Face Sheets Research Articles

The effects of off-axis loading on the compression after impact strength of quasi-isotropic face sheet honeycomb core sandwich structure

This study presents experimental results of compression after impact (CAI) testing of aluminum honeycomb core sandwich structure with face sheets made of co-cured T1100/3960 quasi-isotropic carbon/epoxy when tested at +22.5⁰ and −22.5⁰ with respect to the 0⁰ fibers. In a previous study examining the CAI strengths of honeycomb sandwich structure, it was found that specimens had different CAI strengths, based on a [−45/90/+45/0]S layup, depending on whether they were tested in the 0⁰ direction (face sheet layup of [−45/90/+45/0]S) or 90⁰ direction (face sheet layup of [+45/0/-45/90]S). The CAI strength results showed that the specimens tested in the 90⁰ direction had a 19% drop in CAI strength compared to specimens tested in the 0⁰ direction. This was attributed to the 0⁰ load bearing plies in the 0⁰ direction specimens being “tucked in” at the center of the specimen thus providing more stability against microbuckling. This raised the question as to what CAI strength would specimens tested at +22.5⁰ (face sheet layup of [−22.5/−67.5/+67.6/+22.5]S) and −22.5⁰ (face sheet layup of [−67.5/+67.5/+22.5/-22.5]S) have compared to specimens tested in the 0⁰ and 90⁰ direction. Results presented in this study show that the specimens loaded at +22.5⁰ and −22.5⁰ have a similar average CAI strength compared to the specimens loaded in the 0⁰ direction. The specimens loaded in the 90⁰ direction exhibit 16% lower average CAI strength. Additional specimens were tested in the +45⁰ direction to put the 0⁰ load bearing fibers on the outside of the specimen to see if this would decrease the strength as has been documented for undamaged strength. These specimens have average CAI strength values between the 0⁰ direction average CAI strength values and the 90⁰ direction average CAI strength values.

Microstructural and mechanical characterisation of reinforced aluminium foam sandwich panels made by one-step squeeze casting method

ABSTRACT In just one step, a squeeze casting process was applied to produce aluminium foam sandwich panels. NaCl particles with mean sizes of 4.75 and 6.35 mm were used as space holders in different proportions. In-situ face sheet manufacturing was conducted through previously placed steel meshes in the mould. The effect of NaCl particle sizes and the proportion of each size on porosity, microstructure, and mechanical properties were investigated. Results showed a decline from 59.8% to 43.8% in the porosity amount of samples with increasing the proportion of larger NaCl particles. Accordingly, an increase in compression yield stress (from 3.2 to 6.1 kgf/mm2) and the absorbed energy were observed. Also, at the elevated densities of the sandwich panel core, the plateau stress was enhanced. The presence of Cu, Ni, Fe, and Si elements leads to the formation of brittle intermetallic compounds in the microstructure of cell walls.

Nonlinear static analysis of bi-directional functionally graded sandwich plates in thermal environments by a higher-order finite element model

This study addresses nonlinear static behaviours of bi-directional functionally graded (2D-FGM) sandwich plates lying on an elastic foundation in thermal environments for the first time. The plates comprise two 2D-FGM face sheets and a homogeneous core layer. The material properties of the face sheets change according to the power law in both the longitudinal and thickness directions. Furthermore, the material properties also possess temperature-dependent characteristics in the thermal environment. For the analysis purpose, a higher-order finite element model (FEM) based on Shi’s plate theory that takes into account the geometrically nonlinear von Kármán without regard to material nonlinearity is established. The minimum potential energy principle is used to establish the weak form of governing equations. The four-node rectangular element with the Lagrange and Hermitian interpolation functions is used to discretize the domain of the plate. The nonlinear equilibrium equations are solved by employing Newton–Raphson iterative procedure. The numerical examples comparing the obtained results with the published results in open literature have confirmed the accuracy and convergence of the proposed model. Finally, a few novel parametric studies are conducted to show the effects of the material properties, geometric parameters, temperature, elastic foundation stiffness, and boundary conditions on the nonlinear static behaviours of the 2D-FGM sandwich plates.

Multi-objective optimization of multi-core composite aluminum honeycomb sandwich panels for improved crashworthiness

This article presents the low-velocity impact response of the multi-core aluminum honeycomb sandwich panel with composite face sheets using LS-DYNA-based finite element simulation. The effect of geometric parameters like cell size, cell height, cell thickness, and facesheet thickness, on structural response, is explored. The interaction effect of these geometric parameters, effect of number of layers/cores and characteristic force-time and energy-time curves for multilayer composite sandwich panels are also studied. Finally, these geometric parameters are optimized for better crashworthiness designs, followed by crashworthiness design recommendations.

A sequential mixed-integer programming method for concurrent optimization of core topology and face sheet thickness of a sandwich beam

A method is proposed that allows for the concurrent optimization of core topology and face sheet thickness of a sandwich beam under compliance constraints. The problem is solved using a novel mixed-linear extension of the Topology Optimization of Binary Structure (TOBS) topology optimization method aiming to minimize the total mass of the beam. The method has been demonstrated on a clamped beam example and the results have been compared to results from topology optimization of the core with a range of a priori fixed face sheet thicknesses. It is shown that the new method, starting from a fully populated core, finds a minimum mass that is lower than but in the neighbourhood of the best results from the topology optimization with fixed face sheet thicknesses. By varying the compliance constraint it is shown that the core topology approaches an ideal corrugated geometry as the compliance constraint is relaxed. The trends observed in the results are compared to analytical models for an idealized core.

Enhanced Open-Hole Strength and Toughness of Sandwich Carbon-Kevlar Woven Composite Laminates.

Fiber-reinforced plastic composites are sensitive to holes, as they cut the main load-carrying member in the composite (fibers) and they induce out-of-plane stresses. In this study, we demonstrated notch sensitivity enhancement in a hybrid carbon/epoxy (CFRP) composite with a Kevlar core sandwich compared to monotonic CFRP and Kevlar composites. Open-hole tensile samples were cut using waterjet cutting at different width to diameter ratios and tested under tensile loading. We performed an open-hole tension (OHT) test to characterize the notch sensitivity of the composites via the comparison of the open-hole tensile strength and strain as well as the damage propagation (as monitored via CT scan). The results showed that hybrid laminate has lower notch sensitivity than CFRP and KFRP laminates because the strength reduction rate with hole size was lower. Moreover, this laminate showed no reduction in the failure strain by increasing the hole size up to 12 mm. At w/d = 6, the lowest drop in strength showed by the hybrid laminate was 65.4%, followed by the CFRP and KFRP laminates with 63.5% and 56.1%, respectively. For the specific strength, the hybrid laminate showed a 7% and 9% higher value as compared with CFRP and KFRP laminates, respectively. The enhancement in notch sensitivity was due to its progressive damage mode, which was initiated via delamination at the Kevlar-carbon interface, followed by matrix cracking and fiber breakage in the core layers. Finally, matrix cracking and fiber breakage occurred in the CFRP face sheet layers. The specific strength (normalized strength and strain to density) and strain were larger for the hybrid than the CFRP and KFRP laminates due to the lower density of Kevlar fibers and the progressive damage modes which delayed the final failure of the hybrid composite.

Vibration characteristics of sandwich microshells with porous functionally graded face sheets

This work, for the first time, studies the vibrations of sandwich microshells with porous functionally graded face sheets by not neglecting in-surface curvilinear motions. The governing motion equations are formulated via Hamilton principle where a curvilinear framework for the modified couple stress (MCS) scheme is employed for the length-scale parameter participation. For the curvilinear and normal displacements, the vibration modes are assumed via trigonometric functions and the pertinent natural frequencies of the oscillatory motion are determined numerically. Effects of different layer arrangements, different layer material compositions and porosity-type imperfection on the vibration behaviour of the microsystem are studied. For verification purposes, the effect of size-dependence on the normal and curvilinear motions are cancelled as well as the porosity imperfections and also the nature of being functionally graded, and then the results of this simplified model are compared to an ANSYS based finite element model. Also, other simplified versions of the thin-walled microsystem are compared to the literature. All the verifications showed very good agreement.

A strategy resisting wrinkling of sandwich structures reinforced using functionally-graded carbon nanotubes

Sandwich structures have been widely applied in the wing and the horizontal tail of the aircraft, so face sheets of such structure might occur wrinkling deformation in the process of service, which will largely decrease capability of sustaining loads. As a result, this paper aims at proposing a reasonable strategy resisting wrinkling deformation of sandwich structures. To this end, an enhanced higher-order model has been proposed for wrinkling analysis of sandwich structures. Buckling behaviors of a five-layer sandwich plate are firstly analyzed, which is utilized to assess performance of the proposed model. Subsequently, wrinkling behaviors of four sandwich plates are further investigated by utilizing present model, which have been evaluated by using quasi three-dimensional (3D) elasticity solutions, 3D Finite Element Method (3D-FEM) results and experimental datum. Finally, the present model is utilized to study the buckling and the wrinkling behaviors of sandwich plates reinforced by Carbon Nano Tubes (CNTs). In addition, influence of distribution profile of CNTs on wrinkling behaviors has been analyzed, and a typical distribution profile of CNTs has been chosen to resist wrinkling deformation. Without increase of additional weight, the present strategy can effectively resist wrinkling deformation of sandwich plates, which is rarely reported in published literature.

Nonlinear free vibration modeling of anisogrid lattice sandwich plates based on a weak formulation analysis

This research examines the geometrically nonlinear free vibrations of a type of sandwich plate. This plate consists of an anisogrid lattice core and two nanocomposite face sheets. The lattice core is made up of a set of straight and oblique ribs with rectangular cross-sections that meet at nodes that are not all in the same place (anisogrid nodes). The nanocomposite skins are reinforced with carbon nanotubes (CNTs) based on the various functionally graded models. The displacement field of the sandwich plate is estimated based on the third-order shear deformation plate theory. Also, the nonlinear von-Kármán strain–displacement relations are used. Based on the Hooke and Hamilton laws, the motion equations of the system for nonlinear free vibrations are derived in a weak form. Then, the nodal weak formulation method based on the Lagrange interpolation functions and Gauss–Lobatto–Chebyshev node distribution, for the first time, is employed to calculate the time-dependent equations. The residuals of the obtained equations are minimized by the Galerkin method. Next, a displacement control iterative technique is applied to compute the nonlinear frequency of the sandwich plate. The effect of anisogrid lattice core characteristics and nanocomposite face sheets properties on the linear and nonlinear frequencies and also mode shapes are examined in the results.

Sandwich Structures With Bio-Inspired Viscoelastic Optimized Suture Face Sheets for Blast Mitigation

In this paper, the performance of a novel bio-inspired face sheet in mitigating the effects of blast loading is investigated. The new design of the face sheet consists of an optimized bio-inspired suture structure sandwiched between two aluminium plates. The suture profile is obtained by performing structural optimization using Genetic Algorithm, wherein the dynamic responses obtained using a novel viscoelastic finite element formulation are used. The performance of this optimized suture-based face sheet is experimentally tested in a vertical shock tube which validates the results obtained using commercial finite element software Abaqus. The hybrid sandwich structure composed of an aluminium honeycomb core sandwiched between the developed face sheets is evaluated numerically for its blast resistance performance. The study shows that the proposed face sheet design, not only reduces the stresses in the face sheets significantly, but also reduces the core stresses compared to conventional aluminium face sheet–based sandwich structures. Several parametric studies are presented for axial and transverse shock loading on this hybrid sandwich structures that give more insight into their wave propagation characteristics.

Static Response of Nanocomposite Electromagnetic Sandwich Plates with Honeycomb Core via a Quasi 3-D Plate Theory

This article investigates the static analysis of functionally graded electromagnetic nanocomposite sandwich plates reinforced with graphene platelets (GPLs) under hygrothermal loads. The upper and lower layers of nanocomposite face sheets are made of piezoelectromagnetic material with randomly oriented and uniformly disseminated or functionally graded (FG) GPLs throughout the thickness of the layers, while the core layer is made of honeycomb structures. The effective Young’s modulus of the face sheets of the sandwich plate is derived with the aid of the Halpin–Tsai model. While the rule of mixtures is incorporated to compute Poisson’s ratio and electric-magnetic characteristics of the sandwich plate’s upper and lower layers. The governing equations are obtained by a refined quasi-3-D plate theory, with regard to the shear deformation as well as the thickness stretching effect, together with the principle of virtual work. Impacts of the various parameters on the displacements and stresses such as temperature, moisture, GPLs weight fraction, external electric voltage, external magnetic potential, core thickness, geometric shape parameters of plates, and GPLs distribution patterns are all illustrated in detail. From the parameterized studies, it is significant to recognize that the existence of the honeycomb core causes the plate to be more resistant to the thermal condition and the external electric voltage because of the weak electricity and thermal conductivity of the honeycomb cells. Consequently, the central deflection decreases with increasing the thickness of the honeycomb core. Moreover, with varying the external electric and magnetic potentials, the deflection behavior of the sandwich structures can be managed; raising the electric and magnetic parameters contribute to an increment and decrement in the deflection, respectively.

Reexamination for linear and nonlinear free vibration of porous sandwich cylindrical shells reinforced by graphene platelets

This paper reexamines the linear and nonlinear free vibration responses of porous sandwich cylindrical shells with graphene platelet (GPL) reinforcements surrounded by an elastic medium under temperature conditions. The graphene platelets reinforced composite (GPLRC) core is assumed to be multilayers, and each layer may have different values of porosity coefficient to achieve a piece-wise functionally graded pattern. By introducing an inhomogeneous model instead of the equivalent isotropic model (EIM), the Young’s moduli along with the shear modulus of porous GPLRC core are predicted through a generic Halpin–Tsai model in which the porosity is included. Thermo-mechanical properties of porous GPLRC core and metal face sheets are assumed to be temperature dependent. Motion equations of porous sandwich cylindrical shells reinforced by GPLs are formulated based on the Reddy’s third order shear deformation theory. In the modeling, von Kármán nonlinear strain-displacement relationships, shell-foundation interaction and thermal effect are also taken into account. The analytical solution for the nonlinear vibration problem is obtained by applying a two-step perturbation approach. Numerical studies are performed to compare the results obtained from the present model and the EIM. The results reveal that, in most cases, the difference of the fundamental frequency between the two models is over 10% and in that case the EIM is not suitable for the linear free vibration analysis of porous sandwich cylindrical shells reinforced by GPLs, whereas for the nonlinear-to-linear frequency ratio curves, the difference between the two models is less than 2% when the non-dimensional vibration amplitude reaches 1.0 and in that case the EIM may be valid in the analysis for the same shell.

Energy-based strut stress analysis of 3D lattice cores in sandwich panels

3D strut-based lattices consisting of periodic representative volume elements (RVE) have gained more importance over the last few years since they provide outstanding mechanical performance and an efficient design in lightweight engineering. In sandwich panels, strut-based lattices may replace the established honeycomb cores. In this study, an analytical model is introduced to investigate 3D strut-based lattice cores of sandwich panels. Body-centered cubic and face-centered cubic RVEs reinforced by vertical struts (BCCZ and F2CCZ) are considered as sandwich cores. Since the core stiffness is generally low in comparison with the face sheet stiffness, the derived model considers through the thickness stresses in the core caused by transverse concentrated loads, which are not obtained by the common sandwich theories. Furthermore, stress concentrations near the support points are sufficiently captured thanks to higher-order displacement representations. The analysis of the core behavior is based on the homogenization and dehomogenization of the lattice core methods. Compared with the finite element analysis, the stresses in the lattice struts are determined by the proposed model with ten times less computational effort and adequate accuracy, particularly in the highly loaded vertical struts.

Modeling and Re‐Examination of Nonlinear Vibration and Nonlinear Bending of Sandwich Plates with Porous FG‐GPLRC Core

In this article, an inhomogeneous model is proposed to predict the material properties of porous GPL‐reinforced composite (GPLRC) and to predict the mechanical responses of a sandwich plate which consists of top and bottom metal face sheets and a porous GPLRC core. The GPLRC core is assumed to be multilayers, and each layer may have different values of porosity coefficient to achieve a piece‐wise functionally graded pattern. Young's moduli along with shear modulus of porous GPLRC core are predicted by a generic Halpin‐Tsai model in which the porosity is included. Thermo‐mechanical properties of both metal face sheets and porous GPLRC core are assumed to be temperature‐dependent. The governing equations of motion for porous sandwich plates are solved by applying a two‐step perturbation approach to obtain the analytical solutions for the two cases of nonlinear vibration and nonlinear bending of porous sandwich plates. Numerical studies are performed to compare the results obtained from the present model and the equivalent isotropic model (EIM). The results reveal that, for most cases, the difference of natural frequencies between two models is over 30%, and the vibration frequency–amplitude curves and the bending load–deflection curves are underestimated by using the EIM.

Temperature- and moisture-dependent aeroelastic stability of graphene platelet reinforced nanocomposite lattice sandwich plates subjected to supersonic flow

In this work, we investigate the aero-thermo-elastic properties of graphene platelet (GPL)-reinforced nanocomposite lattice sandwich plates with pyramidal truss cores under supersonic airflow by considering the temperature- and moisture-dependent properties for the first time. GPLs in the face sheets are uniformly distributed or in graded form, and the trusses of the lattice cells are uniformly reinforced by GPLs. The effective modulus of elasticity of the GPL-reinforced nanocomposite is obtained using the Halpin–Tsai model, while the rule of mixture is used to estimate the Poisson's ratio and mass density of nanocomposite. Through the stress and deformation analysis of a pyramidal cell, the equivalent transverse shear modulus of a lattice-sandwich-core layer is determined; then, the composite lattice sandwich plate is transformed into a three-layered-sandwich structure. The kinematic relationships of the face sheets and lattice core layer are established using Kirchhoff plate theory and first-order shear deformation theory, respectively. The aerodynamic load resulting from the supersonic flow is obtained using piston theory. The equations of motion governing the flutter behaviors of simply supported sandwich plates under supersonic flow are derived with the help of a standard Lagrange process and assumed mode method. A comprehensive parameter study is conducted to explore the effects of temperature, moisture, GPL distribution pattern, and GPL weight fraction on the aero-thermo-elastic flutter characteristics of the nanocomposite lattice sandwich plates.

폭발 하중을 받는 그래핀 나노판 보강 다공 코어 샌드위치판의 과도 응답

This paper presents the transient response analysis conducted for sandwich plates subjected to blast load. These plates are made of two isotropic face sheets and a porous core composed of closed-cell foam. The core with functionally graded pore distribution through the thickness direction is uniformly reinforced using graphene nanoplatelets. Three different porosity distributions are considered, namely, two functionally graded symmetric types and a uniform type. In the analytical method of investigating the transient response of sandwich plates, Reddy’s third order shear deformation theory is employed. To verify the reliability of the presented analytical method, the transient responses are compared to those obtained using the ANSYS software. The influences of the weight fraction, geometry of the graphene nanoplatelet, porosity distribution and porosity coefficient on the transient response are examined using a few examples.

Effect of truss core on sound radiation behavior of sandwich plate structures under structure borne excitation

Compared with traditional honeycomb and stiffened sandwich structures, lattice-based structures are considered an ideal substitute for traditional core structures due to their high stiffness and other potential multifunctional application characteristics. Based on that, the sound radiation characteristics of the sandwich plate with different truss cores under structure borne excitation were investigated in the current paper, wherein three truss cores, including pyramidal, tetrahedral and 3D-kagome cores types, are considered. The governing equations for the vibration and sound radiation analysis of the sandwich plate structure are carried out using the static equilibrium method, and then solved via the Navier approach and Rayleigh integral. By comparing the experimental results, the validity of the proposed theoretical model is verified. The theoretical model is subsequently used to investigate the influences of truss core geometric parameters and types, face sheet-core face sheet thickness ratio and aspect ratio on sandwich plate sound radiation characteristics. The results indicate that, in the range and the tetrahedral and 3D-Kagome core types have the maximum natural frequency values, respectively, while the resonance peaks of sound pressure level curves move to lower frequency region with the increase in truss core radius and face sheet-core face sheet thickness ratio, and the sandwich plate with the 3D-kagome core shows better sound radiation characteristics in the wide frequency range.

Higher-order static and free vibration analysis of doubly-curved FGM sandwich shallow shells

Plenty of research papers are available on the static and free vibration analysis of single-layer FG shells, however, literature on the analysis of FGM sandwich shells of double curvature is limited. Especially, the authors have not found any paper on the analysis of hyperbolic and elliptical paraboloid FGM sandwich shells. Therefore, FGM sandwich shallow shells with double curvature are analyzed in this study for the static and free vibration conditions. Face sheets of sandwich shells are made up of functionally graded material whereas the core is made up of isotropic material. A functionally graded material considered herein is a combination of alumina and aluminum. Sandwich shallow shells on rectangular planform are modeled using various equivalent single-layer shell theories via unified formulation considering the effects of shear deformation and rotary inertia. Different shell theories recovered from the present unified formulation satisfy the transverse shear stress-free conditions on the lower and upper surfaces of the shell. Hamilton's principle is applied to the present unified formulation for establishing equations of motion. Solutions to free vibration and static problems of simply-supported sandwich shells are obtained using Navier's technique. The numerical results of frequencies, displacements, and stresses are obtained for various types of shells, different sandwich schemes, different values of the power-law factor, and radii of curvature. The results available in the literature are used for the comparison of the present results and found in good agreement with those. However, many new and useful results are also reported in this paper for the reference of readers.

Re-examination of nonlinear vibration and nonlinear bending of porous sandwich cylindrical panels reinforced by graphene platelets

Abstract This article re-examines the nonlinear vibration and nonlinear bending responses of porous sandwich cylindrical panels reinforced by graphene platelets resting on elastic foundations in thermal environments. The graphene platelet-reinforced composite (GPLRC) core is assumed to be of multilayers, and each layer may have different porosity coefficient values to achieve a piece-wise functionally graded pattern. By introducing an inhomogeneous model instead of the equivalent isotropic model (EIM), the Young’s moduli along with the shear modulus of the porous GPLRC core are predicted through a generic Halpin–Tsai model in which the porosity is included. The thermomechanical properties of metal face sheets and the porous GPLRC core are assumed to be temperature-dependent. Governing equations of motion for sandwich cylindrical panels with porous GPLRC core are formulated based on Reddy’s third-order shear deformation theory coupled with von Kármán nonlinear strain–displacement relationships. In the modeling, the panel–foundation interaction and the thermal effects are also considered. The analytical solutions for the nonlinear vibration and nonlinear bending problems are obtained by applying a two-step perturbation approach. Numerical studies are performed to compare the results obtained from the present model and the EIM. The results confirm that the EIM is not suitable for linear free vibration analysis of sandwich cylindrical panels with the porous GPLRC core, but the EIM may be valid for the cases of nonlinear vibration and nonlinear bending analyses of the same panel resting on Pasternak elastic foundations.

Reconfigurable metatiles with circular maze-like space-coiling-based acoustic metastructure for low-to-mid frequency sound attenuation

This study presents a compact, lightweight, and reconfigurable acoustic metatile for sound mitigation applications. The metatile prototype is designed based on a circular maze-like acoustic metastructure, which utilizes a space-coiling technique for enhanced acoustic performance in low-to-mid frequencies. The proposed labyrinthine acoustic metadisk structure comprises a central hollow front face sheet and two coiling-up backing cavities. Experimental results show that the metadisk has high absorption peaks of 0.81 and 0.75 at 574 and 1436 Hz, respectively, and exhibits high sound transmission loss (STL) values ( ≥25 dB) in separate wide frequency bands between 100–580 Hz and 820–1600 Hz. Based on the metadisk samples, metatiles are constructed, which are 30.5 × 30.5 cm 2 in size. The acoustic performance of these metatiles is investigated in two distinct scenarios. Two prototypes of the acoustic metatile are presented: the metacage and the metapanel. The metacage is a cubical box-shaped structure constructed using five metatiles, which showed a wideband insertion loss of ≥10 dB in 200–16 000 Hz and ≥20 dB over 400–5000 Hz. On the other hand, the metapanel is constructed using nine metatiles, which exhibited STL values of >20 dB over 125–5000 Hz and had a sound transmission class rating of 34. The study highlights the potential of circular maze-like space-coiling-based acoustic disk metastructures to be reconfigured into metatiles and assembled into a metacage or metapanel for practical sound mitigation applications.