Unit Cell Configurations Research Articles

Lateral crushing behavior of tubular lattice structures with triply periodic minimal surface architectures

Tubular lattice structures had attracted significant attention due to their remarkable mechanical performance and lightweight, but there had been few research on their lateral crushing behaviors. In the present work, a novel tubular lattice structure with triply periodic minimal surface (TPMS), named TPMS-T, was manufactured to examine its energy absorption characteristics under lateral crushing through experiments and finite element (FE) simulations, with three types of unit-cell configurations considered for comparison. The numerical predictions of deformation modes and energy absorption responses were in good agreement with the corresponding experimental results. The energy absorption characteristics of three types of TPMS-T were further compared against the single circular tube (SCT) and typical nested tube systems with the same mass and outer size. It was found that TPMS-T exhibited superior crashworthiness performance than the corresponding SCT and typical nested tube system counterparts. The specific energy absorption (SEA) of three types of TPMS-T exceeded that of corresponding SCT by 70% to 133%, and fourfold-tube nested system (FT) by 24% to 61%, respectively. The influence of the relative density, tube thickness and tube outer diameter on the crushing behaviors of TPMS-T was numerically investigated to further understand the crushing mechanism of TPMS-T under lateral crushing. Results showed that the energy absorption performance exhibited an increase trend with increasing the relative density, and crashworthiness indicators could be expressed as a power function of the relative density. Moreover, the tube thickness and tube outer diameter had a large influence on the energy absorption capacity of TPMS-T. Finally, the multi-objective optimization design was further performed to obtain optimized TPMS-T configurations subjected to lateral crushing. The present work was expected to provide researchers and engineers with insights into the design of TPMS-T for structural crashworthiness applications.

Ab initio study of magneto-ionic mechanisms in ferromagnet/oxide multilayers

The application of gate voltages in heavy metal/ferromagnet/Oxide multilayer stacks has been identified as one possible candidate to manipulate their anisotropy at will. However, this method has proven to show a wide variety of behaviours in terms of reversibility, depending on the nature of the metal/oxide interface and its degree of oxidation. In order to shed light on the microscopic mechanism governing the complex magneto-ionic behaviour in $\text{Ta/CoFeB/}\text{HfO}_2$, we perform ab-initio simulations on various setups comprising $\text{Fe/O, Fe/HfO}_2$ interfaces with different oxygen atom interfacial geometries. After the determination of the more stable interfacial configurations, we calculate the magnetic anisotropy energy on the different unit cell configurations and formulate a possible mechanism that well describes the recent experimental observations in $\text{Ta/CoFeB/}\text{HfO}_2$.

Geometric parameterization of unit cell configurations for reduced basis approximation

To facilitate rapid yet accurate multiscale homogenization, we formulate geometric mapping schemes that effectively handles the fiber radius changes of a unit cell to dispense with mesh regeneration. To further expedite unit cell analysis involving fiber radius variation, we integrate the formulated geometric transformations into reduced basis analysis using an empirical interpolation method. For demonstration, we examine the proposed geometric parameterization strategies with the linear elastic analyses of square and hexagonal unit cells under tension and shear loads, respectively. Overall, reduced basis analyses combined with the devised geometric mappings are found more efficient than the corresponding finite element analyses while producing insignificant errors.

A novel approach on design of a fractal inspired logarithmic spiral antenna-based absorber for polarization and incident angle insensitive broadband electromagnetic energy harvesting

Design analysis employing numerical analysis has become one of the most significant components in realizing broadband spiral absorbers in the infrared (IR) region. To date, there have been very few research studies on broadband electromagnetic (EM) wave absorptions by such absorbers in that frequency band. Therefore, in this research, we have proposed a new type of absorber based on gradient log-spiral to enhance the absorption bandwidth in the IR region. The absorbers are designed by considering arrangements of different log-spiral arms in a fractal-like pattern. Investigation was carried out in both unit cell and array configuration. Strong average absorption reaching 88.5% in the unit cell and 83.6% in the array configuration was observed over the bandwidth from 4.5 to 100 THz for a wide range of incident angles. The absorption of the unit cell absorber remains almost stable up to 45° for transverse electric and 75° for transverse magnetic mode polarizations. This unique approach opens a new direction of imaging, EM stealth, camouflaging, and wireless EM energy harvesting applications.

Investigation of AFGSM Methods for Some Electromagnetic Bandgap Structures: From Analysis to Applied Antenna Design

Electromagnetic Bandgap (EBG) structures that exhibit various performances, such as preventing the electromagnetic wave propagation and reflecting an incident wave within the stopband, have unique electromagnetic characterization. Due to this reason, accurate analysis and design of the EBG structures are crucial to enhance the integrated system performance. This paper concentrates on the characterization of some planar EBGs using the Auxiliary Functions of Generalized Scattering Matrix (AFGSM) methods, with particular importance on an in-depth consideration of its bandgaps. The AFGSM method is applied to the planar EBG structures in the literature for the first time. The well-known kinds (symmetric and asymmetric cases) of mushroom type and multilayer EBG structures are considered to verify the presented method. Analysis results are compared with the Conventional Eigenvalue Equation (C-EIV) and the Generalized Scattering Matrix based Eigenvalue Equation (GSM-EIV) methods. Low computation load and accurate results are obtained to analyze the planar EBG structures with the AFGSM method due to using transmission line model. In addition, a design methodology is proposed for a chosen planar EBG structure using the AFGSM methods. Geometrical parameters of interested EBG problems are determined for acquiring the stopband frequency region of interest using the scattering parameters of unit cell configuration. The mushroom EBG model along one and two-dimensional axes is used in an antenna application to decrease mutual coupling between antenna elements. Three different scenarios are simulated in the HFSS electromagnetic simulation design environment to understand the effect of mutual coupling reduction in the antenna problem of the designed EBG structure via the AFGSM method. All designed antennas are manufactured, and the measurement results are in good agreement with the simulation results. The measurement results of the fabricated antenna application example including designed EBG using the proposed AFGSM method is compared with the existing similar problems with the same and different EBG models. It has been demonstrated that bandgap analysis, design of the planar EBG structures and integration of considered EBG model to a design application can be accurately and quickly achieved with the given methodology using the AFGSM method.

A Polarization-Insensitive Dual-Band FSS With High Selectivity and Independently Switchable Characteristics

The investigation of dual-band frequency selective surface (FSS) has attracted a great deal of attention over the past few decades. However, the existing reconfigurable dual-band FSSs exhibit first-order responses with poor frequency selectivity. In this letter, a polarization-insensitive dual-band FSS with a rotationally symmetrical receiver–transmitter unit-cell configuration that exhibits a second-order response is presented. p-i-n diodes are loaded on separate layers to achieve the independent switching of two passbands. The full-wave simulation results are presented, and an equivalent circuits model is also presented to analyze the operation principle of the proposed FSS. The measurement results of our fabricated prototype indicate that the device exhibits two independently switchable polarization-insensitive transmission bands centered at 3.06 and 4.33 GHz. The shape factors representing the selectivity of the transmission band are 2.44 and 2.46, respectively, which are approximately 1/4–1/2 times that of the existing reconfigurable dual-band FSSs. The proposed FSS exhibits angular stability up to 40 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{\circ }$</tex-math></inline-formula> below 5 GHz. This work paves the way toward high selectivity reconfigurable dual-band FSS.

Ab initio structural optimization at finite temperatures based on anharmonic phonon theory: Application to the structural phase transitions of BaTiO3

We formulate a first-principle scheme for structural optimization at finite temperature ($T$) based on the self-consistent phonon (SCP) theory, which accurately takes into account the effect of strong phonon anharmonicity. The $T$ dependence of the shape of the unit cell and internal atomic configuration is determined by minimizing the variational free energy in the SCP theory. At each optimization step, the interatomic force constants in the new structure are calculated without running additional electronic structure calculations, which makes the method dramatically efficient. We demonstrate that the thermal expansion of silicon and the three-step structural phase transitions in ${\mathrm{BaTiO}}_{3}$ and its pressure-temperature ($p\text{\ensuremath{-}}T$) phase diagram are successfully reproduced. The present formalism will open the way to the nonempirical prediction of physical properties at finite $T$ of materials having a complex structural phase diagram.

A Model for Sound Propagation Through Corrugated Porous Structures With Mean Flow

Abstract For applications of porous materials where mean pressure drop is a concern, packaging the material into a corrugated structure is better compared to other geometries such as block or wedge shapes. The goal of this study is to integrate noise reduction functionality within that material, which requires an understanding of sound propagation through corrugated porous structures, including flow effects. The corrugated porous structure involves porous partitions separating inlet and outlet fluid channels. The porous materials considered are periodic octet-truss and body-centered cubic unit cells, and sound propagation across these porous partitions is modeled using the Johnson–Champoux–Allard model. The predicted transmission loss (TL) is benchmarked using designed additively manufactured corrugated structures measured using a flow duct. The laminar flow regime is maintained across the porous structure to reduce flow-noise effects. It is shown that the TL for a given corrugated structure increases with a decrease in porosity, and the impact of flow becomes significant as the porosity decreases. The influence of flow on TL also depends on the unit cell configuration. Furthermore, the model provides insights into pressure and acoustic particle velocity distributions within the corrugated structure and reveals regions of the porous material that effectively participate in noise reduction.

Nonvolatile memory cell using a superconducting-ferromagnetic π Josephson junction

Storage of a single magnetic flux quantum in a superconducting loop containing a Josephson junction represents a promising unit cell configuration for construction of a cryogenic memory of superconducting digital circuits. However, application of a DC bias current is required for operation of such a memory cell to maintain trapping of the flux quantum in the storage loop. In this work, we present a superconducting memory cell that uses a superconducting-magnetic π junction. The cell characteristics show flux quantum hysteresis centering at the zero-bias current. We develop a fabrication process that combines superconductor–ferromagnet–superconductor (SFS) junctions with superconductor–normal metal–superconductor (SNS) junctions. The critical current density of the SFS junctions shows a 0–π oscillation as a function of the ferromagnetic layer thickness. The formation of the π junction is confirmed further by the flux modulation curves of a superconducting quantum interference device made from SNS junctions with an additional SFS junction.

Tailored twist morphing achieved using graded bend–twist metamaterials

This work develops a morphing concept that utilises a metamaterial as the passive morphing device for helicopter blades. The metamaterials are created with bend–twist coupling, which enable the blade twist under prescribed bending loads. Finite element analysis (FEA) is performed to investigate the influence of the unit cell configurations on the coupling properties of the metamaterials. The numerical models are then validated experimentally through a set of bending tests conducted with additively manufactured prototypes. Finally, the validated model is used to design a graded metamaterial, where the cell aspect ratio gradually changes along the blade span, providing unique bend–twist coupling and allowing for tailored twist to be obtained. The results suggest the graded metamaterials are capable of introducing optimised nonlinear twists to the blade during different flight conditions including both hover and forward flight.

Design and Manufacturing of a Metal-Based Mechanical Metamaterial with Tunable Damping Properties.

In the present work, a novel concept for metallic metamaterials is presented, motivated by the creation of next-generation reversible damping systems that can be exposed to various environmental conditions. For this purpose, a unit cell is designed that consists of a parallel arrangement of a spring and snap-fit mechanism. The combination of the two concepts enables damping properties one order of magnitude higher than those of the constituting metal material. The spring element stores elastic energy while the snap-fit allows to absorb and dissipate energy and to reach a second stable state. Different configurations of single unit cells and connected cell assemblies are manufactured by laser powder bed fusion using Ti6Al4V powder. The dimensioning is supported by finite element modelling and the characteristic properties of the unit cells are studied in cyclic compression experiments. The metamaterial exhibits damping properties in the range of polymeric foams while retaining its higher environmental resistance. By variation of selected geometrical parameters, either bistable or self-recovering characteristics are achieved. Therefore, a metamaterial as an assembly of the described unit cells could offer a high potential as a structural element in future damping or energy storage systems operating at elevated temperatures and extreme environmental conditions.

Elastic Metagratings with Simultaneous Highly Efficient Control over Longitudinal and Transverse Waves for Multiple Functionalities

Elastic metagratings that can efficiently manipulate the propagation of bulk longitudinal and transverse waves are highly desired in various applications, such as geological exploration and nondestructive detection. Here, a class of elastic metagratings consisting of a one-dimensional array of elliptical hollow cylinders carved in a steel background is proposed and studied. With the help of elastic-wave-diffraction analysis and a genetic-algorithm-based optimization approach, these intelligently designed metagratings can reflect longitudinal and transverse waves in a highly efficient and spatially separated way. Various wave-front-manipulation functionalities, such as specular reflection for longitudinal waves and anomalous reflection for transverse waves, can be simultaneously achieved with an almost perfect efficiency. Interestingly, different functionalities over both longitudinal and transverse waves can be realized by using the same cylinders with different rotation angles. Due to the nonresonant characteristics, the exhibited functionality shows robustness over both working frequencies and unit-cell configurations, which are beneficial for practical applications.

Properties of AgBiI4 using high through-put DFT and machine learning methods

Silver iodo-bismuthates show promise for optoelectronic and other applications. Within this family of materials, AgBiI4 is a prominent model compound. The complexity of AgBiI4 has prevented a conclusive determination of specific atomic arrangements of metal atoms in the bulk material. Here, we employ high through-put density functional and novel machine learning methods to determine physically relevant unit cell configurations. We also calculate the fundamental properties of the bulk material using newly discovered configurations. Our results for the lattice constant (12.7 Å) and bandgap (1.8 eV) agree with the previous theory and experiment. We report new predictions for the bulk modulus (7.5 GPa) and the temperature-dependent conductivity mass for electrons (m0 at T = 300 K) and holes (7m0 at T = 300 K); these masses will be useful in AgBiI4-based device simulations.

Tuning fork-hammer shaped perfect metamaterial absorber for C-band applications

The new compact octagonal split ring resonator with octagonal close ring resonator-based tuning fork-hammer-shaped metamaterial is a perfect metamaterial absorber (PMA) for C-band application. It is a new arrangement of two octagonal ring resonators with tuning fork-hammer shaped metal strips that are fabricated using FR-4 dielectric material. The numerical results obtained from the Computer Simulation Technology (CST) microwave studio exhibited dual resonance peaks at 4.34 GHz and 5.35 GHz with the absorption of 99.98% and 98.31%, respectively for transverse electric (TE) mode. The obtained numerical results were also authenticated using the high-frequency structure simulator (HFSS) simulator, an equivalent circuit model which shows a very insignificant amount of variation. Agilent N5227 Vector Network Analyzer was also used for measurement in this study. A slight variation at all peak points was observed when the simulation and measurement data were compared. Absorption individualities were also analysed using different polarisation angles and incident angles for TE mode. The projected absorber demonstrated near unity absorptions up to 30° polarisation angle and 60° wide incident angle. The proposed design was optimised using different types of parametric studies such as unit cell configurations, split gap, and width of the octagonal ring resonator. This C-band absorber is frequently used in satellite communication band, defence, security, and stealth coating equipment.

Terahertz Spectroscopy Study of the Stereoisomers of Threonine

The terahertz (THz) band contains a wealth of information about vibration and rotational energy levels, most of the vibration modes between amino acid molecules are in the THz band, so it reflects many unique absorption characteristics in the THz band. The use of terahertz time-domain spectroscopy can not only effectively identify different kinds of amino acids but also distinguish various isomers of the same amino acid due to the varied vibration modes. The absorption spectra of four stereoisomers of threonine were investigated by terahertz time-domain spectroscopy (THz-TDS) and Fourier transform infrared spectroscopy. The results show that the isomers show similarity in the infrared band, while manifest evidently the similarity between enantiomers L-threonine and D-threonine, and between L-allo-threonine and D-allo-threonine, and the difference between diastereoisomer L-threonine/D-threonine and L-allo-threonine/D-allo-threonine in the terahertz band. In order to fully understand the origin of the terahertz absorption characteristics of isomer molecules, simulation calculations were carried out in combination with density functional theory to connect the vibrational modes and molecular structures, Furthermore, the unit cell configurations of L-thr and L-allo-thr and the differences between various low-frequency vibrational modes are analyzed from the perspective of hydrogen bond configuration. By further extracting terahertz optical parameters such as refractive index and imaginary part of the dielectric constant of threonine isomers, the results show that the refractive index spectrum and dielectric loss spectrum can clearly show the response characteristics of the orientation polarization of dipole inside threonine isomers in terahertz band.

Exact wave propagation analysis of lattice structures based on the dynamic stiffness method and the Wittrick–Williams algorithm

This paper proposes two significant developments of the Wittrick–Williams (W–W) algorithm for an exact wave propagation analysis of lattice structures based on analytical dynamic stiffness (DS) model for each unit cell of the structures. Based on Bloch’s theorem, the combination of both the DS and the W–W algorithm makes the wave propagation analysis exact and efficient in contrast to existing methods such as the finite element method (FEM). Any number or order of natural frequencies can be computed within any desired accuracy from a very small-size DS matrix; and the W–W algorithm ensures that no natural frequency of the structure is missed in the computation. The proposed method is then applied to analyze the band gap characteristics and mode shapes of hexagonal honeycomb lattice structures and the results are validated and contrasted against the FE results. The effects of different primitive unit cell configurations on band diagrams and iso-frequency contours are thoroughly investigated. It is demonstrated that the proposed method gives exact eigenvalues and eigenmodes with the advantage of at least two orders of magnitude in computational efficiency over other methods. This research provides a powerful, reliable analysis and design tool for the wave propagations of lattice structures.

A multiscale topological design method of geometrically asymmetric porous sandwich structures for minimizing dynamic compliance

Compared with conventional sandwich structures with two identical face sheets, geometrically asymmetric porous sandwich structures (GAPSSs) can achieve better dynamic performances under certain load and boundary condition, since they provide more choices for geometric design. However, current studies regarding the GAPSSs are mainly analytical- and experimental-based methods with predefined face sheet thicknesses and core configurations, few works investigate the GAPSSs by optimization method. This paper presents a multiscale topology optimization method of the GAPSSs, which is capable of designing both thicknesses of two solid face sheets at macroscale and configurations of porous core at microscale for minimizing dynamic compliance. Specifically, at macroscale, a variable thickness sheet method is employed to optimize the thicknesses of two solid face sheets. Then, at microscale, a parametric level-set method integrated with a numerical homogenization approach is applied to topologically optimize the configuration of the periodic unit cell. The method of moving asymptotes is adopted to update the design variables at both scales. Several 2D and 3D numerical examples are illustrated to show the effectiveness and advantages of the proposed method for designing the GAPSSs. The results indicate that the dynamic compliance of the optimized GAPSSs show superior advantages over the conventional sandwich structures with identical face sheet thicknesses and predefined lattice core.

Bilateral Coupled Epsilon Negative Metamaterial for Dual Band Wireless Communications

This work presents a dual band epsilon negative (ENG) metamaterial with a bilateral coupled split ring resonator (SRR) for use in C and X band wireless communication systems. The traditional split-ring resonator (SRR) has been amended with this engineered structure. The proposed metamaterial unit cell is realized on the 1.6 mm thick FR-4 printed media with a dimension of 10 × 10 mm2. The resonating patch built with a square split outer ring. Two interlinked inner rings are coupled vertically to the outer ring to extend its electrical length as well as to tune the resonance frequency. Numerical simulation is performed using CST studio suite 2019 to design and performance analysis. The transmission coefficient (S21) of the proposed unit cell and array configuration exhibits two resonances at 6.7 and 10.5 GHz with wide bandwidth extended from 4.86 to 8.06 GHz and 10.1 to 11.2 GHz, respectively. Negative permittivity is noted at frequencies between 6.76–9.5 GHz and 10.5–12 GHz, consecutively, with near-zero refractive index and permeability. The optimal EMR value depicts the compactness of the proposed structure. The 1 × 2, 2 × 2 and 4 × 4 arrays are analyzed that shows similar response compared to the unit cell. Measured results of the 2 × 2 array shows the close similarity of S21 response as compared to simulation. The observed properties of the proposed unit cell ascertain its suitability for wireless communications by enhancing the gain and directivity of the antenna system.

Advanced honeycomb designs for improving mechanical properties: A review

Honeycombs are ultra-light materials with outstanding mechanical properties, which mainly originate from their unit cell configurations rather than the properties of matrix materials . Honeycombs are triggering in numerous promising applications in the fields of architecture, automotive, railway vehicle , marine, aerospace, satellite, packaging and medical implants , etc. Here we provide an in-depth overview of some important advances in basic structural design to obtain novel honeycombs with various improved mechanical properties. Firstly, the review introduces the classical honeycomb configurations. Then, a clear classification of advanced designs is established and discussed according to the design scale. The designs on the macro scale are divided into hierarchical, graded and disordered, while the designs on the meso scale are grouped as hybrid, curved ligament and reinforced strut. Moreover, the effects of different designs on the mechanical properties of honeycomb are discussed quantitatively. Finally, we summarize the important potential designs to improve the mechanical properties of honeycombs and the challenges in further research. • Introduction of classic honeycomb configurations and their mechanical properties. • Clear classification of macro-scale advanced designs, i.e. hierarchy, gradient and disorder. • Clear classification of meso-scale advanced designs, i.e. hybrid, curved ligament, and reinforcing strut. • Effects of different honeycomb designs on their mechanical properties.

Scalable, process-oriented beam lattices: generation, characterization, and compensation for open cellular structures.

Additively manufactured lattices are emerging as promising candidates for structural, thermal, chemical, and biological applications. However, achieving a satisfactory prototype or final part with this level of complexity requires synthesis of disparate knowledge from the distinctly digital and physical processing stages. This work proposes an integrated framework for processing self-supporting, open lattice structures that do not require supports and facilitate material removal in post-processing steps. We describe a minimal yet comprehensive design strategy for generating uniform lattice structures with conformal open lattice skins for an arbitrary unit cell configuration. Using continuous liquid interface production (CLIP™) on a Carbon M1, printability is evaluated for five unique bending-dominated lattice structures at unit cell length scales from 0.5 - 3.5 mm and strut diameters ranging from 0.11 - 1.05 mm. Using a cubic lattice as a basis, we further examine dimensional fidelity with respect to 2D lattice void dimensions and part position, finding differences between length scales and within parts, due to physical processing artifacts. Finally, we demonstrate a functional grading strategy based on process control methods to compensate for dimensional deviations. Using an iterative approach based on a naïve process model, deviation of the planar strut radius in a cubic lattice was decreased by approximately 85% after two iterations. These insights and strategies can be readily applied to other structures, characterization techniques, and additive manufacturing processes, thereby improving the exchange of information between digital and physical processing and lowering the energy barriers to producing high-quality lattice parts.