Search

In the present paper, several viscoelastic models are studied for cases when time-dependent viscoelastic operators of Lamé's parameters are represented in terms of the fractional derivative Kelvin–Voigt, Scott-Blair, Maxwell, and standard linear solid models. This is practically important since precisely these parameters define the velocities of longitudinal and transverse waves propagating in three-dimensional media. Using the algebra of dimensionless Rabotnov's fractional exponential functions, time-dependent operators for Poisson's ratios have been obtained and analysed. It is shown that materials described by some of such models are viscoelastic auxetics because the Poisson's ratios of such materials are time-dependent operators which could take on both positive and negative magnitudes. [ABSTRACT FROM AUTHOR]

Auxetics have emerged recently as an exciting class of mechanical metamaterials that are identified by negative Poisson's ratio. The mechanical properties of rationally designed auxetic metamaterials are mostly influenced by their topological characteristics rather than the material properties of their constituent material. The goal of this paper is to use optimization algorithms to achieve the desired target mechanical properties for reentrant auxetic metamaterials. The GEKKO optimization package in Python is used for optimizing the geometry of auxetic lattice structures. Several mechanical properties including Poisson's ratio, relative Young's modulus, relative yield stress, and relative energy absorption capability, as well as the noted properties normalized with respect to relative density and relative Young's modulus are chosen for being minimized or maximized. ANSYS finite element package is also implemented for validation of the results obtained from the optimization algorithm. According to the results, to achieve the maximum magnitude of Poisson's ratio (positive and negative), the size of the unit cell in the lateral direction must be selected to be maximum and the thickness must become minimum. Moreover, to achieve the maximum value of the relative Young's modulus or energy absorption in any direction, the size of the unit cell in that direction must be maximized. Also, to achieve the maximum amount of relative yield stress in both directions, the unit cell must have a maximum thickness and an internal angle close to zero. [ABSTRACT FROM AUTHOR]

This paper is dedicated to the analysis of a foot prosthesis optimization process, with a particular focus on the application of optimization algorithms and unconventional materials, such as auxetic materials. The study aims to enhance prosthesis performance by minimizing the difference between the ground reaction force generated by the prosthetic foot and that of a natural limb. In the initial part of the study, the basic topics concerning the parameterization of the foot prosthesis geometry and the preparation of a finite element model for human gait are discussed. In the subsequent part of the study, the focus is on the optimization process, in which algorithms were applied to adjust the prosthesis structure to the patient's individual needs. The optimization process utilized a finite element method gait model. After validating the FEM, an algorithm generating the prosthesis geometry based on the given parameters was developed. These parameters were optimized using the VOA, comparing FEM gait model data on vertical ground reaction force with experimental results. The results of the foot prosthesis optimization are presented through a comparison of different structural models. The study also demonstrates the application of auxetic materials, which, due to their unique mechanical properties, can enhance foot prosthesis efficiency. Simulations were performed using multi-material topology optimization. The results obtained for different gait phases were compared. [ABSTRACT FROM AUTHOR]

Shunted piezoelectric patches connected to passive electric circuits can be attached to a host structure for effective vibration attenuation. The effect of an auxetic layer to enhance the electromechanical coupling and subsequently the vibration suppression is studied here. Three different configurations are considered for the layer: a classical, a homogeneous auxetic and a layer with microstructure leading to auxetic behavior. First, is presented a modification of the "current-flowing" shunt circuit for multimode vibration control. Two finite element models have been validated, the frequency response graph of the system and the most suitable values of the electric parameters are calculated and a comparison is provided. Furthermore, it is shown that the vibration reduction of the second and third eigenmodes can be enhanced, provided that an auxetic of significant thickness is used. The results demonstrate the effect of the auxetic boosting on vibration suppression. [ABSTRACT FROM AUTHOR]

Copyright of Turkish Online Journal of Design, Art & Communication is the property of Turkish Online Journal of Design, Art & Communication and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

This paper presents an analysis of a sandwich structure featuring an auxetic core, manufactured through Vacuum Assisted Resin Transfer Molding (VARTM) using carbon-aramid hybrid fibers. The core was 3D printed with polylactic acid (PLA). Experimental testing of the structure provided results for validating a cost-effective numerical model. The response surface methodology identified results for different sandwich panel configurations, revealing that auxetic behavior depends on the core's adopted configuration. Increasing the fiber-core contact area induces auxetic behaviors. The structure underwent optimization using the Lichtenberg algorithm, yielding models with 40.32% higher failure load, 36.36% higher natural frequency, and over 20% mass reduction. [ABSTRACT FROM AUTHOR]

Over the years, researchers have investigated active metamaterials because of controllability of their geometry and mechanical properties. However, despite the undeniable appeal and significance of this direction of studies from the perspective of many industries, researchers have struggled to systematically design such structures in an easily reproducible and scalable manner. Herein, it is shown that this can be achieved by harnessing the potential of foldable magnetomechanical metamaterials controlled via the external magnetic field. To this aim, two different configurations of the system with one of them being constrained so that it acts similarly to a linear actuator while the other one allows to fully control its expandability and mechanical properties in all directions are presented. Finally, the possibility of stacking the considered structures to create large systems that may exhibit auxetic properties in multiple planes will be discussed. The magnetomechanical metamaterials discussed are very interesting as they offer multidirectional and finely controlled movement which could be untethered. This is particularly promising for use in actuators, vibration dampers as well as scaffolds in deployable structures and impact resistive materials as the mechanical properties can be fine‐tuned by simply adjusting the external magnetic field and reconfiguring the structure. [ABSTRACT FROM AUTHOR]

Split thickness skin grafting is one of the common skin transplantation techniques employed in treating severe burn injuries. Recent experimental studies have shown that the expansions offered by split thickness skin grafts are much lower than what is claimed by the graft manufacturers. To date, limited studies have investigated the effect of slit patterns on the expansions and the possibility of material innovation. This work has experimentally tested novel auxetic slit patterns on a biofidelic skin simulant to evaluate their effect on skin graft expansion. A wide range of variants of alternating slit (AS), I-shaped (IS), and Rotating Triangle (RT) based auxetic patterns were tested. A biaxial testing device was employed to simulate realistic skin stretching during graft implementation. The deformation, expansion, stress-strain response, void area, and maximum gap size were estimated, and the material behavior was also modeled using hyperelastic curve fit coefficients. The key design parameters were determined, which would produce high expansion with minimal voids. The findings of this study are anticipated to throw light on the possibility of using auxetic slit patterns for generating higher expansions in split thickness skin grafts. [ABSTRACT FROM AUTHOR]

It is known that changes on the microscopic, structural level of materials exert changes on their macroscopic properties, in particular elastic ones. This approach can be used to alter the elastic properties of materials. However, it is not easy to predict the impact of a particular change in the structure of crystals. The recent studies show that depending on the size, shape, and orientation of inclusions used, the resulting elastic properties may differ. Inclusions may enhance, weaken, or eliminate the auxetic properties in a hard sphere model. This study is focused on the influence of the spatial distribution of planar inclusions within the hard sphere crystal and its impact on its elastic properties. Periodic systems containing two nanolayer inclusions in the representative volume element, oriented orthogonally to [001]$\left[\right. 001 \left]\right. $‐direction and in various spatial ordering, are considered. It has been shown that introducing layer inclusions causes the deterioration of auxetic properties in the [110][11¯0]$\left[\right. 110 \left]\right. \left[\right. 1 \bar{1} 0 \left]\right. $‐direction. However, the latter weakly depends on the spatial ordering of the inclusion layers. Moreover, changes in the size of the inclusion particles, combined with different ordering of the inclusion layers, can be used to coarsely and finely tune the elastic properties of the model crystal. [ABSTRACT FROM AUTHOR]

Stress concentrations pose a significant challenge in designing and optimizing composite components, often resulting in structural alterations and crack formation. This study delves into a detailed numerical and analytical analysis of stress concentration factors in composite materials, with a specific focus on the influence of negative Poisson's ratio and out‐of‐plane modulus. By exploring the interaction of these material properties, the objective is to devise effective strategies for alleviating stress concentrations in composite structures. Through analytical and numerical simulations, a robust correlation between the customized Poisson's ratio and modulus and the mitigation of stress concentration factors, particularly in epoxy/graphite‐reinforced composites are established. This tailored approach has the potential to enhance energy absorption capabilities and consequently reduce the risk of crack propagation in perforated laminated composite plates. This research findings offer valuable insights into composite material design, presenting innovative solutions for enhancing structural integrity and reducing susceptibility to stress‐related issues. [ABSTRACT FROM AUTHOR]

This article reports the design of a metamaterial model—made from interconnected units of rotating parallelograms and triangles—in which its Poisson's ratio can range from positive to negative by changing the internal angles between the rotating units. The on‐axes Poisson's ratios are established via geometrical construction while the on‐axes Young's moduli are extracted using energy approach. Results indicate that the metamaterial model manifests auxetic behavior for small and large internal angles, but non‐auxetic properties for intermediate range of internal angle. The results also reveal that a wide range of on‐axes Young's modulus can be adjusted by controlling the internal angle. A sustainable feature of this metamaterial model is its convenient adjustment of the effective mechanical properties over a wide range in order to be redeployed for different applications throughout its entire lifetime. [ABSTRACT FROM AUTHOR]

Cubic lattices based on tetrakaidecahedron cells with re‐entrant ribs are designed and fabricated via additive manufacturing. Poisson's ratio decreased to a negative value as the re‐entrant angle was made smaller. Poisson's ratio decreased as ribs were made more slender. Lattices with convex tetrakaidecahedron (truncated octahedron) cells had a Poisson's ratio near 0.5 in agreement with theory. Rib slenderness had minimal effect on Poisson's ratio in contrast to the re‐entrant lattices. [ABSTRACT FROM AUTHOR]

To obtain materials or metamaterials with desired elastic properties that are tailor-made for a particular application, it is necessary to design a new material or composite (which may be cumbersome) or to modify the structure of existing materials in order to change their properties in the desired direction. The latter approach, although also not easy, seems favourable with respect to parameters like costs and time-to-market. Despite the fact that elastic properties are one of the oldest studied physical parameters of matter, our understanding of the processes at the microstructural level, that are behind these properties, is still far from being complete. The present work, with the help of Monte Carlo computer simulations, aims to broaden this knowledge. The previously studied model crystal of hard spheres, containing a combined nanolayer and nanochannel inclusions, is revisited. This periodic model crystal has been extended to include a degree of disorder in the form of degenerate crystalline phase by introducing a degenerate crystalline phase within its structure. The inclusion has been transformed (without changes to its shape, size, or orientation) by randomly connecting the neighbouring spheres into di-atomic molecules (dumbbells). The impact of this modification on elastic properties has been investigated with the help of the Parrinello–Rahman approach in the isothermal–isobaric ensemble ( N p T ). It has been shown, that the presence of the degenerate crystalline phase of hard dumbbells in the system leads to a significant decrease in the Poisson's ratio in [ 110 ] -direction ( ν = − 0.235 ) and an overall enhancement of the auxetic properties. [ABSTRACT FROM AUTHOR]

This study derives the constitutive relationships between stress resultants and strain measures for 4-star auxetic structures using force-moment analysis of a representative unit cell and the Castigliano strain energy method. To validate the accuracy of the constitutive model, bending analysis is performed for a 4-star auxetic rectangular plate under Levy boundary conditions. The analytical and numerical results show excellent agreement, confirming the validity of the constitutive relations extracted from the generic 4-star unit cell. This work establishes a theoretical framework for designing 4-star auxetic metamaterials and demonstrates their potential for diverse engineering applications. [ABSTRACT FROM AUTHOR]

To advance the design space of electrically‐driven soft actuators, a flexible, architected soft robotic actuator is presented for motor‐driven extensional motion. The actuator comprises a 3D printed, cylindrical handed shearing auxetic (HSA) structure and a deformable, internal rubber bellows shaft. The actuator linearly extends upon applying torque from a servo motor; the rubber bellows shaft is stretchable but resistant to torsional deflection, allowing it to transmit torque from the servo motor to the other end of the HSA. The high flexibility of the HSA and rubber bellows shaft enable the actuator to adaptively extend even when bent. The actuator's two components and its performance are mechanically characterized. Actuation strains of 45% elongation and a maximum blocked pushing force of about 8 N are demonstrated. The actuator's capabilities are showcased in two separate demonstrations: a crawling robot and a sensorized artificial muscle that integrates a microfluidic, liquid metal strain sensor. The architected material design approach for a robust, motor‐driven soft actuator provides several unique features—including a compact form factor and ease of use—over other motorized soft robotic actuators based on HSA assemblies or cable tendon mechanisms. [ABSTRACT FROM AUTHOR]

This paper explores how the examination of additively manufactured auxetic componentry can be applied in human-centred design settings with particular focus on biomedical products. Firstly, the design applications of auxetics are detailed followed by a review of the key problems facing practical researchers in the field with the treatment of boundary conditions identified as a key issue. The testing setup that is then introduced utilises a novel method of part mounting and facilitates optical analysis and real-time force–displacement measurements. A study is advanced that analyses three different auxetic structures (re-entrant, chiral, and semi-rigid), a set of samples of which were additively manufactured in flexible TPU material. A range of parameters were varied across the three designs including interior geometry and wall thicknesses in order to demonstrate the effectiveness of the setup for the examination of the different structures. The results from these examinations are subsequently discussed and a number of suggestions made regarding how this kind of analysis may be integrated into novel design development workflows for achieving human-centred biomedical devices which often require detailed consideration of ergonomic and usability factors. [ABSTRACT FROM AUTHOR]

A perfectly auxetic 2D metamaterial model is described herein by means of interconnected rotating rigid units of three different shapes: hexagons, triangles, and rectangles. The in‐plane Young's modulus is obtained by matching the discrete elastic energy stored at the hinges during relative rotation of the rigid units with the overall elastic energy stored in the metamaterial's continuum equivalence. Results indicate that, in addition to the trivial effects from the rotational spring constant at the hinges, the in‐plane Young's modulus is highly influenced by the internal angle, followed by the relative size of the triangle with reference to the sizes of other rigid units. The effect from the rectangle aspect ratio presents the least influence. As such, there are several parameters to control the Young's modulus, from the coarse tuning by adjusting the internal angle to fine tuning by controlling the rectangle aspect ratio. The capability of the metamaterial's Young's modulus to be tuned from a spectrum of parameters that range from coarse to fine tuning permits this metamaterial's mechanical property to be precisely designed and manufactured in a cost‐effective manner. [ABSTRACT FROM AUTHOR]

Designing a particular change in a system structure to achieve the desired elastic properties of materials for a given task is challenging. Recent studies of purely geometrical atomic models have shown that structural modifications on a molecular level can lead to interesting and desirable elastic properties. Still, the result of such changes is usually difficult to predict. The present work concerns the impact of nanolayer inclusion ordering in hard sphere crystals on their elastic properties, with special attention devoted to their auxetic properties. Two sets of representative models, based on cubic crystals consisting of 6 × 6 × 6 unit cells of hard spheres and containing either neighboring or separated layers of spheres of another diameter, oriented orthogonally to the [001] direction, have been studied by Monte Carlo simulations in the isothermal–isobaric (NpT) ensemble. Their elastic constants have been evaluated using the Parinello–Rahman approach. The Monte Carlo simulations showed that introducing the layer inclusions into a pure face-centered cubic (FCC) structure leads to the system's symmetry changes from cubic symmetry to tetragonal in both cases. Essential changes in the elastic properties of the systems due to layer ordering were found both for neighboring and separated inclusions. It has been found that the choice of a set of layer inclusions allows one to tune the auxetic properties in two crystallographic directions ( [ 110 ] [ 1 1 ¯ 0 ] and [ 101 ] [ 1 ¯ 01 ] ). In particular, this study revealed that the change in layer ordering (from six separated layers to six neighboring ones) allows for, respectively: (i) enhancing auxeticity of the system in the [ 101 ] [ 1 ¯ 01 ] direction with almost loss of auxetic properties in the [ 110 ] [ 1 1 ¯ 0 ] direction in the case of six separated layers, while (ii) in the case of six neighboring layers, keeping the auxetic properties in both auxetic directions independently of the size of spheres constituting inclusions. [ABSTRACT FROM AUTHOR]

Microfluidic valves play a key role within microfluidic systems by regulating fluid flow through distinct microchannels, enabling many advanced applications in medical diagnostics, lab‐on‐chips, and laboratory automation. While microfluidic systems are often limited to planar structures, 3D printing enables new capabilities to generate complex designs for fluidic circuits with higher densities and integrated components. However, the control of fluids within 3D structures presents several difficulties, making it challenging to scale effectively and many fluidic devices are still often restricted to quasi‐planar structures. Incorporating mechanical metamaterials that exhibit spatially adjustable mechanical properties into microfluidic systems provides an opportunity to address these challenges. Here, systematic computational and experimental characterization of a modified re‐entrant honeycomb structure are performed to generate a modular metamaterial for an active device that allows us to directly regulate flow through integrated, multiplexed fluidic channels "one‐at‐a‐time," in a manner that is highly scalable. A design algorithm is presented, so that this architecture can be extended to arbitrary geometries, and it is expected that by incorporation of mechanical metamaterial designs into 3D printed fluidic systems, which themselves are readily expandable to any complex geometries, will enable new biotechnological and biomedical applications of 3D printed devices. [ABSTRACT FROM AUTHOR]

The mechanical properties of metamaterials with different cellular internal structures were experimentally studied when perforated along the normal by a rigid spherical striker. Auxetic and non-auxetic samples of metamaterials with a chiral structure of cells, respectively, in the form of concave or convex hexagons, were produced using a 3D printer from e-PLA plastic. Based on the penetration experiments, the properties of chiral auxetic and non-auxetic samples of the same mass were compared for the cases when there was air inside the cells and when the cells were filled with gelatin. The relative loss of kinetic energy of the striker when perforating gelatin-filled samples was significantly higher for the auxetic metamaterial than for the non-auxetic one. For unfilled ("air") samples, the relative loss of kinetic energy was slightly higher for the nonauxetic. [ABSTRACT FROM AUTHOR]

A generalization of a physically nonlinear Maxwell-type viscoelastoplastic constitutive equation with four material functions is formulated, whose general properties and range of applicability were discussed in a series of our previous studies. In order to expand the range of rheological effects and materials that can be described by the equation, it is proposed to add a third strain component expressed by a linear integral Boltzmann–Volterra operator with arbitrary functions of shear and volumetric creep. For generality and for the convenience of using the model, as well as for fitting the model to various materials and simulated effects, a weight factor (degree of nonlinearity) is introduced into the constitutive equation, which allows combining the original nonlinear equation and the linear viscoelastic operator in arbitrary proportions to control the degree of different effects modeled. Equations are derived for families of creep curves (volumetric, shear, longitudinal, and transverse) generated by the proposed constitutive equation with six arbitrary material functions, and an expression is obtained for the Poisson ratio as a function of time. Their general properties and dependence on loading parameters and characteristics of all material functions are studied analytically and compared with the properties of similar relations produced by two combined constitutive equations separately. New qualitative effects are identified which can be described by the new constitutive equation in comparison with the original ones, and it is verified that the generalization eliminates some shortcomings of the Maxwell-type viscoelastoplastic constitutive equation, but retains its valuable features. It is confirmed that the proposed constitutive equation can model sign alternation, monotonic and nonmonotonic changes in transverse strain and Poisson's ratio under constant stress, and their stabilization over time. Generally accurate estimates are obtained for the variation range, monotonicity and nonmonotonicity conditions of Poisson's ratio, and its negativity criterion over a certain time interval. It is proven that neglecting volumetric creep (the postulate of bulk elasticity), which simplifies the constitutive equation, greatly limits the range of possible evolution scenarios of Poisson's ratio in time: it increases and cannot have extremum and inflection points. The analysis shows that the proposed constitutive equation provides ample opportunities for describing various properties of creep and recovery curves of materials and various Poisson's ratio evolution scenarios during creep. It can significantly expand the range of described rheological effects, the applicability of the Maxwell-type viscoelastoplastic equation, and deserves further research and application in modeling. [ABSTRACT FROM AUTHOR]

To advance the design space of electrically‐driven soft actuators, a flexible, architected soft robotic actuator is presented for motor‐driven extensional motion. The actuator comprises a 3D printed, cylindrical handed shearing auxetic (HSA) structure and a deformable, internal rubber bellows shaft. The actuator linearly extends upon applying torque from a servo motor; the rubber bellows shaft is stretchable but resistant to torsional deflection, allowing it to transmit torque from the servo motor to the other end of the HSA. The high flexibility of the HSA and rubber bellows shaft enable the actuator to adaptively extend even when bent. The actuator's two components and its performance are mechanically characterized. Actuation strains of 45% elongation and a maximum blocked pushing force of about 8 N are demonstrated. The actuator's capabilities are showcased in two separate demonstrations: a crawling robot and a sensorized artificial muscle that integrates a microfluidic, liquid metal strain sensor. The architected material design approach for a robust, motor‐driven soft actuator provides several unique features—including a compact form factor and ease of use—over other motorized soft robotic actuators based on HSA assemblies or cable tendon mechanisms.

This article deals with the development of an elastic tetragonal model for the 2D auxetic rotating units structures in the framework of orthogonal transformations. The existing anisotropy in the structure was first determined by numerical simulations on the samples with different pattern orientation angles. A suitable representative volume element (RVE), which correctly represents the mechanical properties of the whole structure both in macroscale and in microscale, was then proposed by implementation of the kinematic periodic boundary conditions. In the next step, with the help of the orthogonal transformations relations, an anisotropic elastic model was developed, which correctly reflects the present tetragonal symmetry in the structure. Finally, the model parameters were identified and validated with the help of the corresponding experiments [ABSTRACT FROM AUTHOR]

The flowing, jamming, and avalanche behavior of granular materials is satisfyingly universal and vexingly hard to tune: A granular flow is typically intermittent and will irremediably jam if too confined. Here, we show that granular metamaterials made from particles with a negative Poisson's ratio yield more easily and flow more smoothly than ordinary granular materials. We first create a collection of auxetic grains based on a re-entrant mechanism and show that each grain exhibits a negative Poisson's ratio regardless of the direction of compression. Interestingly, we find that the elastic and yielding properties are governed by the high compressibility of granular metamaterials: At a given confinement, they exhibit lower shear modulus, lower yield stress, and more frequent, smaller avalanches than materials made from ordinary grains. We further demonstrate that granular metamaterials promote flow in more complex confined geometries, such as intruder and hopper geometries, even when the packing contains only a fraction of auxetic grains. Moreover, auxetic granular metamaterials exhibit enhanced impact absorption. Our findings blur the boundary between complex fluids and metamaterials and could help in scenarios that involve process, transport, and reconfiguration of granular materials. [ABSTRACT FROM AUTHOR]

Auxetics are materials, metamaterials or structures which expand laterally in at least one cross-sectional plane when uniaxially stretched, that is, have a negative Poisson's ratio. Over these last decades, these systems have been studied through various methods, including simulations through finite elements analysis (FEA). This simulation tool is playing an increasingly significant role in the study of materials and structures as a result of the availability of more advanced and user-friendly commercially available software and higher computational power at more reachable costs. This review shows how, in the last three decades, FEA proved to be an essential key tool for studying auxetics, their properties, potential uses and applications. It focuses on the use of FEA in recent years for the design and optimisation of auxetic systems, for the simulation of how they behave when subjected to uniaxial stretching or compression, typically with a focus on identifying the deformation mechanism which leads to auxetic behaviour, and/or, for the simulation of their characteristics and behaviour under different circumstances such as impacts. [ABSTRACT FROM AUTHOR]

The subject of the work is analysis, which presents a renowned auxetic structure based on so-called rotating polygons, which has been subject to modification. This modification entails introducing pivot points on unit cell surfaces near rectangle corners. This innovative system reveals previously unexplored correlations between Poisson's ratio, the ratio of rectangle side lengths, pivot point placement, and structural opening. Formulas have been derived using geometric relationships to compute the structure's linear dimensions and Poisson's ratio. The obtained findings suggest that Poisson's ratio is intricately tied to the structure's opening degree, varying as the structure undergoes stretching. Notably, there are critical parameter limits beyond which Poisson's ratio turns positive, leading to the loss of auxetic properties. For elongated rectangles, extremely high negative Poisson's ratio values are obtained, but only for small opening angles, while with further stretching, the structure loses its auxetic properties. This observed trend is consistent across a broad category of structures comprised of rotating rectangles. [ABSTRACT FROM AUTHOR]

This study proposes a separate and combined (pyroelectric + piezoelectric) energy harvesting system using a bimetallic beam. Hybrid energy harvesting is achieved by waste thermal energy using thermal expansion mismatch of the bimetallic beam. In this regard, four types of design are proposed, along with various heating/cooling cycles. Auxetic layers in a bimetallic beam are used to enhance the power. The best‐performing design and heating/cooling cycle are considered for further power increment. Two models are simulated based on low (model C3) and high heat consumption (model C5). The load resistance, frequency, bimetallic beam and pyroelectric/piezoelectric layer thickness are varied and maximum power is obtained at 175 MΩ (C3), 75 MΩ (C5), 0.02 Hz (C3 and C5) and 0.4 mm (C3 and C5) as 20 μW (C3) and 40 μW(C5), respectively. The piezoelectric, pyroelectric and combined power variation is the effect of stress and temperature fluctuations on H‐PEH layer. Additionally, five well‐known higher pyroelectric/piezoelectric coefficient materials are studied and found that La‐NBT‐BT‐Ta gives the maximum power of 0.811 and 1.99 mW for combined (pyroelectric + piezoelectric) effect in low (C3) and high (C5) heat consumption model, respectively. The stress generated in both models is analyzed to look at practical feasibility. [ABSTRACT FROM AUTHOR]

The present study analyzes effective elastic properties of two-layered composite tubes from seven-constant tetragonal crystals under tension. Analytical dependencies of Young's modulus and Poisson's ratios on elastic properties and geometric parameters of the tube are obtained. Values of these elastic characteristics compared to the values of Voigt's averages for Young's moduli of rectilinearly-anisotropic crystals and single-layered tubes of corresponding thickness. Auxetic two-layered tubes determined. It is shown that two-layered tubes of seven-constant tetragonal crystals are characterized by linear Poynting's effect – the longitudinal strain is proportional to the torque angle. [ABSTRACT FROM AUTHOR]

Triboelectric generators are integrated into curved architected materials to realize triboelectric metamaterials that simultaneously harvest electricity from wasted mechanical energy and perform mechanical energy absorption. Novel triboelectric mechanical metamaterials (TMMs) of distance‐changing, angle‐changing, and mixed modes are designed, fabricated, and tested under a cyclic compressive load. The open‐circuit voltage and short‐circuit current of lightweight TMMs are found to be as high as 40 V and 10 nA. The introduced TMMs can effectively harvest energy under loadings from two distinctive directions. A theoretical model for predicting the energy harvesting properties of TMMs is developed, and the role of auxeticity on the energy harvesting figure‐of‐merit (FOMes) is elicited. The introduced TMMs exhibit enhanced FOMes enabled by a decrease in their negative Poisson's ratio and an increase in their resilience. The FOMes of curved architected TMMs surpasses by more than 16 times the FOMes of triboelectric materials with conventional architectures (i.e., triangular, quadrilateral, and hexagonal cell topologies). An intelligent skateboard with integrated TMMs is fabricated as a proof of concept to demonstrate motion sensing, shock‐absorbing, and energy harvesting functionalities of multimodal triboelectric metamaterials. The introduced design strategy for triboelectric metamaterials unlocks their applications in self‐powered and self‐monitoring sports equipment, smart soft robots, and large‐scale energy harvesters. [ABSTRACT FROM AUTHOR]

Production of piezo‐resistive auxetic sensors is usually carried out through mixing and coating methods. Although these methods are beneficial, Young's modulus of mixed sensors becomes high because of using a high percentage of sensing elements while the durability of coated sensors gets low due to the separation of sensing elements from the sensor surface. This article presents a new core–shell metamaterial model to address the mentioned problems. The shell and the core are produced of polydimethylsiloxane (PDMS) rubber and a mixture of PDMS/graphite powders (73.45 wt% graphite powders), respectively. A finite element model is developed via COMSOL software to predict the electromechanical behaviors of the created sensor and verified by an experimental study. Scanning electron microscope imaging is conducted to detect the separations of the graphite particles. The main important feature of this meta‐sensor is to possess a linear sensitivity due to having zero Poisson's ratio. The advantage of this method is that Young's modulus of the sensor does not decrease (unlike the mixing method), and the sensor‐coated particles do not separate from the sensor after a while (unlike the coating method). The introduced model has advantages that promote potential applications such as using sensory gloves to detect, for instance, human hand movements. [ABSTRACT FROM AUTHOR]

This study is focused on the deformation behaviour of composites formed by auxetic lattice structures acting as a matrix based on the re-entrant unit-cell geometry with a soft filler, motivated by biomedical applications. Three-dimensional models of two types of the auxetic-lattice structures were manufactured using filament deposition modelling. Numerical finite-element models were developed for computational analysis of the effect of the filler with different mechanical properties on the effective Poisson's ratio and mechanical behaviour of such composites. Tensile tests of 3D-printed auxetic samples were performed with strain measurements using digital image correlation. The use of the filler phase with various elastic moduli resulted in positive, negative, and close-to-zero effective Poisson's ratios. Two approaches for numerical measurement of the Poisson's ratio were used. The failure probability of the two-phase composites with auxetic structure depending on the filler stiffness was investigated by assessing statistical distributions of stresses in the finite-elements models. [ABSTRACT FROM AUTHOR]

This article summarizes the latest advances in manufacturing of negative Poisson's ratio materials, so-called auxetic materials, their structural mechanics and energy absorption capabilities. Diverse examples of application of these materials have been provided. Due to their excellent energy absorption capability and functional performance, these materials have generated significant interest in several applications including medical implants, automobile and sport items. It is possible to tailor their macroscopic properties by tuning the architectural features at the meso-scale, thanks to developments in 3D printing technologies. The current research is dominated by auxetic materials with periodic or ordered microstructures. The main focus of this review paper is to discuss the progress in energy absorption performance of re-entrant structures manufactured by fused deposition modeling 3D printing process. After highlighting the knowledge gaps in existing literature, this review provides an outlook on the possible future research. It is expected to gain new insights into the design and modeling of re-entrant structures for different industrial applications. [ABSTRACT FROM AUTHOR]

Some properties of metamaterials with a negative Poisson's ratio (auxetics) have been studied experimentally when punched along the normal by a rigid spherical impactor. Samples of a metamaterial with a chiral structure (hexachirals honeycomb) are made of e-PLA plastic using a 3D printer. In experiments, a deviation of the direction of movement of the impactor after leaving the punched sample from the approach direction (normal to the side surface) is observed. The dependence of the impactor projection direction on the orientation of the elements of chiral symmetry of the samples is established. A FE model for calculating the penetration of a chiral structure has been developed. Numerical results are presented and their agreement with experimental data is noted. [ABSTRACT FROM AUTHOR]