Symmetric Sandwich Structures Research Articles

Dynamic response-oriented multiscale topology optimization for geometrically asymmetric sandwich structures with graded cellular cores

Compared with conventional symmetric sandwich structures with two identical face-sheets and uniform core, geometrically asymmetric sandwich structures (GASSs) enable better dynamic performance due to the expanded design space provided by two unidentical face-sheets and graded cellular cores (GCCs). This paper proposes a dynamic response-oriented multiscale topology optimization method for the GASSs, which is capable of designing the thicknesses of two solid face-sheets, graded distribution of GCCs and their topological configurations to minimize dynamic compliance. Specifically, at macroscale, a variable thickness sheet method is employed to optimize the thicknesses of two solid face-sheets and then generate an overall free distribution of cellular cores within sandwich layers. At microscale, a parametric level set method combining with a numerical homogenization approach is adopted to progressively optimize multiple representative cellular cores (RCCs), achieving their similar topological configurations. Benefitting from the level set-based topology description of RCCs, a shape interpolation technology is conveniently applied to interpolate the shapes of these RCCs to generate the configurations of GCCs. Moreover, a Kriging metamodel constructed by some cellular cores as sample points is adopted to predict the effective properties of each cellular core within sandwich layers, significantly reducing the computational burden. Several 2D and 3D numerical examples are illustrated to show the validity of the proposed method. The results indicate that the optimized GASSs show superior dynamic performances over the conventional sandwich structures designed by microscale and macroscale topology optimization, as well as filled with the commonly applied lattice and honeycomb cores.

A refined stiffened plate element for stability analysis of the stiffened sandwich structures

The stiffened plate is one type of representative thin-walled structure consisting of plates reinforced by varying stiffeners to strengthen its resistance to buckling deformation. However, the mechanism by which stiffeners could efficiently improve the buckling resistance of such structures has not been well reported. Therefore, to investigate the mechanism and predict buckling behavior of the structures accurately, a refined Reddy-type global-local stiffened plate model (RGLS) is established firstly, which can fulfill continuous condition of in-plane displacement and transverse shear stress among the adjacent layers between the plate and stiffeners. In conjunction with the established model, a refined triangular element which can accommodate the continuous condition of transverse shear stresses at the interface between the plate and the stiffeners is constructed. Subsequently, on the basis of the unidirectional stiffened sandwich structures, the symmetric grid-stiffened sandwich structures are proposed. The buckling behavior of grid-stiffened structures is further explored by the present element. A full experimental study and a system of numerical examples have been performed to assess the reliability of the established elements. These examples demonstrate that the buckling loads could be captured by the developed element with high computational efficiency and accuracy. However, the models will significantly overrate buckling loads of the stiffened sandwich structures without considering the continuum condition of the transverse stresses at the plate-stiffener interface.

Mixed-Type Skyrmions in Symmetric Pt/Co/Pt Multilayers at Room Temperature.

We demonstrate the generation of mixed-type skyrmions (all are about 200 nm) that are primarily Bloch-type, hybrid-type, and a negligible amount of Néel-type in symmetric Pt/Co(1.55)/Pt multilayers at room temperature. The magnetic field dependence of skyrmion evolution is reversible. Brillouin light-scattering is used to quantitatively quantify the Dzyaloshinskii-Moriya interaction constant D in order to comprehend the mechanism. Interestingly, the D value is high enough to generate skyrmions in a symmetric sandwich structure. Micromagnetic simulations show that Néel-type skyrmions transform into Bloch-type skyrmions as the D value decreases. The interface-induced non-uniform D may be the cause to generate mixed-type skyrmions. This work broadens the flexibility to generate skyrmions by engineering skyrmion-based devices with nominally symmetric multilayers without the requirement of very large DMI.

Room temperature skyrmions in symmetric multilayers

We experimentally demonstrate the generation of skyrmions (166 ± 42 nm) by controlling and modulating the skyrmion stability parameter κ through changing the Co layer thickness in the multilayer with repetitions of the symmetric Pt/Co/Pt trilayer. The magnetic field dependence of skyrmion evolution is reversible. Dzyaloshinskii–Moriya interaction constant D is quantitatively measured by Brillouin spectroscopy to understand the mechanism. Surprisingly, the D value is high enough to generate skyrmions in a symmetric sandwich structure although the value is smaller than that in an antisymmetric Pt/Co/Ta trilayer. The decreased D value with the increase in the Co layer thickness indicates that the Dzyaloshinskii–Moriya interaction is still contributed from the interfaces. This work broadens the flexibility to generate skyrmions by engineering skyrmion-based devices with nominally symmetric multilayer without the requirement of very large DMI.

Microstructure and mechanical properties of gradient ultrafine-grained Mg-Gd-Zr alloy

Fabricating gradient structure (GS) is increasingly adopted in structural engineering materials due to the unique combinations of mechanical properties. In the present work, a gradient ultrafine-grained structure with a change in the grain size up to three orders of magnitude from nano to micrometer is prepared on Mg-2Gd-0.4Zr (wt.%) alloy sheet with coarse grains (CG) by sliding friction treatment (SFT). Compared with homogeneous CG structure, the yield strength of GS + CG + GS is improved from 126 MPa to 243 MPa, acquiring an increase of 93%, and maintains an excellent ductility of 25%, showing an extraordinary strength-plasticity combination among the reported GS-containing Mg alloys. Two-beam condition TEM and EBSD analyses reveal that the refined grain structure and grain orientation modification can promote the activation of multiple dislocation slip systems such as <a> and <c+a> slipping in the GS area, which improves the coordinate ability of plastic deformation. In addition, GS + CG + GS owns higher strength, ductility and better strain hardening ability than that of the GS + CG sample. This is because the symmetrical sandwich structure can alleviate the strain localization so exhibits a better work hardening effect and resistance for failure. This work sheds light on the deformation behavior of heterostructured material and provides a new strategy to improve the mechanical properties of wrought Mg alloys.

The Moisture Diffusion Equation for Moisture Absorption of Multiphase Symmetrical Sandwich Structures

When hydrophilic materials (such as natural fiber, epoxy resin or concrete) compose sandwich structures, the moisture absorption from hydrothermal environments may significantly affect their mechanical properties. Although some experimental works were carried out, few mathematical efforts have been made to describe the moisture diffusion of multiphase symmetrical sandwich structures thus far. In this paper, the moisture diffusion equation was developed to effectively predict the moisture diffusion behavior of multiphase symmetrical sandwich structures as the function of aging time. Both finite element analysis (FEA) and experimental works were carried out to validate the accuracy of the analytical method, and the analytical results show a good agreement with FEA and experimental data. The effect of the interface condition on the concentration at the interfaces was discussed; the difference between concentration and normalized concentration was illustrated; the correct interface condition, which is a continuous normalized concentration condition, was explained for the moisture diffusion behavior of sandwich structures.

Asymmetrical magnetic domain wall motion in symmetrical heavy metal/ferromagnet multilayers

In a heavy metal (HM)/ferromagnet (FM)/HM multilayer with a symmetrical sandwich structure, the absence of an interfacial Dzyaloshinskii-Moriya interaction (iDMI) is generally expected. A zero iDMI constant is usually verified by the motion of a magnetic domain wall (DW) with a velocity that is symmetrical to the longitudinal magnetic field (symmetrical DW motion). However, asymmetrical DW motion was still experimentally observed in a symmetrical HM/FM/HM multilayer, and its mechanism is unclear. In this letter, we theoretically prove that even in a perfectly symmetrical HM/FM/HM multilayer with a zero total iDMI constant, asymmetrical DW motion is still possible owing to the exchange coupling between the neighboring FM atomic layers in the FM medium. This work paves a way to unraveling unique DW motion in HM/FM multilayers from a deep perspective of the subtle magnetic interactions within a FM medium.

An investigation of the Effect of Asymmetry on the Free Vibration Behavior of Sandwich Structure

This study presents a free vibration analysis of asymmetric sandwich structures comparatively. Sandwich structures were manufactured by the hand-layup vacuum bagging method. Symmetric and asymmetric sandwich structures were analyzed experimentally to evaluate the effect of asymmetry on the free vibration characteristic of sandwich structures. Free vibration analysis was performed by using VIBXPERT II under clamped-free boundary conditions. The frequency response function (FRF) is obtained from the modal test. Modal parameters of sandwich structures were obtained from analysis by curve fitting to FRF using Matlab. The effects of asymmetry on the natural frequency of the sandwich structures are investigated and results are comparatively presented. The finite element method (FEM) was also implemented by using COMSOL Multiphysics® for verifying the selected system parameters and analyzing the experimental results. By the experimental study the accuracy of the model being having proven, it also has potential for the investigations of vibration behavior of the various applications including asymmetric sandwich structures.

Transparent oxygen barrier nanocellulose composite films with a sandwich structure

Transparent gas barrier materials have extensive applications in packaging, pharmaceutical preservation, and electronics. Herein, we designed transparent films with a symmetric sandwich structure using layer-by-layer assembly of biaxially oriented polypropylene (BOPP) and acrylic resin (AR) followed by a cellulose nanoparticle (CNP) layer. The BOPP as a substrate created a barrier to hinder the transmission of water molecules to the adhesive AR layer and gas barrier functional CNP layer. The aspect ratio of the CNPs was shown to affect the film microstructure, resulting in different values for the oxygen transmission rate (OTR). The well-organized CNP layer exhibited lower OTR when compared with the network layer. The thickness, density, and porosity of the CNP layer exhibited correlations with OTR. The water molecules were able to flow in through an additional pathway, thus increasing the water vapor transmission rate (WVTR). Moreover, these sandwiched cellulose composite films showed excellent light transmittance and tensile strength.

Computation of mode I strain energy release rate of symmetrical and asymmetrical sandwich structures using mixed finite element

The use of composite materials is on the rise in different engineering fields, the main advantage of these materials for the aerospace industry is their low weight for excellent mechanical qualities. The analysis of failure modes, such as delamination, of these materials has received great attention from researchers. This paper proposes a method to evaluate the mode I Strain Energy Release Rate (SERR) of sandwich structures. This method associated a two-dimensional mixed finite element with virtual crack extension technique for the analysis of interfacial delamination of sandwich beams. A symmetrical Double Cantilever Beam (DCB) and asymmetrical Double Cantilever Beam (UDCB) have been analyzed in this study. The comparison of the results obtained by this method and those found in the literature shows efficiency and good precision for the calculation of Strain Energy Release Rate (SERR).

Experimental investigations on the thermal insulation performance of SiC opacifier doped silica aerogel at large temperature difference

Drawn on the experience of the heat-flow-meter method, an experimental system is designed, established and validated to investigate the thermal insulation performance of SiC opacifier doped silica aerogel at large temperature difference. The test section of the experimental system is a symmetrical sandwich-structure, while the temperature of the cold surface keeps constant by circulating the water from the thermostat, NESLAB ThermoFlex2500, and the hot surface is electrically heated. Compared with the traditional method of the back-surface temperature response, the cold surface temperature is quantitatively controllable. Based on the present experimental system and method, the thermal insulation performance of insulator can be qualitatively and quantitatively evaluated by analyzing the temperature responses of the hot surface through the whole heat transfer process including the initial steady-state, the unsteady-state, and the final new steady-state. Qualitatively, the higher the temperature of the hot surface is and the faster the temperature rise of the hot surface is, the better the thermal insulation performance of the material has; quantitatively, the effective thermal conductivity of the materials is obtained based on the semi-infinite medium model and Fourier's Law. The present results show that the effective thermal conductivities of silica aerogel doped by 3.5 μm SiC opacifier with different volume fraction of 0%, 1%, and 5.84% are about 0.02559 W m−1 K−1, 0.02844 W m−1 K−1, and 0.03305 W m−1 K−1, respectively, at small temperature difference (<15 K) and low temperature (288–308 K). At large temperature difference (120–812 K) and high temperature (>400 K), the thermal insulation performance of silica aerogel doped by 3.5 μm SiC particle with a volume fraction of 1% is good enough, though 3.5 μm SiC particle with a volume fraction of 5.84% has a marginal improvement in blocking the thermal radiation within silica aerogel.

Debonding identification of Nomex honeycomb sandwich structures based on the increased vibration amplitude of debonded skin

Accurate debonding identification is essential for subsequent structural repair, however, the research on debonding identification on both sides of the symmetrical honeycomb sandwich structures by means of the vibration-based method faces certain challenges. To address this, a two-step strategy based on the uniform load surface (ULS) curvature method is presented. Specifically, the potential debonding area is first identified through the global debonding detection of the entire sandwich structure. Then, the specific debonding information on both sides is identified by introducing the local constraint to the boundary of the above potential debonding area. The corresponding mechanism is that the local constraint improves the relative vibration amplitude of debonded skin. On this basis, further study on the applicable scope of the ULS curvature method indicates that the debonding of Nomex honeycomb sandwich structures with relatively low-modulus skin and large honeycomb core phenolic resin thickness is relatively easy to be identified. This work is helpful to guide the precise debonding detection of honeycomb sandwich structures.

Mixed‐Cation MAxCs1−xPbBr3 Perovskite Single Crystals with Composition Management for High‐Sensitivity X‐Ray Detection

High‐sensitivity X‐ray detection is widely applied in medical imaging, environmental monitoring, and scientific research. Perovskite single crystals have been demonstrated to be a kind of potential material in X‐ray detection because of their large X‐ray attenuation coefficient, low crystal growth cost, and excellent photoelectric properties. In this study, a successful method to grow mixed‐cation MAxCs1−xPbBr3 single crystals with the assistance of an antisolvent and a highly sensitive X‐ray detector with symmetrical sandwich structure is reported. The analysis of absorption spectra of these crystals indicates the bandgap is tunable from 2.25 to 2.16 eV with increasing methylammonium (MA) compositions. Through the space‐charge‐limited current measurement, these mixed‐cation crystals achieve significantly improved electronic properties involving trap density, mobility, and conductivity, compared with the MAPbBr3 and CsPbBr3 crystals. The optimized electronic properties enable a high‐sensitivity X‐ray detector and a systematical analysis of crystal composition and detection performance. The X‐ray detectors based on mixed‐cation crystals exhibit a higher sensitivity and lower detection limit. The MA0.6Cs0.4PbBr3‐based detector achieves a sensitivity of up to 2017 μC Gyair−1 cm−2 and a detection limit of 1.2 μGyair s−1 under an applied voltage of 1 V, which is much better than our prepared MAPbBr3‐ and CsPbBr3‐based detectors.

Electrically Tunable Liquid-Crystal–Polymer Composite Laser with Symmetric Sandwich Structure

This study investigates and develops for the first time an electrically tunable liquid-crystal–polymer composite laser with a symmetric sandwich structure. This structure includes two identical pol...

Dual-phase-lagging heat conduction and associated thermal shock fracture of sandwich composite plates

This paper studies a penny-shaped crack at the middle of a symmetric sandwich structure and is under a sudden temperature gradient on the crack faces. The temperature field of the structure and the stress intensity factor (SIF) at the crack tip are derived based on the dual-phase-lag non-Fourier heat conduction theory. Two specific factors are identified to predict the coupling effects of the thermal property differences between the skin and the core (including the differences in thermal conductivity, thermal diffusivity, heat flux lag and temperature gradient lag) on the thermoelastic behavior. If the coefficient of thermal expansion (CoTE) of the skins is greater than that of the core, the peak value of transient SIF increases. If the CoTE of the skins is smaller than that of the core, the transient SIF is scarcely influenced. The results are useful for the designs and applications of sandwich structures work under thermal shock.

Effect of hot rolling on microstructure and properties of high-chromium cast iron hardfacing cladding plate

In this study, a high-chromium cast iron (HCCI) material hardfacing on a low-carbon steel (LCS) plate is fabricated into a symmetrical sandwich structure, which is subjected to multi-pass hot rolling on a rolling mill at a reduction of 15%, 25%, and 35%. The effects of different rolling reductions on the microstructure (interface and HCCI layer) and mechanical properties (bonding strength, hardness, and wear resistance) of the composite sheets are studied. Results show that the cracks in the hardfacing layer disappear. HCCI and LCS exhibit coordinated thermal deformation, and the shape of the composite plate after hot rolling is good without warpage. HCCI/LCS composite panels are successfully prepared. As the rolling reduction increases, the microstructure defects at the original bonding interface are eliminated. Moreover, a carbide-free zone and intergrowth are observed, and the bonding strength increases. Hot rolling results in the breakdown of a eutectic carbide chain network of hardfacing HCCI cast iron layer and the dispersion distribution of carbides. The precipitation of secondary carbides is observed in the HCCI layer after hot rolling at 35%, which results in increased hardness of HCCI.

Observation of highly stable and symmetric lanthanide octa-boron inverse sandwich complexes

While boron forms a wide range of metal borides with important industrial applications, there has been relatively little attention devoted to lanthanide boride clusters. Here we report a joint photoelectron spectroscopy and quantum chemical study on two octa-boron di-lanthanide clusters, Ln2B8- (Ln = La, Pr). We found that these clusters form highly stable inverse sandwich structures, [Ln-B8-Ln]-, with strong Ln and B8 bonding via interactions between the Ln 5d orbitals and the delocalized σ and π orbitals on the B8 ring. A (d-p)δ bond, involving the 5dδ and the antibonding π orbital of the B8 ring, is observed to be important in the Ln-B8 interactions. The highly symmetric inverse sandwich structures are overwhelmingly more stable than any other isomers. Upon electron detachment, the (d-p)δ orbitals become half-filled, giving rise to a triplet ground state for neutral La2B8 In addition to the two unpaired electrons in the (d-p)δ orbitals upon electron detachment, the neutral Pr2B8 complex also contains two unpaired 4f electrons on each Pr center. The six unpaired spins in Pr2B8 are ferromagnetically coupled to give rise to a septuplet ground state. The current work suggests that highly magnetic Ln…B8…Ln inverse sandwiches or 1D Ln…B8…Ln nanowires may be designed with novel electronic and magnetic properties.

Symmetrical Sandwich-Structured SiN/Si/SiN Composite for Lithium-Ion Battery Anode with Improved Cyclability and Rate Capacity

Substoichiometric silicon nitride/silicon/substoichiometric silicon nitride (SiN/Si/SiN) composite with a symmetrical sandwich structure is investigated as the anode of thin-film lithium-ion battery. The SiN film between the Si film and electrolyte acts as a clamping layer to suppress the anode volume expansion and stabilize the formation of solid-state interphase, while the SiN film between the Si film and current collector works as a buffer layer to enhance the mechanical strength and electrical contact at the anode/current collector interface. Compared with the pure Si anode, the SiN/Si/SiN anode exhibits improved mechanical stability and keeps intact even after cycling. Consequently, the SiN/Si/SiN anode shows much better cycling stability (1702 mAh g−1 at 0.6 C with a capacity fading rate of 0.1% per cycle) and higher rate performance than the Si anode (3222 mAh g−1 with a fading rate of 0.7% per cycle). Moreover, because the SiN buffer layer can effectively alleviate the stress (caused by anode volume change) accumulated at the anode/current collector interface and also the stress compensation effect can be created by using this symmetrical sandwich structure, the SiN/Si/SiN anode presents higher mechanical stability and thus better electrochemical performance than the anode with a structure of SiN/Si on the current collector.

Optimization of energy product and reversal process for Nd2Fe14B/α″-(FeCo)16N2/Nd2Fe14B exchange-spring trilayer films

The influence of the structure on the energy product for the exchange-coupled Nd2Fe14B/α″-(FeCo)16N2 films has been investigated, and the magnetic properties of the trilayer system have been analyzed based on the micromagnetic simulation software object-oriented micromagnetic framework. The numerical results show that the energy product and the magnetic properties change with the bilayers to trilayers, where the maximum energy products of the bilayer and trilayer systems are 589 and 833.52kJ/m3, respectively. The dependence of the magnetic properties on the structure of the trilayer system was also considered. The symmetrical sandwich structure was observed to achieve a large energy product. In addition, hysteresis loops, energy product, and magnetic reversal process for the exchange-spring trilayer system were investigated. The optimal thickness of the soft phase is 7nm with the largest maximum energy product. (BH)max was observed to decrease with the increase of β, where β is the angle between the applied field and the easy axis. The trilayer system achieves the largest maximum energy product when β=0°.

Symmetrical napcore and honeycomb sandwich structures under impact load

Sandwich structures are state of the art for construction of lightweight structures with high mechanical load capacity. Beside established core materials like foams, honeycombs or wood, napcore material is an interesting option when economic production costs or the possibility to integrate extra functionality are criteria in addition to specified mechanical values. Napcore material is made from a two-dimensional resin-impregnated knitted fabric that is shaped into a threedimensional nap structure and stabilized by the cured resin matrix. In this paper the mechanical behavior under low velocity impact loads of sandwich structures made with napcores from aramide textile and phenolic resin is investigated and compared with components containing honeycomb cores.