Unit Cell Configurations Research Articles

A Dual-Band Orthogonally Polarized Contour Beam Transmitarray Design

A dual-band (12.5/14.25 GHz) orthogonally polarized contour beam transmitarray has been designed to accomplish the contour beam pattern for a European coverage region in both transmit and receive frequency bands. A quad-layer transmitarray unit cell configuration is optimized theoretically for an efficient dual-band operation. An orthogonally polarized 3-dipole unit cell with 360° phase range is used to control transmission phases at both frequencies independently. A modified intersection approach with a fast Fourier transform algorithm is used to optimize the aperture phase distribution of transmitarray at each frequency independently. An antenna prototype has been fabricated and tested. The measured contour beam patterns are in good agreement with the calculated patterns in both frequency bands. The measured minimum gains in the required coverage region are 25 and 25.3 dBi at 12.5 and 14.25 GHz, respectively.

Dynamic crushing analysis of a circular honeycomb with leaf vein branched characteristic

For further improving the mechanical properties of the traditional honeycomb structure, a new excellent energy absorption structure named as the leaf vein branched honeycomb (LVBH) is proposed based on lotus leaf vein branched structure. The in-plane dynamic crushing of LVBH is numerically studied by LS-DYNA. Meanwhile, the crushing behavior of the regular circular honeycomb is carried out for comparison. The introduction of leaf vein branched characteristics affects the local deformation zone and the deformation mechanism of LVBHs can be classified into three categories: quasi-static mode, transition mode and dynamic mode. The mode classification map is further given. The comparison indicates that LVBH has better energy absorption performance with 79.94% higher specific energy absorption per unit mass (SEAm) than regular circular honeycomb (RCH). Following that, the influence of microstructure, relative density and crushing velocity on the energy absorbing characteristics of LVBH is systematically studied. The LVBHs with moderate structural parameters exhibit better mechanical properties. The quasi-static plateau stress depends upon the unit-cell configuration. Based on one-dimensional shock theory, the empirical equations of dynamic plateau stress for LVBHs with different relative densities are given by using least-square fitting. Furthermore, the numerical results show that the part of the outer-circle and branch-vein cell walls have higher energy absorption efficiency. The specific energy absorption per unit mass(SEAm)of LVBH can increase by 12.4% through the thickness matching between main-vein and the branch-vein cell walls. The results of this study will be helpful to design a novel honeycomb structure with better energy absorption ability.

Prediction of elastic moduli and ultimate strength of fiber/yarn-reinforced elastic–plastic matrix using Fourier series approach and cuboidal/wedge sub-volumes

Homogenization of mechanical properties of a heterogeneous material using analytical/semi-analytical micromechanics approaches is computationally less expensive than that through numerical techniques. However, analytical methods cannot be easily applied to a complex distribution of the microstructure in a unit cell (or a representative volume element). Here, we alleviate this by accommodating cuboidal and wedge shaped sub-volumes in the Fourier series approach (FSA). This is akin to using penta- and hexa-hedral elements to discretize the geometry in 3-dimensional finite element analysis (FEA). The technique is applied to study the elasto-plastic response of unidirectional fiber/yarn-reinforced composites with square, circular and star shaped fibers to transverse loading. It is shown that (i) predicted transverse elastic modulus and the shear moduli are sensitive to the fiber shape and the unit cell configuration, (ii) the stress–strain curves for the homogenized composite agree with those reported in the literature found by using the FEA, and (iii) the presently computed elastic constants for plain and 2/2 twill weave fabrics are close to those found by other methods and deduced from the test data. A linear softening model based on plasticity approach is implemented within the FSA to predict failure and progressive softening in the yarn and the resin. It captures the nonlinear response and provides the ultimate strength under tensile loading.

A priori determination of the elastic and acoustic responses of periodic poroelastic materials

Designed periodic foam structures have the potential for achieving predictable acoustic performance. For foam structures that have small strand thickness and made of low-modulus material, rigid-porous assumption may be inaccurate owing to strong fluid-structure coupling. The goal of this study is to design and predict the elastic and acoustic responses of periodic poroelastic structures. Three surface based and one truss based unit cell configurations are considered for study. Elastic properties of these unit-cell configurations are estimated using the homogenization approach and validated experimentally. The predicted elastic properties are used along with the predicted five microscopic Johnson-Champoux-Allard (JCA) parameters for estimating the response of periodic foam structures using the Biot-UP formulation. Parametric study reveals that the effect of structural modes on acoustic absorption coefficient increases with increase in porosity as well as with decrease in pore size. A soft porous periodic lattice sample is fabricated and its predicted absorption coefficient as well as sound-induced displacement response are experimentally validated. The approach presented here will be useful to design soft periodic foam structures for desired structural and acoustic response, and to control the effect of structural modes on acoustic response.

Quasi-static crushing behavior of novel re-entrant circular auxetic honeycombs

Honeycombs with re-entrant (RE) unit cells were found to have negative Poisson's ratios (NPRs) and enhanced energy absorption capabilities than regular hexagon honeycombs. For further improved energy absorption capacity, a novel re-entrant circular (REC) honeycomb configuration is proposed in this work by replacing the sloped cell wall of the regular re-entrant honeycomb with double circular arc cell walls, which can dissipate extra energy due to more formed plastic angles during the crushing process. The in-plane quasi-static crushing response of the REC honeycomb with large deformation was investigated theoretically and numerically. The numerical modeling methods in LS-DYNA software were validated by using the quasi-static crush test data of 3D-printed REC honeycomb specimens. Experiment and numerical simulations both revealed that quasi-static load can result in an “X” mode deformation of the REC honeycomb. Based on the simulated deformation profiles of the representative unit, theoretical models were derived to predict the REC honeycomb's crushing strength, i.e. plateau stress. Good agreements were found between the theoretical and the numerical results within a relative error about 9%. Parametric analyses further showed that the unit cell configuration has a great effect on the crushing strength of the REC honeycomb. The REC honeycomb presents a varied Poisson's ratio with the global strain of the honeycomb; smaller radius to height ratio and length to height ratio were found to yield more pronounced NPR effect. Moreover, compared with the regular RE honeycomb, the specific energy absorption of the REC honeycomb is much higher due to the introduction of the double circular arc cell walls.

Architecture design of periodic truss-lattice cells for additive manufacturing

The development of additive manufacturing (AM) technology exhibits potential for the design and manufacturing of complex lattice structures. Herein, a novel design strategy is proposed for the lattice unit cell configurations, including triangular prism (T), quadrangular prism (Q) and hexagonal prism (H), by considering the tight spatial arrangement and manufacturing constraints. Moreover, the influence of altering the degree of freedom of nodes, caused by additional struts, on mechanical performance and energy absorption capacity is systematically investigated by theoretical modeling, experimental characterization and finite element method. A series of lattice core sandwich panels is designed and manufactured by selective laser melting (SLM). X-ray micro-computed tomography (μ-CT) is carried out to obtain the realistic geometrical information. Quasi-static uniaxial compressive tests are performed to investigate the failure mechanism and mechanical performance. The results reveal that the joint connectivity of the unit cell increased with the increase of the number of the struts, resulting in superior compressive modulus and ultimate strength. The main deformation mode of cells is gradually changed from bending-dominated to stretch-dominated with the increase of the joint connectivity. The proposed design ensures the performance consistency of the manufactured struts and facilitates the theoretical predictions and analysis. Furthermore, the specific energy absorption of the structure also increased with the increase of joint connectivity. In the case of unit cells with different configurations, T series rendered superior specific strength and specific energy absorption, whereas Q series exhibited excellent specific stiffness.

Homogeneous enhancement of near-fields in all-dielectric metasurfaces with cluster-based unit cells.

To construct a dielectric analog of a spaser, we study several configurations of cluster-based unit cells for an all-dielectric metasurface characterized by resonant conditions of the trapped mode excitation. Excitation of the trapped mode is realized by performing either specific displacement of particles in the cluster or perturbation of the equidistantly spaced particles by off-centered holes. The latter approach is more advantageous for enhancement of the electric near-field with homogeneous distribution in-plane of the structure and strong field localization outside the high-refractive-index dielectric particles. This feature opens prospects for realization of subwavelength flat lasing structures based on strong near-field interaction with substances exhibiting nonlinear characteristics and properties of gain media.

Mechanical performance of topology-optimized 3D lattice materials manufactured via selective laser sintering

Topology optimization is a useful tool for designing microstructures of cellular materials, especially for 3D lattice materials. In this study, 3D lattice material of 21 types of unit cell configurations are designed by bidirectional evolutionary structural optimization method, and their mechanical performance are comprehensively compared with three typical truss lattice materials. The elastic modulus, compression strength, bulk modulus, shear modulus, anisotropy of the optimized and truss lattice materials coupled with their deformation and failure modes are characterized and compared through numerical and experimental methods. Results show, for high relative density, the optimized lattices have higher stiffness and the strength than the typical truss lattices. This work demonstrated that, with topology optimization, new optimized light-weighted lattices exhibit better mechanical performance than the traditional truss lattices. Moreover, these novel optimized lattice unit cells bring inspiration to the design of novel 3D lattice materials.

Collapse response of two-dimensional cellular solids by plasticity and cracking: application to wood

The competition between fracture and plasticity in periodic hexagonal honeycomb structures subjected to (i) intercell cracking, (ii) intrawall cracking and (iii) transwall cracking is examined, and their effect upon the macroscopic collapse response is explored using dedicated FEM analyses of unit cell configurations. These three cracking mechanisms are regularly observed in wood microstructures, and insight into their influence on the macroscopic collapse behavior is necessary for adequately designing timber structures against failure. The numerical results are presented by means of collapse contours in the hydrostatic-deviatoric stress space, illustrating the effects of wall slenderness, relative fracture (versus yield) strength, and the relative size of the plastic zone at the crack tip. Both the hydrostatic and deviatoric collapse strengths of the honeycomb strongly increase in the transition from brittle cell walls with low relative fracture strength to ductile cell walls with high relative fracture strength. This strength increase typically changes the shape of the collapse contour, and is the largest for transwall cracking, followed by intercell cracking and finally intrawall cracking. The ultimate collapse strength of the honeycomb is significantly more sensitive to the fracture strength than to the fracture toughness of the cell walls, and correctly approaches the plastic yield surface under increasing relative fracture strength. The numerical results may serve as a useful guideline in the experimental calibration of the local fracture and yield strengths of cell walls in wood.

Inverse design and deep learning for phononic crystals

In this talk, I will present some results pertaining to the inverse design of phononic crystals. First, I will consider the design of 3-D phononic crystals exhibiting large omnidirectional bandgaps within the context of topology optimization using the SIMP method. The problem will be presented in the traditional forward-inverse paradigm where the forward solver is a mixed-variational scheme implemented on multiple graphical processing units. The inverse solver is a large scale adjoint based optimization framework with highly efficient vectorized operations for sensitivity calculations. In the second part of the talk, I will consider a deep learning based surrogate model for the forward problem. The neural network architecture is based on convolutional neural networks (CNN) and is trained to predict the phononic eigenvalues of unseen unit cell configurations. I show that a CNN based model easily outperforms traditional neural network architectures such as the multi layer perceptron (MLP) on both efficiency of learning and generalization capabilities. Strong and highly efficient surrogate models such as CNN coupled with topology optimization provide a an important way forward for the inverse design of phononic crystals and metamaterials.

LTE Wide Band Metamaterial Antenna

This paper presents the effect of Metamaterials (MTMs) unit cells on the performance of LTE wide band antenna. It is a conventional rectangular patch antenna with dimension of 10 mm x 6 mm. It is implemented on FR-4 lossy substrate material with relative permittivity of εr = 4.4, thickness of 1.6 mm and loss tangent of 0.025. It is feed through a microstrip line with dimension of 22 mm x 3 mm. The overall dimension of the antenna is 35 mm x 35 mm x 1.6 mm. It operates over a frequency band from 2.1914 GHz to 2.6802 GHz with central frequency 2.4392 GHz and 4.831 GHz to 5.5642 GHz with central frequency 5.1573 GHz. The simulated gain at frequency 5.1573 is 3.834 dBi. MTMs cells are supporting the truncated ground layer as it widens the band and increases the gain. The antenna ground layer is replaced with different unit cell configurations, truncated ground with different array of patches, truncated ground with array of Ring Resonator (RR), truncated ground with different array of patches with vias, truncated ground with array of RR with vias, and truncated mushroom grounded. The antenna is supported with MTMs conducts gain 5.6 dBi. The design is implemented through CST microwave studio and measured through network analyzer.

Design of periodic foam structures for acoustic applications: Concept, parametric study and experimental validation

Reticulated foams are widely used in noise control applications. However, difficulties in controlling the foaming process makes it difficult to control the microstructure and consequently the absorption characteristics of stochastic foams. Present work is a step toward designing an appropriate microstructure of foam to obtain desired acoustics performance using the concept of periodic foams. Periodic foam structures are created from designed unit cells by uniform spatial repetition of the unit cell in the three-dimensional domain. Three different unit cell configurations are created, characterized and compared using the Johnson-Champoux and Allard (JCA) model. These three unit cell configurations are further used to create periodic foam structures and prototypes of those structures are fabricated using additive manufacturing techniques. The measured characteristic impedance, complex wave propagation constant, absorption coefficient, and transmission loss for the fabricated samples are in close agreement with the JCA model predictions. Designed periodic foam structures proposed in this study offer a novel way to tailor the absorption spectrum to specifications in noise control and other acoustics applications.

In-plane crushing response of tetra-chiral honeycombs

Tetra-chiral (TC) honeycombs are a type of auxetic materials showing negative Poisson's ratio (NPR), which are promising for impact energy absorption. This study aims to analyze the in-plane crushing response of the TC honeycombs under both quasi-static and dynamic loading conditions through numerical simulations and theoretical analyses. The numerical modeling techniques in LS-DYNA were validated using the quasi-static crush test data of TC honeycomb specimens obtained with a 3D-printer. Numerical simulations revealed the “Z” mode deformation and the “bulge effect” under quasi-static crushing, and the row-by-row “I” mode deformation under dynamic crushing of the TC honeycombs. By referring to the simulation results, theoretical models were derived based on the representative unit for predicting the TC honeycomb's crushing strength, i.e. plateau stress, under both quasi-static and dynamic loads. Good agreement was found between the theoretical and the numerical predictions with a maximum relative error of 7%. Parametric analyses showed that the unit cell configuration has a great effect on the crushing strength of TC honeycomb under in-plane crushing. The dynamic sensitivity index was defined to quantitatively evaluate the sensitivity of the TC honeycomb's crushing strength to the loading speed, and was found to depend on the unit cell geometry. The TC honeycomb was found to present a varied effective Poisson's ratio (EPR) with strain under both quasi-static and dynamic in-plane crushing; smaller radius ratio and lower crushing speed were found to yield more obvious NPR effect. Moreover, relative dimension of the TC honeycomb represented by the orthogonal array ratio do not affect its plateau stress under either quasi-static or dynamic crushing, while large orthogonal array ratio results in more obvious “bulge” and NPR effects under quasi-static crushing.

Impact energy absorption performances of ordinary and hierarchical chiral structures

Auxetic chiral structures consisting of circular ring nodes and tangentially connected ligaments are engineered systems that exhibit excellent flexibility, vibration attenuation, impact resistance performances. In this paper, the out-of-plane dynamic crash behaviors of anti-tetrachiral, hexachiral and hierarchical chiral structures are studied. The energy absorption efficiency, plateau stress, peak stress of chiral structures with different ligament length, node radius, ligament thickness and level of structural hierarchy under different external crashing conditions are compared, and identical mass per area of different unit cell configurations are assumed. It is found that anti-tetrachiral structure is able to generate higher plateau stress and better energy absorption efficiency than hexachiral structures. Making use of the mechanical benefits of structural hierarchy, novel hierarchical chiral structures are proposed for improving the crash energy absorption abilities of chiral structures, and relations between crash energy absorption performances and unit cell geometries are explored, such as: energy absorption efficiency, plateau stress of chiral structures. Based on systematical analysis, optimized chiral and hierarchical chiral cellular structure can be designed for impact energy absorption in protective sandwich structures.

Sensitivity of responses of three micro-mechanics approaches to changes in unit cell configuration and inclusion shape

We use three micromechanics approaches, namely, the method of cells (MoCs), the Fourier series approach (FSA) and the transformation field analysis (TFA) to delineate the sensitivity of their predictions to changes in the unit cell configuration. The three approaches are formulated to have identical final expressions relating the local sub-cell strains to the homogenized strains in the unit cell. The cubic-edge, the hexagonal close-packed and the cubic-diagonal unit cell configurations are employed to study the elasto-plastic response of a Boron-Aluminum fiber reinforced composite under transverse loading, and compare present results with Brockenbrough et al.’s findings who used the finite element method. The elasto-plastic response predicted from the cubic-edge unit-cell shows the most hardening while that from the cubic-diagonal unit-cell the least hardening. Of the three approaches employed, the elasto-plastic response predicted by the MoCs is closest to that of Brockenbrough et al. The effect of fiber cross section on the elasto-plastic response analyzed using only the TFA approach is found to be significant for both elastic constants and plastic hardening.

Determination of energy absorption in different cellular auxetic structures

This paper deals with the effect of unit cell configuration on the energy absorption response of different cellular auxetic structures subjected to quasi-static and dynamic loadings through the experimental and numerical methods. Among the various structures, a re-entrant structure was selected due to its fundamental properties underlying the main characteristics of an auxetic material. Computer simulation techniques using ABAQUS software validated by experimental testing were used to conduct the evaluation of such devices. Several re-entrant structures with different geometrical parameters were modeled and compared with the conventional ones. Standard compression tests were carried out on the different structures produced by the 3D printing machine to evaluate the influence of auxeticity phenomenon in the energy absorption capability. It is discovered that the auxetic structures are superior to non-auxetic structures in terms of all studied impact resistance and energy absorption indicators due to their ability to withstand quasi-static axial impact loads. The primary outcome of this research is to extract design information for the use of auxetic materials as energy absorbers where quasi-static loading is expected.

Depiction and analysis of a modified theta shaped double negative metamaterial for satellite application

Abstract In this article, a modified Theta shaped, compact double negative metamaterial structure is designed and presented for satellite communication. Two oppositely faced E-shaped resonators are connected with the substantial Theta to complete the structure. A low profile dielectric substrate FR-4 is used to design the 9 × 9 mm2 unit cell which has a succinct structure where the attainment of the resonator is explored both integrally and experimentally. The proposed metamaterial has a transmission coefficient of 13 GHz (bandwidth) with a 500 MHz band gap at the middle. A correlation is made between the basic unit-cell and array structures, and a comparison is shown among 1 × 2, 2 × 2, and 4 × 4 array structures with 1 × 2, 2 × 2, and 4 × 4 unit-cell configurations to validate the performance of the proposed metamaterial. It has also been observed by the Nicolson–Ross–Weir approach at the resonating frequencies. The effective electromagnetic parameters retrieved from the simulation of the S-parameters imply that the metamaterial structure shows negative refraction bands. The structure shows negative permittivity at 2.60 to 5.16 GHz, 6.63 to 9.31 GHz and 13.03 to 16.18 GHz and negative permeability at 7.74 to 13.07 GHz and 13.88 to 16.55 GHz, respectively. It exhibits double-negative phenomena at X and Ku bands with a frequency range of about 1.17 GHz (8.14 – 9.31 GHz) and 1.42 GHz (13.80 – 15.22 GHz), respectively. Having an auspicious design and wide range double negative characteristics, this structure can be applied to satellite communication.

Multistable kirigami for tunable architected materials

In nature, materials such as ferroelastics and multiferroics can switch their microstructure in response to external stimuli, and this reconfiguration causes a simultaneous modulation of its material properties. Rapid prototyping technologies have enabled origami and kirigami-inspired architected materials to provide a means for designing shape-shifting structures, and here we show how multistable structures inspired by kirigami provide novel design criteria for preparing mechanical metamaterials with tunable properties. By changing the geometry of kirigami unit cells, we obtain multistable kirigami lattice structures endowed with a bistable snap-through mechanism. We demonstrate the precise control of material stiffness, along with the ability to tune this property in situ by locally and reversibly switching the unit cell configurations. We anticipate these mechanical metamaterials will provide a platform to achieve in situ tunable electrical, optical, and mechanical properties for a variety of applications in multifunctional materials, two-dimensional materials, and soft robotics.

Long‐Term Operation of Perovskite‐Catalyzed Bifunctional Air Electrodes in Rechargeable Zinc‐Air Flow Batteries

AbstractRechargeable zinc‐air flow batteries are investigated as promising stationary energy storage system due to compact system design and low cost materials. Bifunctional air electrodes employing a La0.6Sr0.4Co0.2Fe0.8O3 perovskite catalyst for O2‐reduction and O2‐evolution are manufactured in small scale (4 cm2) and in up‐scaled sizes (50–55 cm2) and tested in unit cell configuration with flowing electrolyte. Stable operation of 1000 h is achieved in the small set‐up with an overall 1800 h of operation over 700 cycles at high voltage efficiencies of >50 % (air electrode) at 50 mA cm−2. The up‐scaled flow cell reaches nearly the same performance for 320 h and 130 h, respectively, proving the successful scale‐up. Slowly decreasing hydrophobicity is found to be the main reason of initially increasing but then decreasing performance. This is confirmed by electrochemical impedance spectroscopy. Although many problems are suppressed with flowing electrolyte, zinc morphology proves to be the major challenge especially in larger cells in long‐term operation of a few hundred hours.

Microfabrication of terahertz frequency-selective surface by short- and ultrashort laser ablation

This paper presents the method of fabricating a frequency-selective surface (FSS) filter for terahertz frequency by short- and ultrashort laser ablation process. The FSS consists of a capacitive screen made up of the copper metallic structure supported by a Teflon dielectric substrate. The dielectric properties of the Teflon substrate at terahertz frequencies were evaluated using THz-time-domain spectroscopy technique. The numerical simulation of the designed structure was performed using Computer Simulation Technology microwave studio software with unit cell configurations and the resonance was observed at 0.135 THz. The desired metallic microstructure was created in copper using an 8-ns solid-state Nd: YAG and 150 fs Ti:sapphire laser operating at the second harmonic wavelength of 532 and 800 nm, respectively. The proposed structure provides polarization-independent operation. The experimental verification of the fabricated FSS using femtosecond ablation process was done using THz-TDS technique, and it is observed that the measured data well match with the simulated result.