Auxetic Performance Research Articles

Machine learning analysis/optimization of auxetic performance of a polymeric meta-hybrid structure of re-entrant and meta-trichiral

In the ever-evolving field of materials science, the quest for innovative design solutions continues. One such area of research is the optimization of auxetic performance in meta-hybrid structures. This study delves into the application of machine learning analysis to enhance the properties of a specific meta-hybrid structure combining re-entrant and meta-trichiral designs. The polymeric meta-hybrid structure is subjected to finite element (FE) analysis to determine its Poisson's ratio. Various machine learning methods, such as linear, quadratic, and non-linear approaches, are employed to accurate forecast the Poisson's ratio of the polymeric meta-hybrid structure. Although both linear and quadratic machine learning algorithms have shown some success in predicting Poisson's ratios, they cannot be considered outstanding. As a result, a non-linear machine learning algorithm has been developed to enhance the accuracy of Poisson's ratio prediction. The results indicate that non-linear machine learning techniques out performs both linear and quadratic methods when it came to predicting Poisson's ratio. Specifically, the non-linear model achieved an impressive R-sq value of 93.68%, demonstrating its exceptional accuracy and overall excellence in fitting the data. The optimum conditions for the meta-hybrid structure are determined to be a thickness of 4.35 mm, a temperature of 15 °C, and a strain rate of 1%. Under these specific parameters, the Poisson's ratio of the meta-hybrid structure is measured to be −3.899, well outside isotropic stability limit but acceptable for auxetic structures.

Finite element analysis and surrogate-optimized design of a nature-inspired auxetic stent

Prior studies have revealed that the structural design of stents is critical to reducing some of the alarming post-operative complications associated with stent-related intervention. However, the technical search for stents that guarantee robustness against stent-induced post-intervention complications remains an open problem. Along this objective, this study investigates a re-entrant auxetic stent's structural response and performance optimizations. In pursuit of the goal, a nonlinear finite element analysis (FEA) is employed to uncover metrics characterizing the auxetic stent’s mechanical behavior. Subsequently, the non-dominated sorting genetic algorithm (NSGA-II) is implemented to simultaneously minimize the stent’s von Mises stress and the elastic radial recoil (ERR). Results from the FEA revealed a tight connection between the stent’s response and the features of the base auxetic building block (the rib length, strut width, and the re-entrant angle). It is observed that the auxetic stent exhibits a much lower ERR. Besides, larger values of its rib length and re-entrant angle are noticed to favor smaller von Mises stress. The Pareto-optimal front from the NSGA-II-based optimization scheme revealed a sharp trade-off in the simultaneous minimization of the von Mises stress and the ERR. Moreover, an optimal combination of the auxetic unit cell’s geometric parameters is found to yield a much lower maximum von Mises stress of ≈ 403 MPa and ERR of ≈ 0.4 % .

In-Plane Compressive Responses of Non-Homogenous Re-Entrant Honeycombs Fabricated by Fused Deposition Modelling.

Auxetic structures, with re-entrant (inverted hexagonal or bow tie) unit cells, have received considerable interest due to their negative Poisson's ratio property that results in superior mechanical properties. This study proposes a simple method to create non-homogeneous re-entrant honeycombs by modifying the size of chevron crosslinks. The various structural designs were conceived by changing the geometrical dimensions of the crosslinks, namely the length (lcl) and the thickness (tcl), while maintaining the parameters of the re-entrant cell walls. The influence of the design parameters of chevron crosslinks on the mechanical behaviour of additively manufactured re-entrant honeycombs was investigated experimentally and numerically. The structures were fabricated using the Fused Deposition Modelling (FDM) technique from polylactic acid (PLA) plastic. In-plane quasi-static compression tests were conducted to extract the elastic, plastic, and densification properties of the structures. Furthermore, a Finite Element (FE) model was developed via LS-DYNA R11.0 software, validated experimentally, and was then used to obtain a deeper insight into the deformation behaviour and auxetic performance of various designs. The obtained results revealed that the mechanical performance of re-entrant honeycombs can only be tuned by controlling the geometrical configuration of chevron crosslinks.

An experimental and numerical study on an innovative metastructure for 3D printed thermoplastic polyurethane with auxetic performance

AbstractMetamaterials are specifically designed materials that possess unique properties that cannot be found in naturally occurring substances. These remarkable materials have the capability to bring about a significant transformation across a wide range of industries. Auxetic structures are a recent area of research possess a distinctive characteristic known as a negative Poisson's ratio. Unlike conventional materials that contract when stretched, auxetic structures actually expand in two dimensions. In this study, a new auxetic structure was introduced, and thermoplastic polyurethane samples were 3D printed using a fused filament fabrication method. The samples are then subjected to strains ranging from 5% to 50% and Poisson's ratios are measured both experimentally and numerically using finite element method in Ansys software. By comparing the results of the experimental research and simulation, it is evident that applying strains within this range causes the Poisson's ratio of the samples to change from −0.81 to −0.14 and it showed that the newly introduced structure is auxetic. According to the analysis of root mean square error, the hexagonal mesh with a size of 0.7 mm consistently produced the most accurate results, aligning closely with the experimental sample. Given that this is an entirely novel auxetic structure within the category of arrow‐head auxetic structures, there is potential for future research to be conducted in order to further develop and enhance this model.

A Novel Barbell‐Shaped Perforated Auxetic Metastructure with Superior Auxetic Effect

Recently, artificial perforated auxetic metastructures with negative Poisson's ratio have attracted considerable attention with the superior mechanical properties. The concepts and design methods of various types of perforated auxetic metastructures have developed rapidly. However, the effective Poisson's ratio exhibited by the majority of current perforated auxetic metastructures has a lower limit of −1, which restrains their potential applications significantly. Herein, a novel 2D perforated auxetic metastructure is proposed by arraying orthogonal barbell‐shaped holes onto a sheet structure. The mechanical properties and the underlying deformation mechanism of the proposed auxetic metastructure are then investigated by performing experimental tests and finite element simulations. Results show that the lower limit of the Poisson's ratio exhibited by the proposed auxetic metastructure surpasses −1 remarkably and the proposed design hence exhibits much better auxetic performance compared with existing designs. It is also revealed that the proposed design exhibits relatively low local stress distribution. The results of this study will broaden the potential applications of perforated auxetic metastructures in many engineering fields.

A novel elliptical annular re-entrant auxetic honeycomb with enhanced stiffness

Auxetic structures have sparked a lot of interest due to their excellent mechanical characteristics and unusual negative Poisson's ratio influence. However, the large porosity property results in low stiffness, which limits their application scope. Current approaches to augment stiffness often result in diminished auxetic performance. Therefore, it is challenging to simultaneously enhance the auxetic, stiffness and energy absorption capacity of auxetic structures. To address this issue, this paper proposes a novel elliptical annular re-entrant honeycomb (EARE) by introducing an elliptical annular structure into a conventional re-entrant honeycomb (RE) cell, which can provide extra longitudinal support without hindering lateral deformation. The plateau stress, Poisson's ratio and energy absorption properties of the EARE are investigated by quasi-static compressive tests and finite element modeling. The results show that the EARE honeycomb has two plateau stages due to the supporting effect of the elliptical annular structure. Compared with the conventional RE honeycomb with the same wall thickness, the EARE honeycomb not only exhibits a stronger auxetic effect embodied in the Poisson's ratio reduced by 5.19%, but also has a higher average plateau stress and a higher specific energy absorption of 171.63% and 28.03%, respectively, which significantly improves the stiffness and energy absorption performance. The parameterized analysis reveals that by appropriately adjusting the geometric parameters, the stiffness, energy absorption, and auxetic effect of the EARE honeycomb can be enhanced. The research findings address the conflict between auxetic performance and honeycomb stiffness, providing new insights into the optimization of honeycomb structure design.

A sequential cross-product knowledge accumulation, extraction and transfer framework for machine learning-based production process modelling

ABSTRACT Machine learning is a promising method to model production processes and predict product quality. It is challenging to accurately model complex systems due to data scarcity, as mass customisation leads to various high-variety low-volume products. This study conceptualised knowledge accumulation, extraction, and transfer (KAET) to exploit the knowledge embedded in similar entities to address data scarcity. A sequential cross-product KAET (SeqTrans) is proposed to conduct KAET, integrating data preparation and preprocessing, feature selection (FS), feature learning (FL), and transfer learning (TL). The FS and FL modules conduct knowledge extraction and help address various practical challenges such as changing operating conditions and unbalanced datasets. In this paper, sequential TL is introduced to production modelling to conduct knowledge transfer among multiple entities. The first case study of auxetic material performance prediction demonstrates the effectiveness of sequential TL. Compared with conventional TL, sequential TL can achieve the same test mean square errors with 300 fewer training examples when facing data scarcity. In the second case study, balancing anomaly detection models were constructed for two gas turbines in the same series using real-world production data. With SeqTrans, the F1-score of the anomaly detection model of the data-poor engine was improved from 0.769 to 0.909.

Comparative Geometrical Analysis of In Situ Mechanical Performance of 2-D Woven In-Plane Auxetic Structures

Abstract Recently, the production of auxetic textiles using conventional yarns and equipment has captured the attention of researchers. Yet only uni-stretch 2-D woven auxetic structures and auxetic knitted textiles have been produced using conventional yarns. This paper reports a comparative analysis of auxetic and mechanical performance (tensile strength, tear strength, and stretch and growth %) performance of auxetic foldable and auxetic honeycomb (AHC) structures using common elastic and nonelastic yarns and conventional weaving technologies. For this purpose, five different auxetic weave designs including α-vertical pointed twill (S1), β-vertical pointed twill (S2), α-horizontal pointed twill (S3), β-horizontal pointed twill (S4), and honeycomb structure (S5) were selected, and their corresponding woven fabrics were developed with equal thread density at the weaving stage. Auxeticity of woven structures was measured in both the warp and weft directions. The results showed that when the developed fabrics were stretched along the warp and weft directions, the foldable geometry showed −1 and −0.07 maximum negative Poisson’s ratio (NPR), respectively. Although the AHC structure showed maximum NPR −0.94 and −0.21 along warp and weft directions, respectively. AHC showed better mechanical performance as compared with foldable geometry and has higher dimensional stability. AHC structure showed the highest tensile strength as compared with the others. S3, S4, and S5 remained untorn along the warp direction because these have longer float lengths that produce a bunch having a greater number of yarns, whereas S1 and S2 showed tear strength of 47 and 43.2 N, respectively. In the weft direction, only S1 remained untorn, whereas the other four samples showed the tear strength value.

The compressive responses and failure behaviors of composite graded auxetic re-entrant honeycomb structure

This paper investigated the quasi-static compressive performances and failure behaviors of composite auxetic re-entrant honeycomb sandwich structure. Three types of auxetic re-entrant honeycomb structures with different gradient configurations were manufactured and tested. The carbon/epoxy prepreg was used to fabricated the auxetic re-entrant honeycomb sandwich structure. The reaction force and displacement of the test fixture were collected by the transducer of the universal testing machine. The compressive processes of specimens were recorded by high-resolution camera. Combined with the stress–strain curves and deformation processes of composite structures, the compressive responses and deformation mechanisms was analyzed. The Poisson’s ratio, energy efficiency and plateau stress of re-entrant honeycomb sandwich structures were defined and utilized to study the auxetic performance and energy absorption ability. Furthermore, a high-fidelity numerical model was established and finite element software ABAQUS was used to analyzed the compressive performances and failure behaviors of composite graded auxetic re-entrant honeycomb structure. The compressive failure modes of the numerical simulation results were compared with that of the experimental results, and the numerical simulation results agreed well with the experimental results. The results indicated that average structure exhibited best energy absorption performance and bidirectionally graded auxetic honeycomb structure had the best negative Poisson’s ratio performance.

Auxetic effects in the large deflection bending characteristics of FG GRMMC shallow arches

Shallow arches are ubiquitous in long-span buildings, bridge structures, and tunnels. Uniform loading can cause large deflections characterized by snap-through, which can ultimately lead to structural failure. In this work, nonlinear bending of functional graded graphene reinforced metal matrix composite (GRMMC) shallow arches with auxetic properties is studied using higher order shear constitutive equations based on the von Kármán nonlinear theory of beam. Asymptotic solutions for an arch with simply and clamped supported ends are derived using a two-perturbation approach. The results show that the the initial thermal stress-induced deformation of the members increases with increasing environmental temperature. The generated auxetic effect in the structures due to Negative Poisson's Ratio (NPR) will act to resist bending deformation. This strengthening effect is more noticeable with increase in the deflection, especially in the first snap-through. In addition, the elastic foundation increases the global stiffness of the arch, thus enhancing its bending stiffness. The current work not only provides a theoretical foundation for understanding the interrelationship between the auxetic effect and mechanical performance of structures but also offers guidance for designing metamaterial structures.

In-Plane Quasi-Static Crushing Behaviors of a Novel Reentrant Combined-Wall Honeycomb

Abstract The innovative design of microstructure topology is of great significance to improve the energy absorption performance of honeycombs. In this paper, by embedding a hexagonal substructure in each inclined wall of the traditional reentrant (RE) honeycomb, a novel auxetic honeycomb, called reentrant combined-wall (RCW) honeycomb, is developed for improving energy absorption. Combining the experimental methods, numerical simulations, and analytical analyses, we systematically studied the in-plane quasi-static behaviors of the proposed honeycomb structure. It can be found that the deformation of the RCW honeycomb has a transitional stage, which makes a significant stress enhancement. During the crushing process, the Poisson’s ratio of the RCW honeycomb keeps negative and is lower than that of the RE honeycomb. Besides, numerical and analytical analyses show that the stress-strain response of the RCW honeycomb has a good designability. Further, the analysis of specific energy absorption (SEA) is also performed, in which the RCW honeycomb shows a significant superiority over the RE honeycomb, with the SEA value almost twice that of the latter. Therefore, it can be concluded that the proposed novel structure has tangible improvements in the crushing strength, auxetic performance, and energy absorption, which deserves more attention in future work.

Mechanical behaviors of a novel auxetic honeycomb characterized by re-entrant combined-wall hierarchical substructures

The current focus of the metamaterials is to further improve their performance by the unit cell innovation, while for the auxetic metamaterials, the compromise between the mechanical properties and auxetic effect still needs more efforts. Given this issue, here we developed a novel auxetic honeycomb, named re-entrant combined-wall (RCW) honeycomb, by introducing four hierarchical substructures to the RE cell. Analytical expressions were derived and used to study the in-plane elastic constants of the RCW honeycomb, which were well confirmed by the established finite element model. Further, we investigated its crushing behaviors under large deformation by the explicit numerical method, and the quasi-static crushing experiments were also carried out by the 3D-printed specimens. Results show that the properties of the proposed RCW honeycomb have a high degree of orthogonality and tunability. Compared with the traditional RE honeycomb, the Young’s modulus of the RCW honeycomb in the y direction increases by more than 120%, and the Poisson’s ratio decreases by about 43%. Besides, behaviors of the cell wall contact induced by the adding substructure can lead to an interesting stress enhancement phenomenon under large deformation, which significantly increases its crushing strength, up to 140%, compared with the RE honeycomb. Therefore, the results in this work effectively demonstrate the improved mechanical properties and auxetic performance of the proposed RCW honeycomb. Besides, the adopted design strategy of hierarchical substructure also exhibits great potential for developing novel and excellent auxetic mechanical metamaterials.

Investigation of Auxetic Performance and Various Physical Properties of Fabrics Woven with Braid Yarns

In this study, the auxetic performance and various physical properties of fabrics woven with braid yarn were examined. Fabrics were woven in plain weave with conventional warp and braid weft yarn. Experimental results showed that fabrics woven with braid weft yarn exhibited an auxetic behavior by giving Negative Poisson's Ratio (NPR) up to a certain elongation value under tension in the warp direction. In addition, it was observed that the NPR of fabric was affected by the thickness of the braid yarn and the tightness (compactness) of the fabric. It was found that the use of braid yarn in woven fabric improved various physical properties such as tensile strength, thermal resistivity and abrasion resistance. Use of braid yarns increased the tensile strength in the weft direction where braid yarns were used, increased thermal resistivity values at the fabric woven with thick and bulky braid yarns and also increased abrasion resistance.

Anisotropy in conventional and uniaxially thermoformed auxetic polymer foams

Auxetic (negative Poisson's ratio) open cell polymeric foams have been traditionally produced using thermoforming techniques coupled with the volumetric compression of conventional (pristine) off-the-shelf foams. The anisotropy of the pristine foam plays however a strong role on the performance of thermoformed auxetic foams, and this is the subject studied in this work. Micro computed tomography (μ-CT) scans indeed show that the cell structures of the pristine foam exhibit an elongated convex polyhedron shape, with more ribs oriented along the foam rising direction d 1 . The modulus along d 1 can be up to 70% larger than the one along the transverse directions, and the Poisson's ratios in different planes are also noticeable different, ranging from 0.2 to 0.85. Three types of auxetic foams uniaxially thermoformed along the three orthogonal directions of the conventional porous material are here developed, and all show different mechanical performances due to the anisotropy of the conventional baseline foam. The auxetic foams show significant auxetic behavior along the thermoforming direction only when thermoformed along the transverse directions of the pristine foam (d 2 or d 3 ) and loaded along the foam rising direction (d 1 ); in that case, the negative Poisson's ratio reach values as low as −1. In other cases, the auxetic performance is not evident, and the Poisson's ratio is close to 0. Finite Element models of the pristine and auxetic foams have been also here developed based on the 3D models from μ-CT, and the stiffness-strain curved derived from those coincide well with the experimental results. The simulated deformation mechanisms of the cell structures inside different foams help to explain the anisotropic performance of the foams and the differences between compressive and tensile properties.

Modification of hexachiral unit cell to enhance auxetic stent performance

Stents possess disadvantages such as foreshortening and dogboning. Material selection alongside topological modifications are two main approaches to overcome these issues. Stents with negative Poisson’s ratio, also known as auxetic property, can lead to promising performance improvement. Therefore, in this study, three stents were designed by improving a hexachiral unit cell and underwent numerical and experimental investigations using CoCr 605 L and SS 316 L alloys. The modified auxetic stents presented higher auxeticity and NPR of −0.64 and −0.69. Higher radial expansion, stress tolerance and yield strain of these modified stents help them overcome drawbacks of non-auxetic stents.

Effect of Different Yarn Combinations on Auxetic Properties of Plied Yarns

Abstract This study presents the effects of a novel plied yarn structure consisting of different yarn components and yarn twist levels on the Poisson's ratio and auxetic behavior of yarns. The plied yarn structures are formed with bulky and soft yarn components (helical plied yarn [HPY], braided yarn, and monofilament latex yarn) and stiff yarn components (such as high tenacity [HT] and polyvinyl chloride [PVC]-coated polyester yarns) to achieve auxetic behavior. Experimental results showed that as the level of yarn twist increased, the Poisson's ratios and the tensile modulus values of the plied yarns decreased, but the elongation values increased. A negative Poisson's ratio (NPR) was obtained in HT–latex and PVC–latex plied yarns with a low twist level. The plied yarns formed with braid–HPY and braid–braid components gave partial NPR under tension. A similar result was achieved for yarns with HT–latex and PVC–latex components. Since partial NPR was seen in novel plied yarns with braided and HPY components, it is concluded that yarns formed with bulky–bulky yarn components could give an auxetic performance under tension.

Investigation of the effect of pique weave on auxetic performance and related fabric properties

Re-entrant structures are known to perform an auxetic effect. It was aimed in this paper to investigate the Poisson's ratio of the woven pique structures because of the re-entrant lozenge effect of the weave. Sound absorption, air permeability, and synclastic properties of the fabrics concerning the auxetic properties of pique structures were also examined. In evaluating the synclastic behavior, the drape ability and bending rigidity of the fabrics were assessed. Pique weave fabrics were subjected to tensile tests, and the changes in fabric unit cell at different tensile strains were observed in the warp and weft directions. Experimental results showed that the auxetic effect was produced in the warp direction. A high negative Poisson's ratio (NPR) was obtained in cotton pique fabrics under low elongation values. It was observed that the yarn floating lengths and the yarns intersecting region widths forming the re-entrant lozenge pattern had effects on fabrics' NPR.

Graphene Origami-Enabled Auxetic Metallic Metamaterials: An Atomistic Insight

Auxetic metamaterials with programable negative Poisson's ratio (NPR) are of great significance in engineering applications. These materials, however, are either mechanically weak or heavily reliant on their specially designed architecture/topology to be auxetic. Consequently, their auxetic performance may be deteriorated or even lost due to local structural deformation or failure under excessive external loading. Developing materials simultaneously possessing auxetic characteristics and excellent mechanical properties still remains a great challenge. Herein, we report a class of graphene origami (GOri)-enabled metallic metamaterials with a highly tunable NPR as well as improved mechanical properties using molecular dynamics study. Simulation results reveal that embedding more GOri into Cu matrix can not only lead to a bigger NPR but also an increased elastic modulus of the nanocomposite. The NPR of the nanocomposite with 3.35 wt % Miura-patterned GOri can reach –0.2796 at room temperature. The application of pressure and temperature can further enhance the auxetic behavior of the nanocomposite. It is also found that GOri/Cu nanocomposite significantly outperforms its counterpart reinforced with pristine graphene in terms of NPR.

Investigating the influence of the core material on the mechanical performance of a nitinol wire wrapped helical auxetic yarn

Helical Auxetic Yarns (HAYs) can be used in a variety of applications from healthcare to blast and impact resistance. This work focuses on the effect of the use of different core materials (e.g. rubber, polyurethane, polytetrafluoroethylene/teflon, polypropylene, polyetheretherketone, polycarbonate, acetal) with a nitinol wire wrap component on the maximum Negative Poisson Ratio (NPR) produced and thus the auxetic performance of Helical Auxetic Yarns (HAYs). From the analytical model, it was found that an acetal core produced the largest NPR when compared to the other six materials. The trend obtained from the experimental tensile tests (validation) correlated closely with the theoretical predictions of the NPR as axial strain was increased. The experimental method presented a maximum NPR at an average axial strain of 0.148 which was close to the strain of 0.155 predicted by theory. However, the maximum experimental NPR was significantly lower than that predicted by the analytical model.

Indentation resistance of brittle auxetic structures: Combining discrete representation and continuum model

This work investigates the indentation resistance of the auxetic/non-auxetic honeycomb ceramics layer by combining a discrete numerical representation and a continuum analytical model. The indentation depth of honeycombs with varying cell-wall angle and cell-wall thickness are measured. The indentation depth of auxetic honeycombs with negative Poisson’s ratio (NPR) is smaller than that of conventional ones in the premise of the same cell-wall thickness. But on the condition that the honeycombs possess the same relative density, there exists an inverse tendency. The corresponding mechanisms responsible for those trends are revealed. The stress intensity factor (SIF) is defined to characterize the stress singularity at the corner of the punch. The influence of NPR on the magnitudes of SIF is evaluated. A punch toughness concept is defined by a novel multi-scale method as the failure criterion of brittle honeycomb ceramics in indentation tests. The effects of NPR on the punch toughness of honeycombs are discussed. And the empirical formulas of punch toughness varying with the cell-wall thickness and the relative density are summarized. Finally, the NPR effects on the collinear punches problem are detected; the results show that the auxetic performance can significantly weaken the interaction between the punches.