Thickness Gradient Research Articles

Simulation Study on Quasi-Static Compressive Mechanical Properties of Honeycomb Structures with Negative Poisson’s Ratio

Negative Poisson’s ratio structures are widely used in military, aerospace, vehicles, air-dropped packages, and medical devices due to their high strength, stiffness, energy absorption, and impact resistance, but their mechanical properties are significantly affected by geometric parameters. In this paper, for the compression energy-absorbing mechanism of the re-entrant hexagonal honeycomb structure, numerical simulations are used to study the effects of wall thickness and angular gradient on its compression failure mode and energy-absorbing performance. The results show that the wall thickness has little impact on the compression failure mode, while the angular gradient has a significant impact on the compression failure mode. The wall thickness has a significant effect on the energy-absorbing performance, and the larger the wall thickness is, the better the energy-absorbing property is; the energy-absorbing performance of the symmetric negative gradient structure is significantly better than that of the other three angular gradients, and all the four kinds of angular gradient structures have better energy-absorbing performance than that of the conventional re-entrant honeycomb. The results obtained provide a reference for improving the energy absorption performance of negative Poisson’s ratio honeycomb structures under quasi-static compression.

In-plane crashing behavior and energy absorption of graded re-entrant honeycombs reinforced by catenary

Re-entrant honeycombs have demonstrated promising applications in engineering fields attributed to better manufacturability and adaptability. Herein, the reinforced re-entrant honeycombs (RRH) with thickness and reinforced strut gradients are proposed, respectively, and their in-plane energy absorption is investigated through experiment, theoretical analysis, and simulation. Metallic specimens of uniform and graded RRH are manufactured through a step-by-step method. The deformation mode and plateau stress (PS) of these specimens are significantly influenced by both thickness and catenary gradients. Two PSs of RRH specimens are predicted theoretically with a relative error of less than 7.0 %. Subsequently, the deformation mode and energy absorption of uniform and graded RRH, subjected to varying load rates, are investigated thoroughly with a numerical method verified against the experimental data. The results show that both thickness and catenary gradients can inspire multi-step deformation mode and prevent instability deformation. Compared to uniform RRH (U-RRH), the specific energy absorptions (SEA) of thickness gradient (TG)-123 (123 represents that the thicknesses from impact end to fixed end are t1, t2, t3) and TG-213 are enhanced by 52.2 % and 49.8 %, respectively, under quasi-static compression, the SEA of TG-132 and TG-123 are raised by 41.6 % and 20.2 %, respectively, under low-velocity impact, and the SEA of strut gradient (SG)-123 is 66.7 % higher than that of U-RRH under high-velocity impact. More significant auxetic deformation does not always mean better energy absorption because there is also an increased tendency of global instability and earlier densification points. This work provides a novel approach to designing high-energy absorption honeycombs with low structural complexity.

Deposition characteristics and influence regularity of sputtered products in a low-power RF gridded ion thruster

RF ion thruster has a broad prospect of development via its simple structure, high specific impulse and electrode-less discharge. During the RF operating process, an important but ignored generally issue is that the high-energy metal particles generated by sputtering grid system can diffuse towards the ionization chamber and then deposit on the wall to form a conductive film. As a result, the RF power feed can be significantly inhibited which affects the thruster perform and reliability. In this work, the axial distribution of film thickness on the ionization chamber has been measured which has the characteristic of nonlinear distribution. And the film roughness can be significantly affected due to the gradient of film thickness and deposition temperature. Another corresponding measurement of plasma parameter evolutions in the ionization chamber has been carried out within 6000 min of thruster operation. Results shows that the presence of the deposited film can result in a ∼30 % decrease in the plasma density, which weakens the grid sputtering erosion and subsequently leads to a decrease in film growth rate. The combination of contributed to the degradation of the thruster performance, with the effect of deposited film accounting for about 41 %.

Analysis of low-velocity impact responses in bidirectional gradient double-arrow auxetic metamaterials

In this paper, we propose double-arrowed auxetic metamaterials (DAMs) featuring bidirectional wall thickness gradients. Their crushing behavior under low-velocity impact is systematically investigated using a validated finite element (FE) model. The results show that the bidirectional gradient design effectively improves the transverse necking effect observed in uniform DAMs. The bidirectional gradient configuration contributes to improved energy absorption and impact strength. Specifically, there is a 60.7% increase in specific energy absorption (SEA) and a 40.5% improvement in plateau stress, compared to uniform DAMs. Furthermore, the bidirectional gradient configuration reduces the initial peak stresses generated during the dynamic crushing process and exhibits higher crash load efficiency (CLE).

Spatial distribution of soil depth in relation to slope as a consequence of erosion-accumulation processes in loess lowland hills of Slovakia

Abstract In our study, we examined the influence of slope gradient on erosion processes and present soil formation and change on loess hills. We analysed data from the two study areas and found that slope gradient is a significant factor influencing soil depth as well as humus horizon thickness. At the Báb locality, we observed a negative correlation between slope gradient and soil depth (r = –0.206, p < 0.05) and a negative correlation between slope gradient and humus horizon thickness (r = –0.227, p < 0.01). At the Nová Vieska locality, there was a negative correlation between slope gradient and soil depth (r = –0.334, p < 0.02), as well as between slope gradient and humus horizon thickness (r = –0.356, p < 0.01). These findings confirm that slope gradient is a key factor influencing soil formation in loess hills, and has a significant impact on its depth and soil profile. The analysis revealed that a critical slope of 3° significantly influences soil formation, with shallower soils and a thinner humus horizon occurring on steeper slopes. Our findings have important implications for planning erosion control measures and soil management depending on the location and slope gradient. Overall, our work provides insights into soil formation processes in loess hills and contributes to a better understanding of the interactions between slope gradient and erosive processes.

Optical constants and thickness gradients for light intensities in glass substrate layers and for complex electric fields in indium tin oxide (ITO) transparent conductor thin film layer, using Bode and Nyquist wavelength-dependent diagrams

Abstract The optical constants of a thin film layer of transparent conductive indium tin oxide (ITO), deposited on a thick glass substrate (G), were obtained for an ITO-G sample using a multilayer matrix method by fitting the regular transmittance and specular reflectance measurements to the collimated equations of the four-flux model, once the optical constants of the glass substrate layer had previously been obtained from a G sample of a single glass substrate layer. The accuracy of the results was validated using a feedback system called the three-extinction matching requirement, that is, obtaining the same extinction coefficients from the optical constants and from the forward and backward collimated differential equations of the four-flux model. To calculate the extinction coefficients of the glass from optical constants, the compression of the wavelength of the incident light in the glass substrate and in the thin ITO film layer was demonstrated, with a thickness less than the wavelength of incident light. The thickness gradients of the forward and backward collimated light intensities, on the glass substrate of the ITOG sample, were compared with those obtained for the single-layer G sample. Then, the thickness gradients for the complex forward and backward electric fields of the ITO thin film layer were obtained and plotted using spectral magnitude and phase Bode plots and new Nyquist plots, i.e. imaginary versus real parts, dependent on wavelength.

Biomimetic structural design and performance study of 3D-printed graded minimal surface bone scaffolds with enhanced bioactivity

Bone bionics and structural engineering have played a vital role in bone regeneration, with artificial scaffolds generating widespread interest. However, the mechanical properties and bone regeneration potential of biomimetic structures remain unclear. Herein, biodegradable polymer composites based on poly(butylene adipate-co-terephthalate)/poly(lactic acid) (PBAT/PLA) were 3D-printed into lattice structures as tissue engineering scaffolds. For structural design, graded diamond (D) minimal surfaces were proposed and designed to mimic the natural bone structure. The graded topologies were realized by designing gradient thickness either radially from center to edge or vertically from top to bottom. The mechanical performance of these graded samples displayed better load-carrying and energy absorption capacity than the uniform counterparts. No obvious damage was detected in the internal microstructure of the compressed samples using computed tomography. Subsequently, platelet-rich plasma (PRP), containing diverse cytokines, was loaded on the graded scaffolds. The PRP-loaded D-scaffold reported improved in vitro cell proliferation and osteoblast differentiation. Finally, femoral condyle defect repair results indicated that the PRP-loaded D-scaffold effectively promoted early-stage bone regeneration. Overall, this work provides insights into fabricating artificial scaffolds with bioactive factors and biomimetic lattice structures.

Wavelength calibration using MEMS-enabled double filter configuration for air gap sensing in the tunable Fabry–Pérot filter

We fabricate a microelectromechanical systems (MEMS)-based device configuring the tunable air gap Fabry–Pérot filter (FPF) with a static gradient thickness filter on the same platform. The proposed double filter configuration offers a wavelength calibration approach that accurately estimates the air gap dimension in the tunable air gap FPF. The wavelength calibration is performed by utilizing the spectrally-selective and spatially-resolved transmission characteristics of the tunable air gap FPF and the static gradient thickness filter, respectively. The MEMS-compatible chip-level integration of the static gradient thickness filter facilitates device miniaturization to enable its use in handheld devices.

Mechanical Behavior of 3D-Printed Thickness Gradient Honeycomb Structures.

In order to obtain a lightweight, high-strength, and customizable cellular structure to meet the needs of modern production and life, the mechanical properties of four thickness gradient honeycomb structures were studied. In this paper, four types of honeycomb structure specimens with the same porosity and different Poisson's ratios were designed and manufactured by using SLA 3D-printing technology, including the honeycomb, square honeycomb, quasi-square honeycomb, and re-entrant honeycomb structures. Based on the plane compression mechanical properties and failure mode analysis of these specimens, the thickness gradient is applied to the honeycomb structure, and four structural forms of the thickness gradient honeycomb structure are formed. The experimental results show that the thickness gradient honeycomb structure exhibits better mechanical properties than the honeycomb structure with a uniform cellular wall thickness. In the studied thickness gradient honeycomb structure, the mechanical properties of the whole structure can be significantly improved by increasing the thickness of cell walls at the upper and lower ends of the structure. The wall thickness, arrangement order, shape, and Poisson's ratio of the cell all have a significant impact on the mechanical properties of the specimens. These results provide an effective basis for the design and application of cellular structures in the future.

High‐Throughput Screening of Low‐Bandgap Organic Semiconductors for Photovoltaic Applications: In the Search of Correlations

Low‐bandgap nonfullerene acceptors (NFAs) offer a unique potential for photovoltaic (PV) applications, such as transparent PV and agrivoltaics. Evaluating each new PV system to achieve the optimum thickness, microstructure, and device performance is, however, a complex multiparametric challenge with large time and resource requirements. Herein, the PV potential of low‐bandgap donor and NFA materials by combining high‐throughput screening and statistical methods is evaluated. The use of thickness gradients (20–600 nm) facilitates the fabrication of more than 2000 doctor‐bladed devices from 24 different low‐bandgap blend combinations. The corresponding power conversion efficiencies varies significantly, from 0.06% to 10.45% across materials and thicknesses. The self‐consistency of the large dataset allows to perform a parameter sensitivity study as well as parameter correlation analysis. These reveal that the choice of materials and energy alignment‐related features (i.e., electron affinity offset, ionization energy offset, bandgap, and energy loss) has the largest influence on final device performance, while processing conditions appear less important for the final efficiencies. Our study demonstrates that high‐throughput experimentation is a perfect match for correlation analyses in order to gain a statistically meaningful understanding of these systems, potentially accelerating the discovery of new materials.

Symmetry breaking and high-order instability of telephone cord buckles triggered by in-plane gradients of thin films

Buckle delaminations are ubiquitous in natural and artificial systems including tectonic plates, biological tissues, film structures, and two-dimensional materials. Although the formation and manipulation of various buckle delaminations have been extensively investigated in the past few decades, understanding complex buckle delamination patterns is still a challenge. Here we report on the symmetry breaking and high-order instability of telephone cord (TC) buckles triggered by in-plane gradients of thin films, including strain gradient and thickness gradient. Our experimental observations reveal the unique morphological profile of the asymmetric TC buckle due to the strain gradient. Impact of in-plane strain gradient on morphological changes of TC buckles is elucidated by simulations of buckling and delamination of the thin film. Phase diagrams for five buckle delamination morphologies, i.e., symmetric TC buckle, asymmetric TC buckle, symmetric bilateral branched buckle, asymmetric bilateral branched buckle, and unilateral branched buckle with respect to average strain and strain gradient (or thickness gradient) have been numerically established. This work could promote a better understanding of complex buckle delamination patterns with symmetry breaking and high-order instability and controllable fabrication of ordered buckle delamination patterns by regulating in-plane gradients of thin films.

Effect of Carbon Fiber Paper with Thickness Gradient on Electromagnetic Shielding Performance of X-Band.

Flexible paper-based materials play a crucial role in the field of flexible electromagnetic shielding due to their thinness and controllable shape. In this study, we employed the wet paper forming technique to prepare carbon fiber paper with a thickness gradient. The electromagnetic shielding performance of the carbon fiber paper varies with the ladder-like thickness distribution. Specifically, an increase in thickness gradient leads to higher reflectance of the carbon fiber paper. Within the X-band frequency range (8.2-12.4 GHz), reflectivity decreases as electromagnetic wave frequency increases, indicating enhanced penetration of electromagnetic waves into the interior of the carbon fiber paper. This enhancement is attributed to an increased fiber content per unit area resulting from a greater thickness gradient, which further enhances reflection loss and promotes internal multiple reflections and scattering effects, leading to increased absorption loss. Notably, at a 5 mm thickness, our carbon fiber paper exhibits an impressive average overall shielding performance, reaching 63.46 dB. Moreover, it exhibits notable air permeability and mechanical properties, thereby assuming a pivotal role in the realm of flexible wearable devices in the foreseeable future.

Numerical Analysis of Temperature and Stress Field of Side Dam on the Twin‐Roll Strip Casting

Side sealing technology is crucial in the twin‐roll casting process. The performance of side dam materials directly determines twin‐roll casting (TRC) process stability and safety. An orthogonal test is conducted to study the effects of thermal conductivity, linear expansion coefficient, and elastic modulus on the maximum thermal stress of side dams. The elastic modulus and linear expansion coefficient are identified as key factors affecting the maximum thermal stress of the side dam. The fluctuation of maximum thermal stress under different elastic modulus and linear expansion coefficients is 79.7 and 75.8 MPa, respectively, whereas it is only 11.2 MPa under different thermal conductivity. A side dam with heterogeneous material along height and thickness gradient is designed. The results indicate that the maximum thermal stress in the contact area between the side dam and the molten metal decreases to 31.2 MPa, representing a reduction of more than 30 MPa compared to the homogeneous side dam with optimal performance. The results provide important guidance in improving the stability of the TRC process.

Optimized design of armored vehicle door structure with negative Poisson’s ratio core layer under blast impacts

Abstract The door is the main part of the side wall of the vehicle body and the key to the side explosion protection of the vehicle. First of all, this paper takes a certain type of armored vehicle as the research object to analyze the side blast protection performance of the equivalent composite door based on the sandwich structure with a negative Poisson’s ratio core layer. Then, the effect of the cell length, cell height, cell angle, cell thickness, and thickness gradient of the core layer on the explosion protection performance of the sandwich structure is studied. Finally, the door structure is optimally designed based on the basic structural parameters of the cells. The results show that the side blast protection of the vehicle is improved compared with the original door, and the shoulder and head injuries of the occupants under side blast impact are reduced to a great extent.

High‐Throughput Optimization of Magnetoresistance Materials Based on Lock‐In Thermography

AbstractWith the giant magnetoresistance (GMR) effect serving as a vital component in modern spintronic technologies, researchers are dedicating significant efforts to improve the performance of GMR devices through material exploration and design optimization. However, traditional GMR measurement approaches are inefficient for comprehensive material and device optimization. This study proposes a high‐throughput current‐in‐plane GMR measurement technique based on thermal imaging of Joule heating utilizing lock‐in thermography (LIT). This LIT‐based technique is advantageous for efficiently evaluating films with varying compositions and thickness gradients, which is crucial for ongoing material exploration and design optimization to enhance the GMR ratio. First, it is demonstrated that using CoFe/Cu multilayers, the simple Joule heating measurement based on LIT enables quantitative estimation of the GMR ratio. Then, to confirm the usefulness of the proposed method in high‐throughput material screening, a case study is shown to investigate the GMR of CoCu‐based granular films with a composition gradient. These techniques allow to determine the optimum composition with maximum GMR ratio using the single composition‐gradient film and reveal Co22Cu78 as the optimal composition, yielding the largest GMR ratio among the reported polycrystalline CoCu‐based granular films. This demonstration accelerates the material and structural optimization of GMR devices.

In-plane and out-of-plane compressive properties of regular and graded cellular cores of sandwich panels fabricated by additive manufacturing

Cellular materials with a gradient of properties become appealing as cores of the sandwich panels due to the possibility of improving strength and absorbed energy in lightweight components. 2D cellular structures designated by honeycombs have an anisotropic behaviour when loaded under in- and out-plane. Thus, when proposing new designs, it is essential to analyse how the in-plane arrangement with a gradient in cell wall thickness affects in-plane and out-of-plane mechanical properties. This work aims to study graded cellular structures in comparison with regular hexagonal honeycombs. Structures were manufactured by laser powder bed fusion using an aluminium alloy. Regular arrangements were formed with cells with the same thickness, while graded structures possessed a radial gradient of cell thickness. Three types of innovative gradients, where cell length varies radially along concentric layers, were analysed. The compressive properties of regular and graded structures were evaluated when loaded both under in-plane and out-of-plane conditions. Compression behaviour was assessed, both experimentally and by numerical modelling. Even though there is a mismatch between numerical and experimental results, they exhibit the same trends. All graded samples showed an increased mechanical performance when loaded under out-of-plane conditions in comparison with the results from tests under in-plane loading with values, for example, of stiffness four hundred times larger, absorbed energy around thirty times higher and with yield stress four times larger. The results showed that the graded samples attain higher values of strength, stiffness and absorbed energy in comparison with regular hexagonal honeycombs, for the same relative density.

The Air Permeability and the Porosity of Polymer Materials Based on 3D-Printed Hybrid Non-Woven Needle-Punched Fabrics.

The possibility of controlling the porosity and, as a result, the permeability of fibrous non-woven fabrics was studied. Modification of experimental samples was performed on equipment with adjustable heating and compression. It was found that the modification regimes affected the formation of the porous structure. We found that there was a relationship between the permeability coefficient and the porosity coefficient of the materials when the modification speed and temperature were varied. A model is proposed for predicting the permeability for modified material with a given porosity. As the result, a new hybrid composite material with reversible dynamic color characteristics that changed under the influence of ultraviolet and/or thermal exposure was produced. The developed technology consists of: manufacture of the non-woven needle-punched fabrics, surface structuring, material extrusion, additive manufacturing (FFF technology) and the stencil technique of ink-layer adding. In our investigation, we (a) obtained fibrous polymer materials with a porosity gradient in thickness, (b) determined the dependence of the material's porosity coefficient on the speed and temperature of the modification and (c) developed a model for calculating the porosity coefficient of the materials with specified technological parameters.

Dysphagia in Head and Neck Radiotherapy: The Influence of Pharyngeal Constrictor Anatomy and Dosimetry.

The goal of this study was to identify which anatomical and dosimetric changes correlated with late patient-reported dysphagia throughout the course of head and neck chemo-radiotherapy treatment. The patient cohort (n = 64) considered oropharyngeal and nasopharyngeal patients treated with curative intent, exhibiting no baseline dysphagia with a follow-up time greater than one year. Patients completed the MD Anderson Dysphagia Inventory during a follow-up visit. A composite score was measured ranging from 20 to 100, with a low score indicating a high symptom burden; a score ≤60 indicated patient-reported dysphagia. The pharyngeal (PCM) and cricopharyngeal constrictor muscles (CPM) were contoured on a planning CT image and adapted to weekly cone-beam CT anatomy using deformable image registration and dose was accumulated using weighted dose-volume histogram curves. The PCM and CPM were examined for volume, thickness, and dosimetric changes across treatment with the results correlated to symptom group. Anatomical evaluation indicated the PCM thickness increased more during treatment for patients with dysphagia, with base of C2 vertebrae (p = 0.04) and superior-inferior middle PCM (p = 0.01) thicknesses indicating a 1.0-1.5mm increase. The planned and delivered mean dose and DVH metrics to PCM and CPM were found to be within random error measured for the dose accumulation, indicating delivered and planned dose are equivalent. The PCM and CPM organs were found to lie approximately 5mm closer to high dose gradients in patients exhibiting dysphagia. The volume, thickness, and high dose gradient metrics may be useful metrics to identify patients at risk of late patient-reported dysphagia.

Energetics of the Transmembrane Peptide Sorting by Hydrophobic Mismatch.

Hydrophobic mismatch between a lipid membrane and embedded transmembrane peptides or proteins plays a role in their lateral localization and function. Earlier studies have resolved numerous mechanisms through which the peptides and membrane proteins adapt to mismatch, yet the energetics of lateral sorting due to hydrophobic mismatch have remained elusive due to the lack of suitable computational or experimental protocols. Here, we pioneer a molecular dynamics simulation approach to study the sorting of peptides along a membrane thickness gradient. Peptides of different lengths tilt and diffuse along the membrane to eliminate mismatch with a rate directly proportional to the magnitude of mismatch. We extract the 2-dimensional free energy profiles as a function of local thickness and peptide orientation, revealing the relative contributions of sorting and tilting, and suggesting their thermally accessible regimes. Our approach can readily be applied to study other membrane systems of biological interest where hydrophobic mismatch, or membrane thickness in general, plays a role.

Falling film hydrodynamics and heat transfer under vapor shearing from various orientations

Vapor shearing is a common issue encountered in the operations of falling film heat exchangers. The vapor stream effect depends on its orientation. This study investigates liquid film hydrodynamics and heat transfer performance under the influence of vapor streams from different orientations. The results indicate that both orientation and velocity of vapor determine the encountering time and position of the films on the tube's two sides. The liquid film thickness uniformity and the liquid column deflection vary significantly depending on the orientation and velocity of the vapor. Zones of accelerated liquid film, climbing liquid film, liquid stagnation, and transition of liquid film flow pattern are observed. The gradient of film thickness along the tube axis and the deflection in time-averaged peripheral film thickness increase as the vapor orientation varies from 0° to 90° and subsequently decrease as the vapor orientation varies from 90° to 180°. Vapor streams have more pronounced effects on time-averaged peripheral film thickness in regions close to the liquid inlet and outlet. Vapor streams result in changes in peripheral heat transfer coefficients toward the downstream side depending on the orientation and velocity of the vapor. The impact of vapor streams on the overall heat transfer coefficient does not directly correlate with the velocity of the vapor when maintaining the same orientation.