cementitious polymer mortar – auxetic foam composites suitable for wall rendering. The deformation behaviour of the composites with the high negative Poisson’s ratio (NPR) auxetic foam embedded in polymer cement mortar matrix has been experimentally evaluated and compared to the commonly used mortar composites containing fiberglass mesh layers (that exhibits positive Poisson’s ratio). It is shown that the auxetic embedded polymer cement composite outperformed other composites through elimination of delamination and minimisation of brittleness. The auxetic foams used in this research were manufactured from the inexpensive open cell polyurethane foam through a thermo-mechanical process and characterised through lab tests; the maximum Poisson’s ratio determined was −1.5. Sixty composite specimens tested under axial compression exhibited a maximum NPR of −0.11. The results were validated numerically. The numerical model was then used to examine the effectiveness of the relative thickness of the auxetic foam layers and the polymer cement matrix for maximising the NPR of the composites. The results show that through auxetic material embedment, delamination and brittleness suffered by the cementitious composites could be eliminated.

Low-velocity impact response of multilayer orthogonal structural composite with auxetic effect

Composites with auxetic effect have been drawing more attention in recent years due to their unique features and wide applications. In this study, a kind of novel multilayer orthogonal structural composite with auxetic effect was fabricated for energy absorption and impact resistance. Non-auxetic composite with the same raw materials but with different reinforcement structure was also fabricated for comparison in order to better understand the effect of reinforced structure on the deformation mechanism and mechanical responses of the composites. Since the quasi-static compressive properties and Poisson’s ratio effect of composites had been investigated previously, this paper only focused on the study of the low-velocity drop-weight impact tests of composites under impact energies from 7.25J to 65.25J. It was shown that both auxetic and non-auxetic composites were strain rate sensitive from 34.23s−1 to 111.48s−1, but they exhibited totally different mechanical responses due to different deformation and damage mechanism. Pull out tests demonstrated that strong interfacial bonding between the reinforcements and matrix could ensure the desired deformation of structural reinforcements and auxetic effect of the composite. It was also concluded that the auxetic composite had better energy absorption performance in medium strain range.

The development of novel auxetic woven structure for impact applications

Nowadays, the auxetic materials, auxetic reinforcement as well as auxetic composite are under the great attention of scientific research due to having excellent mechanical properties. In the current research work, the impact resistance of composite was improved by modifying the four layer through the thickness woven structure, that was used as reinforcement. A comparison was made between the standard four layer through the thickness and modified four layer through the thickness woven structure in terms of auxeticity and penetration. The modified four layer through the thickness woven structure showed auxetic behavior in both warp and weft direction having less penetration resistance as compared to standard sample. The four layer through the thickness non-auxetic woven structures and modified auxetic four layer thorough the thickness woven structures were used as reinforcements to develop composite for impact resistance comparison. The result showed that impact resistance of auxetic composite was 6.7% greater as compared to the non-auxetic composite. It was concluded that the greater value of impact resistance was due to the auxetic behavior of modified four layer through the thickness woven structure.