In this study, failure modes of foam core sandwich composites are investigated by using embedded Fiber Bragg Grating sensors. Sandwich specimens with Fiber Bragg Grating sensors, embedded inside the face sheet, are manufactured using vacuum infusion process and then subjected to a static and a cyclic loading under the three-point bending mode. Different failure modes are monitored utilizing the wavelength shift and the spectrum of Fiber Bragg Grating sensors. It is shown that the responses of the Fiber Bragg Grating sensor differ depending on damage modes thereby making structural health monitoring of sandwich structures possible.
In this study, the influence of 3D fabric on the flexural behavior of cementitious composites has been investigated. Three 3D fabric samples were produced with different spacer yarn orientation angles of 65°, 55°, and 47°. The cementitious matrix was fabricated by cement and waste stone powder. After casting of all samples, flexural test was carried out on all specimens. Results showed that cementitious sample reinforced by 3D fabric with less spacer yarn orientation angle proposed the highest flexural strength among all samples (reinforced and unreinforced samples). Moreover, finite element method was used to predict the flexural behavior of textile reinforced concrete. Finite element method results showed good agreement with the experimental data. Consequently, the maximum spacer yarn stress derived by finite element analysis was used to calculate the efficiency reinforcement factor for all the textile reinforced concrete samples.
Based on classical dynamic cylindrical cavity expansion theory, a nine-step penetration and perforation process of aluminum foam sandwich targets by truncated cone-nosed projectiles are developed theoretically. In the theoretical model, the friction, shear strength, and the force for tearing the cells in the core at the periphery of the projectile are considered, and the resistance force and instantaneous velocity are achieved from this process. On this basis, the effects of the geometry of projectiles, core thickness, and impact velocities of projectiles on absorbed energy are also analyzed. Simple composite failure criteria will be applied in the fracture and perforation of the face sheet, core, and back sheet. It is shown that the diameter of projectile and core thickness have significant influence on the ballistic velocity of the projectile, which is important for the impact response and absorbed energy of the sandwich. Numerical simulation at various impact velocities is also performed, and there is a good agreement between the numerical predictions and the analytical measurements.
In order to investigate the impact resistance of the Nomex honeycomb sandwich structures skinned with thin fibre reinforced woven fabric composites, both drop-weight experimental work and meso-mechanical finite element modelling were conducted and the corresponding output was compared. Drop-weight impact tests with different impact parameters, including impact energy, impactor mass and facesheets, were carried out on Nomex honeycomb-cored sandwich structures. It was found that the impact resistance and the penetration depth of the Nomex honeycomb sandwich structures were significantly influenced by the impact energy. However, for impact energies that cause full perforation, the impact resistance is characterized with almost the same initial stiffness and peak force. The impactor mass has little influence on the impact response and the perforation force is primarily dependent on the thickness of the facesheet, which generally varies linearly with it. In the numerical simulation, a comprehensive finite element model was developed which considers all the constituent materials of the Nomex honeycomb, i.e. aramid paper, phenolic resin, and the micro-structure of the honeycomb wall. The model was validated against the corresponding experimental results and then further applied to study the effects of various impact angles on the response of the honeycomb. It was found that both the impact resistance and the perforation depth are significantly influenced by the impact angle. The former increases with the obliquity, while the latter decreases with it. The orientation of the Nomex core has little effect on the impact response, while the angle between the impact direction and the fibre direction of the facesheets has a great influence on the impact response.
The flexural behavior of the 3D integrated woven spacer composites with carbon fiber-reinforced polymer face sheets are performed with epoxy and polyester matrix in the warp direction. The static loading of foam core carbon/epoxy and carbon/polyester composite sandwich panels in three-point flexural test was characterized at room temperature (23℃) and at liquid nitrogen temperature (–40℃). Macro-fracture morphology and progress have been examined to understand the deformation and failure mechanism. Significant increases in the flexural strength with brittle type core shear failure were observed at low temperatures as compared with the corresponding room temperature behavior. The performance of the epoxy-based composite is compared to the polyester one. Significant changes in the flexural properties of the composites have been found, first related to the temperature and then to the resin type. The flexural properties of the epoxy-based composites were affected greatly by temperature and exhibited higher flexural performance than polyester-based composites at low temperatures.
A simplified three-unknown shear and normal deformations nonlocal beam theory for thermo-electro-magneto mechanical bending analysis of a nanobeam with a functionally graded material core and two functionally piezomagnetic layers is studied in this paper. The assumed structure is subjected to mechanical, thermal, electrical, and magnetic loads. An initial applied voltage and magnetic load is considered on the functionally graded piezomagnetic material layers. Eringen’s nonlocal constitutive equations are considered in the analysis. Governing equations are derived according to the present refined theory using the principle of virtual displacements. The numerical results including the deflection, electric, and magnetic potential distribution are calculated in terms of important parameters of the problem such as applied electric and magnetic potentials, two parameters of temperature distribution, and nonlocal parameter. The numerical results indicate that increase in applied electric potential increases the deflection unlike the applied magnetic potential that decreases the deflection. Furthermore, it can be concluded that increasing the nonlocal parameter leads to increase in the deflection.
In this paper, hydraulic performance and self-healing capacity of geotextile clay liners containing various amounts of nano-clay are studied. Nano-clay was employed as a substitute of a portion of bentonite. For comparison, the hydraulic performance and self-healing capacity of common geotextile clay liners (geotextile clay liners without any additives) were also experimentally studied. A novel instrument was developed to evaluate the self-healing capacity of geotextile clay liners samples. Atterberg limits and free swell index of neat and modified clayey samples were also measured. Experimental results showed that nano-clay considerably reduces the hydraulic conductivity of geotextile clay liners. It also improves the self-healing capacity of geotextile clay liners. The free swell index and liquid limits of bentonite specimens containing nano-clay were considerably higher than that of normal specimens. It can be concluded that factors affecting the free swell index of bentonite can change the self-healing capacity of geotextile clay liners. In this study, the specimen containing 15% nano-clay showed the best performance in hydraulic conductivity and self-healing capacity among all specimens. In the final, the effect of nano-clay inclusion on the permeability of bentonite is showed through an analytical model using the results of surface tension measurements.
This paper focuses for the first time on the static analysis of sandwich plates with functionally graded faces and homogeneous core by an nth-order shear deformation theory and meshless global collocation method. The meshless global collocation method approximates the solutions of governing differential equations based on the nth-order shear deformation theory using all nodes in the problem domain. The deflection and stress of a simply supported sandwich plates under sinusoidal load are calculated to verify the accuracy and efficiency of the present theory.
The behavior of a simple and innovative multi-layer sandwich panels having a polypropylene honeycomb core has been investigated carefully, theoretically and experimentally. A four-point bending test was performed to detect the mechanical characteristics of the multi-layer core. The experimental results emphasize a better rigidity of the multi-layer structure compared to the weakness displayed by the single-layer configuration. In fact, a small increase in the final weight of the component leads to a significant increase of the mechanical properties. In the second part of this study, analytical and numerical homogenization approaches were developed to compute the effective properties of the single polypropylene honeycomb core. The numerical model complies with the experimental protocol, and the simulation conducted is aiming to reproduce a typical four-point bending test on a polypropylene honeycomb multi-layer sandwich panel. Both numerical and experimental results are presented in details and a good correlation between them is highlighted.
A novel technique to improve the Mode I and Mode II interlaminar fracture toughness of woven carbon-fiber polymer matrix composite face sheets using zinc oxide nanowires is proposed. Zinc oxide nanowires are directionally synthesized on dry carbon fabrics that are used to manufacture the laminate. The influence of zinc oxide nanowires on interlaminar fracture toughness is compared against regular interfaces using double cantilever beam and end-notched flexure tests to provide fracture toughness values. A significant improvement in the Mode I and Mode II interlaminar fracture toughness values is observed with zinc oxide nanowires. Therefore, zinc oxide nanowire interlaminar reinforcement has been proven to enhance the interlaminar fracture toughness of textile composites.
In this study, nonlinear free and forced vibration analysis of an embedded functionally graded sandwich micro-beam with a moving mass is investigated. The velocity of moving mass is assumed constant. The structure is resting on nonlinear Pasternak foundation. The governing equation of motion is obtained using Hamilton's principle based on the Euler–Bernouli model with considering nonlinear terms in strain–displacement relation. Strain gradient elasticity theory is used to model the small scale effects. The micro-beam contains a homogenous core and two integrated functionally graded face-sheets. Mechanical properties except Poisson ratio are assumed to be variable based on the power-law distribution along the thickness direction. Galerkin's decomposition technique is implemented to convert nonlinear partial differential equation to a nonlinear ordinary differential equation. Multiple times scale method is applied to derive closed form approximate solution for free and forced vibration and nonlinear natural frequencies of the micro-beams. Accuracy of the obtained results using current issue may be justified by comparing with those obtained by existing results of the literature. The effect of some important parameters such as length scale parameter, power gradient index, nonlinear elastic foundation, aspect ratio, position, and velocity of moving mass and boundary conditions is studied on the various responses of the micro-beam such as nonlinear natural frequency, frequency response, and force–response curves.
The boundary layer hygrothermal stresses in the thick sandwich cylinder with laminated face are investigated. Uniform and through the thickness steady-state distribution for temperature and moisture content can be considered in the analysis. A displacement based layer-wise formulation is presented for analysis of thick sandwich composite cylinders subjected to hygrothermal loading conditions. Considering a general displacement field and employing a displacement based layer-wise theory, the governing equations of thick laminated sandwich cylinder are obtained. The displacement based formulation is derived for thick sandwich cylinder, which is subjected to non-uniform hygrothermal loading conditions. The faces of the sandwich cylinder are made of laminated composite with general layer stacking. The governing equations of the system include a set of coupled differential equations on the displacement components of the numerical surfaces. A semi-analytical solution is developed and the governing equations are solved for free edge boundary conditions. The accuracy of the numerical results is validated by the results of the finite element simulation and good agreements are seen between the predicted results. The free edge interlaminar stresses distributions are presented for thin and thick sandwich composite cylinders for uniform and non-uniform loading conditions. It is concluded that the presented layer-wise formulation is efficient and accurate method for analysis of thermal and hygroscopic stresses in thick and thin sandwich cylinders with general layer stacking.
This paper presents experimental and analytical investigations about the elastic and viscoelastic (creep) behaviour of sandwich panels made of glass-fibre reinforced polymer faces and a polyethylene terephthalate foam core, produced by vacuum infusion for civil engineering structural applications. First, the elastic response of the panels’ constituent materials (glass-fibre reinforced polymer and polyethylene terephthalate) in tension, compression and shear was experimentally assessed; shear tests on the foam were carried out using a novel test method, the diagonal tension shear test. The creep behaviour in shear of the polyethylene terephthalate foam was evaluated for different load levels. The effective flexural properties of the full-scale sandwich panels as well as their flexural behaviour up to failure were experimentally assessed. Flexural creep and subsequent recovery experiments were also conducted for different load levels, to characterise the viscoelastic behaviour of the full-scale sandwich panels. Creep deformations of the polyethylene terephthalate foam and of the sandwich panels were found to be significantly lower than those corresponding to polyurethane foam and balsa wood reported in the literature; unrecoverable viscoelastic deformations were observed in the full-scale panels. In the analytical study, the creep response of the panels was modelled using Findley’s power law and the composite creep modelling approach. The composite creep modelling predictions were reasonably accurate and allowed assessing the relative contributions of bending and shear deformations to the total sandwich panel creep deflections.
An experimental investigation on sandwich composite materials composed of glass-fiber face sheet and polyvinyl-chloride foam core has been carried out. The research demonstrates improvement in mixed-mode delamination fracture toughness values of samples under mixed-mode bending condition. The improvement is recorded with addition of a certain percentage by weight of multiwalled carbon nanotubes in comparison to conventional samples. An easy and cost-effective methodology of multiwalled carbon nanotube insertion through sonication of epoxy resin followed by mixing with hardener and vacuum resin infusion technology for manufacturing of sandwich composites has been utilized in this study. The study also identifies the optimum weight percentage of multiwalled carbon nanotube addition in the resin system for maximum performance gain in mixed-mode fracture toughness. The results of observations in this study have been supported by field emission scanning electron microscope studies as well as high-resolution transmission electron microscope analysis.
In this study, the low-velocity impact response of sandwich composites consisting of different foam core configurations is investigated experimentally. Polyvinyl chloride foam core and glass fibers were used as core material and face sheets, respectively. A number of tests under various impact energy levels for three different sandwich composite configurations were conducted with Ceast 9350 Fractovis Plus impact testing machine in order to improve the energy absorption capacity of sandwich composites panels. Absorbed energies, maximum loads and the maximum deflection of sandwich panels were obtained for each impact energy level. As the impact energy was increased, fiber fractures at face sheets, delaminations between glass-epoxy layers, core fractures, and indentations failures were observed by visual inspection. According to the obtained results, the sandwich composite with proposed new foam core design with two internal face sheets exhibits high energy absorption capacity compared to sandwich panels formed by sandwiching a polyvinyl chloride foam core between glass fabric face sheets. Maximum contact force values decrease by increase of number of core material.
Sandwich panel which has a design involving acoustic comfort is always denser and larger in size than the design involving mechanical strength. The respective short come can be solved by exploring the impact of core geometry on sound transmission characteristics of sandwich panels. In this aspect, the present work focuses on the study of influence of core geometry on sound transmission characteristics of sandwich panels which are commonly used as aircraft structures. Numerical investigation has been carried out based on a 2D model with equivalent elastic properties. The present study has found that, for a honeycomb core sandwich panel in due consideration to space constraint, better sound transmission characteristics can be achieved with lower core height. It is observed that, for a honeycomb core sandwich panel, one can select cell size as the parameter to reduce the weight with out affecting the sound transmission loss. Triangular core sandwich panel can be used for low frequency application due to its increased transmission loss. In foam core sandwich panel, it is noticed that the effect of face sheet material on sound transmission loss is significant and this can be controlled by varying the density of foam.
In the present work, for the first time, the accuracy of the Refined Zigzag Theory in reproducing the static bending response of sandwich beams is experimentally assessed. The theory is briefly reviewed and an analytical solution of the equilibrium equations is presented for the boundary and loading conditions under investigation (four-point bending). The experimental campaign is described, including the material characterization and the bending tests. The experimentally measured deflections and axial strains are compared with those provided by Refined Zigzag Theory and by the Timoshenko Beam Theory with an ad hoc shear correction factor. The Refined Zigzag Theory is shown to be more accurate than the Timoshenko Beam Theory, in particular for beams with higher face-to-core thickness and stiffness ratios and with a reduced slenderness.
Free vibration of laminated composite and soft core sandwich plates resting on Winkler–Pasternak foundations using four-variable refined plate theory are presented. The theory accounts for the hyperbolic distribution of the transverse shear strains through the plate thickness, and satisfies the zero traction boundary conditions on the surfaces of the plate without using shear correction factors. Equations of motion are derived from the dynamic version of the principle of virtual work. Navier technique is employed to obtain the closed-form solutions of antisymmetric cross-ply, angle-ply, and soft core laminates or soft core sandwich plates resting on elastic foundations. Numerical results obtained using present theory are compared with three-dimensional elasticity solutions and those computed using the first-order and the other higher-order theories. It can be concluded that the proposed theory is not only accurate, but also efficient in predicting the natural frequencies of laminated composite and soft core sandwich plates resting on Winkler–Pasternak foundations.
In this paper, we study the nonlinear dynamic response of higher order shear deformable sandwich functionally graded circular cylindrical shells with outer surface-bonded piezoelectric actuator on elastic foundations subjected to thermo-electro-mechanical and damping loads. The sigmoid functionally graded material shells are made of the metal–ceramic–metal layers with temperature-dependent material properties. The governing equations are established based on Reddy’s third-order shear deformation theory using the stress function, the Galerkin method and the fourth-order Runge–Kutta method. Numerical results are given to demonstrate the influence of geometrical parameters, material properties, imperfection, elastic foundations, and thermo-electro-mechanical and damping loads on the nonlinear dynamic response of the shells. Accuracy of the present formulation is shown by comparing the results of numerical examples with the ones available in literature.
The increasing use of sandwich composites for structural applications brings with it a need to establish a reliable inspection and monitoring method to ensure structural integrity and safe operation throughout the service life. Since optical fibre-based photonic sensing technologies are increasingly common for structural health monitoring of composite structures, selection of optical fibre Bragg grating sensors could be one possible choice for this purpose. In this paper, performance characterisation of sandwich composite with embedded silica fibre Bragg grating sensor is reported. Experimental tests were performed on a carbon fibre foam core sandwich composite embedded with a silica fibre Bragg grating sensor to extract the structural health monitoring parameters such as strain and temperature. The current study found that sandwich composite exhibits foam relaxation; however, its impact on strain measurement is negligible. Another important finding from the theoretical and the experimental thermal modelling was that although the constituent components of the sandwich composite have entirely different thermal expansion coefficients, its effect on the embedded fibre sensor can be minimal if the sensors are embedded between the face sheets. These results can initiate further research in this area and can lead to the development of state-of-the art structural health monitoring techniques for sandwich composite structures.
This study investigates the dynamic performance of the partially treated magnetorheological elastomer tapered composite sandwich plates. Various partially treated tapered magnetorheological elastomer laminated composite sandwich plate models are formulated by dropping-off the plies longitudinally in top and bottom composite face layers to yield tapered plates as the face layers. The uniform rubber and magnetorheological elastomer materials are considered as the core layer. The governing differential equations of motion of the various partially treated magnetorheological elastomer tapered composite sandwich plate configurations are derived using classical laminated plate theory and solved numerically. Further, silicon-based magnetorheological elastomer and natural rubber are being fabricated and tested to identify the various mechanical properties. The effectiveness of the developed finite element formulation is demonstrated by comparing the results obtained with experimental tests and available literature. Also, various partially treated magnetorheological elastomer tapered laminated composite sandwich plates are considered to the study the effect of location and size of magnetorheological elastomer segment on various dynamic properties under various boundary conditions. The effects of magnetic field on the variation of natural frequencies and loss factors of the various partially treated magnetorheological elastomer tapered laminated composite sandwich plate configurations are analysed at different boundary conditions. Also, the effect of taper angle of top and bottom layers, aspect ratio, ply orientations on the natural frequencies of different configurations are analysed. Further, the transverse vibration responses of three different partially treated magnetorheological elastomer tapered laminated composite sandwich plate configurations under harmonic excitation are analysed at various magnetic fields. This analysis suggests that the location and size of the magnetorheological elastomer segments strongly influence the natural frequency, loss factor and transverse displacements of the partially treated magnetorheological elastomer tapered laminated composite sandwich plates apart from the intensities of the applied magnetic field. This shows the applicability of partial treatment to critical components of a large structure to achieve a more efficient and compact vibration control mechanism with variable damping.
Shock tube experiments were performed to investigate the blast response of corrugated steel cellular core sandwich panels filled with a silicone based syntactic foam at room and high temperatures. The syntactic foam filler was prepared by mixing a two-part silicone mixture with glass microspheres; its microstructure, and mechanical properties were also characterized. The syntactic foam-filled sandwich panels were loaded via air shock pressure by using the shock tube with a fixture capable of testing materials at temperatures up to 900℃. High-speed photo-optical methods, digital image correlation techniques, were used in tandem with optical band-pass filters and high intensity light sources for providing sufficient contrast at elevated temperatures. Back-face deformation images were captured using two synchronized high-speed cameras while a third camera captured the side view deformation images. The shock pressure profiles and digital image correlation analysis were used to obtain the impulse imparted to the specimen, transient deflection, in-plane strain and out-of-plane velocity of the back-face sheet. It was observed that using the syntactic foam as a filler material decreased the front face and back face deflections by 42% and 27%, respectively, as compared to the empty sandwich panel. At high temperatures, the silicone-based syntactic foam decomposes into silica, a stable and non-hazardous byproduct. The highest impulse was imparted to the specimen at room temperature and subsequently lower impulses with increasing temperatures were observed. Due to the increased ductility of steel at high temperatures, the specimens demonstrated an increase in back face deflection, in-plane strain and out-of-plane velocity with increased temperatures, with weld failure being the primary form of core damage.
In this study, the flexural behavior of sandwich composite beams made of fiber-reinforced polymer (FRP) skins and light-weight cores are studied. The focus is on the comparison of natural and synthetic fiber and core materials. Two types of fiber materials, namely glass and flax fibers, as well as two types of core materials, namely polypropylene honeycomb and cork, are considered. A total of 105 small-scale sandwich beam specimens (50 mm wide) were prepared and tested under four-point bending. Test parameters were fiber types (flax and glass fibers), core materials (cork ad honeycomb), skin layers (0, 1, and 2 layers), core thicknesses (6–25 mm), and beam spans (150 and 300 mm). The load–deflection behavior, peak load, initial stiffness, and failure mode of the specimens are evaluated. Moreover, the flexural stiffness, shear rigidity, and core shear modulus of the sandwich composites are computed based on the test results of the two spans. An analytical model is also implemented to compute the flexural stiffness, core shear strength, and skin normal stress of the sandwich composites. Overall, the natural fiber and cork materials showed a promising and comparable structural performance with their synthetic counterparts.
In this paper, classical as well as various refined plate finite elements for the analysis of laminates and sandwich structures are discussed. The attention is particularly focussed on a new variable-kinematic plate element. According to the proposed modelling approach, the plate kinematics can vary through the thickness within the same finite element. Therefore, refined approximations and layer-wise descriptions of the primary mechanical variables can be adopted in selected portions of the structures that require a more accurate analysis. The variable-kinematic model is implemented in the framework of the Carrera unified formulation, which is a hierarchical approach allowing for the straightforward implementation of the theories of structures. In particular, Legendre-like polynomial expansions are adopted to approximate the through-the-thickness unknowns and develop equivalent single layer, layer-wise, as well as variable-kinematic theories. In this paper, the principle of virtual displacements is used to derive the governing equations of the generic plate theory and a mixed interpolation of tensorial components technique is employed to avoid locking phenomena. Various problems are addressed in order to validate and assess the proposed formulation, including multi-layer plates and sandwich structures subjected to different loadings and boundary conditions. The results are compared with those from the elasticity theory given in the literature and from layer-wise solutions. The discussion clearly underlines the enhanced capabilities of the proposed variable-kinematic mixed interpolation of tensorial component plate elements, which allows, if used properly, to obtain formally correct solutions in critical areas of the structure with a considerable reduction of the computational costs with respect to more complex, full layer-wise models. This aspect results particularly advantageous in problems where localized phenomena within complex structures play a major role.
Absorbing impact energy at subsystem level is an attractive idea that is emphasized by new composite reinforcement techniques such as stitching or pinning. This paper reports experimental results of medium velocity impact tests carried out on several arrangements of reinforced foam/braided composite structures. The tests consisted of a steel ball shot at a velocity of 110 m/s from a gas gun impacting the structures on their leading edge. Post-mortem tomography analysis delivered very rich information which shed light on the damage mechanisms that the composite structures underwent. In addition, two fast-speed cameras were used to derive the energy absorption during the impact. Absorption capabilities were also compared with those of dynamic crushing tests (reported in a companion paper) and some designs clearly exhibited promising behavior as shock absorbers.
The objective of this study is to enhance the out-of-plane tensile and compressive performances of foam core sandwich composite via structural core modifications considering the ease of application and time consumption. The performances of single core perforated, single core stitched, divided core perforated, and divided core stitched sandwich composites are compared with each other and reference plain foam core sandwich composites. Results indicate that "perforated and stitched core" sandwich composites have superior strength, and in terms of performance modification, dividing the core is found very efficient for plain (non-perforated and non-stitched) core sandwich composites.
Composite sandwich structures are devised to work in a wide frequency of the microwave band. The microwave absorbing properties of composite sandwich structures are studied in 2–18 GHz frequency band. The sandwich structures were manufactured from E-glass fiber/epoxy composites filled with carbon nano-materials and para-aramid honeycomb cores. The complex permittivity of E-glass/epoxy nanocomposites and adhesive films are determined in 8–12 GHz frequency range using free-space measurement setup. The complex permittivity data were used to design the sandwich structures by varying composition and thickness of nanocomposite sheets using a simulation tool Computer Simulation Technology Microwave Studio. In the designing process, the thickness of honeycomb sheets was also varied to get best spacer thickness for the cancellation of reflected and transmitted microwaves. The simulated and measured results have shown that the designed structure can be used for –10 dB Reflection coefficient over a wide frequency ranges in the microwave region. The results of flexural strength of the sandwich structure and tensile strength of facing sheets are also presented.
Used the Reddy's higher-order shear deformation plate theory, the nonlinear dynamic analysis and vibration of imperfect functionally graded sandwich plates in thermal environment with piezoelectric actuators (PFGM) on elastic foundations subjected to a combination of electrical, damping loadings and temperature are investigated in this article. One of the salient features of this work is the consideration of temperature on the piezoelectric layer, and the material properties of the PFGM sandwich plates are assumed to be temperature-dependent. The governing equations are established based on the stress function, the Galerkin method, and the Runge–Kutta method. In the numerical results, the effects of geometrical parameters; material properties; imperfections; elastic foundations; electrical, thermal, and damping loads on the vibration and nonlinear dynamic response of the PFGM sandwich plates are discussed. The obtained natural frequencies are verified with the known results in the literature.
In this paper, a modified analytical model has been presented for clamped circular composite sandwich panels subjected to low-velocity impact by spherical impactor. The composite sandwich panel is symmetric and its composite face sheets consist of cross-ply laminates. The principle of minimum total potential energy has been used for obtaining the contact force and contact force–indentation relation. In this analytical model, the strain membrane stretching energy and the strain energy due to the bending of face sheet have been determined and the core considered as rigid perfectly plastic. Also, the two degree-of-freedom spring–mass model has been used for determining the contact force history and deflection of the sandwich panel. The analytical contact force and the deflection of sandwich panel have been compared with published experimental and numerical results and good agreement has been observed.
The paper presents the experimental and numerical studies of sandwich panels with a hybrid core. The sandwich panel consists of external steel facings and a core, which is made of polyurethane foam or mineral wool or a combination of those two materials. The polyurethane foam material has a low weight and high thermal insulation properties, while the mineral wool material can provide high acoustic insulation and excellent fire resistance. Various proportions of the core materials are taken into account. It is assumed that a proper combination can provide the benefits of both materials. The structural behavior of a sandwich structure with a hybrid core is observed during laboratory tests. The failure mechanism is investigated in a four-point bending test. The material parameters of the core and facings are determined in standardized tests. The obtained parameters are used for FE simulations of the four-point bending tests. The criteria of damage initiation and propagation are defined in the interface layer of the numerical model. A satisfactory correlation between laboratory tests and numerical results is reported. Additionally, the sensitivity analysis of the numerical model response to the variation of the parameters of the interface is presented.
The nonlinear torsional buckling and post-buckling of ceramic functionally graded material (C-FGM-M) stiffened cylindrical shell surrounded by Pasternak elastic foundation in thermal environment are investigated in this paper. The C-FGM-M cylindrical shell is reinforced by ring and stringer stiffeners system in which the material properties of shell are assumed to be continuously graded in the thickness direction. Based on the classical shell theory, theoretical formulations are derived with the geometrical nonlinearity in von Karman sense and the smeared stiffeners technique. The three-term approximate solution of deflection is chosen more correctly and the explicit expression for finding critical load and post-buckling torsional load–deflection curves are given. The effects of geometrical parameters, temperature, stiffeners and elastic foundation are investigated.
Free vibration analysis of a sandwich beam with soft core and carbon nanotube reinforced composite face sheets, hitherto not reported in the literature, based on extended high-order sandwich panel theory is presented. Distribution of fibers through the thickness of the face sheets could be uniform or functionally graded. In this theory, the face sheets follow the first-order shear deformation theory. Besides, the two-dimensional elasticity is used for the core. The field equations are derived via the Ritz-based solution which is suitable for any essential boundary conditions. Chebyshev polynomials multiplying boundary R-functions are used as admissible functions and evidence of their good performance is given. A detailed parametric study is conducted to study the effects of nanotube volume fraction and their distribution pattern, core-to-face sheet thickness ratio, and boundary conditions on the natural frequencies and mode shapes of sandwich beams with functionally graded carbon nanotube reinforced composite face sheets and soft cores. Since the extended high-order sandwich panel theory can be used with any combinations of core and face sheets and not only the soft cores that the other theories demand, the results for the same beam with functionally graded carbon nanotube reinforced composite face sheets and stiff core are also provided for comparison. It is concluded that the sandwich beam with X and V distribution figures of face sheets, no matter what the boundary conditions, has higher vibration performance than the beam with UD-CNTRC face sheets.
For exploring lightweight protective structures, a type of lattice sandwich plate and its preparation method are introduced, and anti-penetration capability is investigated. This lattice sandwich structure includes two metal sheets and multimetal pyramidal lattice trusses, which is prepared based on the three-dimensional interlocking process, parallel ceramic rods, and hybrid fillers mixed with chopped glass fibers and epoxy resins. Mechanical properties of lattice structures with different relative densities are compared. Then, penetration experiments with ball-shaped projectiles on this plate and a contrast structure with no ceramic rods are performed, so that the failure modes, anti-penetration mechanisms, and impact absorption efficiency can be initially explored. The results indicate that this process introduced is suitable for any conductive material, with no requirement of strong plasticity and liquidity; deformation processes of lattice structures with different truss width are similar, and inelastic buckling model can describe the failure mode and compression limit well. Owing to the macrobending deformation of the main metal sheets, severe plastic deformation, and shear reaming of the lattice trusses, smash and failure of ceramic rods and hybrid fillers occur, which bring about much energy absorption. A significant anti-penetration capability of this plate is shown with energy absorption rate being 75% approximately.
In this work, biaxial buckling analysis of sandwich plates with symmetric composite laminated core and two functionally graded nanocomposite face sheets is carried out by a new improved high-order theory. The nanocomposite face sheets are carbon nanotube (CNT)-reinforced nanocomposites and the material properties of the nanocomposites plates are graded along the thickness and are estimated though the Mori–Tanaka approach. CNTs are assumed randomly oriented and aggregated into some clusters. The same third order theory is used for modeling of core and the faces sheets. The theory has third and second orders of z for in-plane and normal displacements, respectively. The principle of minimum potential energy is used to derive the equations of motion and boundary conditions. Analytical solution for static analysis of simply supported sandwich plates under biaxial in-plane compressive loads is presented using Navier’s solution. The effects of CNT volume fraction, CNT aggregation states, CNT distribution, biaxial loads ratio, and geometric dimensions of sandwich plate are investigated on the overall buckling of functionally graded carbon nanotube-reinforced nanocomposite sandwich plates.
Mechanical stability of the functionally graded rectangular plates bonded with functionally graded piezoelectric layers is studied in the present research. Classical plate theory is employed in the description of three components of the plate displacement. Geometric nonlinearity is considered in the strain–displacement relations using the von Karman equation. The constitutive relations are developed for the functionally graded material core and functionally graded plate material layers in the general state. The top and bottom of the piezoelectric layers is assumed to be short-circuited. Three equilibrium equations as well as Maxwell equation are constituted of four governing differential equations of the problem. Some important nondimensional parameters representing the geometries of the sandwich plate incorporated with nonhomogeneous index of the material properties are considered for this analysis. The results indicate that these parameters have important effects on the buckling loads.
In the present study, a simple four-variable trigonometric shear deformation theory considering the effects of transverse shear deformation and rotary inertia is evaluated for the free vibration analysis of antisymmetric laminated composite and soft core sandwich plates. The theory is displacement-based equivalent single-layer theory in which the in-plane displacements use trigonometric function in terms of thickness coordinate, for calculating out-of-plane shearing strains. The number of unknown variables involved in the present theory is only four as against five or more than five in case of other higher order theories. The equations of motion are obtained using the principle of virtual work. A closed-form solution for equations of motion is obtained using the Navier’s solution technique. The effects of side-to-thickness ratio, modular ratio and fibre angle are critically assessed for several problems of laminated composite and sandwich plates. The natural frequencies obtained by using present theory are verified by comparing the results with those of other theories and exact elasticity solution wherever applicable.
A simplified layer-wise sandwich beam model to capture the effects of a combination of geometric taper and variable stiffness of the core on the static response of a sandwich beam is developed. In the present model, the face sheets are assumed to behave as Euler beams and the core is modelled with a first-order shear deformation theory. With geometrical compatibility enforced at both upper and lower skin/core interfaces, the beam’s field functions are reduced to only three, namely the extensional, transverse and rotational displacements at the mid-plane of the core. The minimum total potential energy method is used in combination with the Ritz technique to obtain an approximate solution. Geometrically nonlinear effects are considered in the present formulation by introducing von Kármán strains into the face sheets and core. Two types of sandwich beams, uniform and tapered, with different boundary conditions are studied. Results show that the proposed model provides accurate prediction of displacements and stresses, compared to three-dimensional finite element analysis. It is found that due to the axial stiffness variation in the core, displacements of beams and stresses of face sheets and core are significantly affected. The potential design space is shown to be expanded by utilizing variable stiffness materials in sandwich constructions.
Springback is one of the most important defects which occur in sheet metal forming processes specially bending process. Springback occurs because of an elastic behavior during unloading process and leads to variation of bending angle from intended angle. In order to increase geometrical accuracy of formed parts, bending parameters should be chosen properly to compensate the springback effect on the arbitrary geometry. In the present study, the springback behavior of Al2024/Polyurethane-glass reinforced/Al2024 sandwich sheet has been investigated by using experimental, finite element and theoretical procedures. The results show that presented theoretical and finite element procedures can be used to predict amount of springback in sandwich sheets. Also results show that the amount of springback in sandwich sheets decreased by increasing process temperature and core thickness of sandwich sheets.
Polyurethane foam-based sandwich structures were developed for radar absorbing properties together with load bearing capability. The sandwich construction of radar absorbing structure comprised glass fiber epoxy matrix composite containing carbon black as front face skin, carbon fiber epoxy matrix composite as back face skin, and polyurethane foam reinforced with carbonyl iron or graphite powder as the core material. The quantity of carbon black in the front face skin composite was varied from 6 wt.% to 8 wt.%, while the loading of carbonyl iron and graphite powder in foam core was varied from 30 wt.% to 55 wt.% and 5 wt.% to 30 wt.%, respectively. The compression molding technique was used to prepare face skins and sandwich structures. Different combinations of the sandwich structures were characterized for a frequency range of 2–18 GHz using free space measurement method. The maximum attenuation of –31.85 dB was observed in a combination containing 6 wt.% carbon black in front face skin and 20 wt.% graphite powder in the foam core. The total reduction in radar cross-section demonstrated almost zero radar signature at nine different frequencies, whereas broad range attenuation of –10 dB was achieved.
Building, naval, and automotive industries have deep interest in eco-friendly, lightweight, stiff and strong materials. In addition, materials with low thermal conductivity are desirable in many applications where energy savings and thermal comfort are needed. In response to these requirements, sandwich panels were manufactured using glass and jute fiber composite skins bonded to different cores: balsa wood, Divinycell® and honeycombs. These honeycombs, as well as the skins, were manufactured by the vacuum infusion technique using polyester resin and jute, glass and carbon fiber fabrics. In this work, the thermal properties and density of the sandwich panels were measured and compared.
This paper presents the free vibration analysis of composite thick rectangular plates, based on Reddy’s higher order shear deformation theory (HSDT). The plate theory ensures a zero shear-stress condition at the top and bottom surfaces of the plate and do not requires a shear correction factor. Although the plate theory is quite attractive, it could not be used in the finite element analysis. This is due to the difficulties associated with the satisfaction of the
An accurate discrete model and analytical solutions thereof are presented for shear-deformable web-core sandwich plates. The face-plates are analyzed using the equations of three-dimensional elasticity, while the webs are accurately modelled using the classical plate theory with a plane stress solution for transverse bending and a Levy-type methodology for lateral bending. It is shown that this obviates the need for a complete three-dimensional analysis of the sandwich plate. Results obtained by this approach are used to highlight the effect of shear deformation of the face-plates.
Honeycomb sandwich structures are increasingly used in the automotive, aerospace and shipbuilding industries where fuel savings, increase in load carrying capacity, vehicle safety and decrease in gas emissions are very important aspects. The aim of this study was to develop the theoretical methods, initially proposed by the authors and by other researchers for the prediction of low-velocity impact responses of sandwich structures. The developed methods were applied to sandwich structures with aluminium honeycomb cores and glass-epoxy facings for the assessment of impact parameters and for the prediction of limit loads. The values of model parameters were compared with data reported in literature and the predictions of the limit loads were validated by means of the experimental data. Good achievement was obtained between the results of the theoretical models and the experimental data. The failure mode and the internal damage of the sandwich panels have been investigated using 3D computed tomography, which allowed the evaluation of parameters of energy balance model, and infrared thermography, which allowed the detection of the temperature evolution of the specimens during the tests. The experimental and theoretical results demonstrated that the use of glass-epoxy reinforcement on aluminium honeycomb sandwiches enhances the energy absorption and load carrying capacities.
The paper studied the dynamic response of square aluminum corrugated sandwich panels under projectile impact. The aluminum foam projectile was utilized to apply the impulse on the sandwich panels. In order to increase the applied impulse under controlled impact velocity (V < 200 m/s), a cylindrical Nylon mass was adhered to the back of foam projectile. Corrugated sandwich panels with two different configurations were tested and their typical deformation modes were obtained in the experiment. Based on the experiment, corresponding numerical simulations were presented. The energy absorption and deformation mechanism of corrugated sandwich panels were studied through the simulation. The influence of impact velocity, thickness of face sheet and wall thickness of corrugated core were discussed. The results indicated that the corrugated sandwich panels with smaller core height produce larger deformation than the panels with larger core height. The face sheets of corrugated sandwich panel absorbed comparable amount of energy with the corrugated core. The velocity histories show that under the combined action of aluminum foam projectile and nylon back mass, a second peak velocity of front face sheet can be produced during the impact process, which is defined as "accelerating impact stage" in current study. The influence of "accelerating impact stage" to the response of structures is sensitive to the impact velocity.
Bridge damage due to over-height vehicle collisions is a major issue throughout the transportation network with damages ranging from minor distortion or spalling in fascia girders to almost complete bridge destruction. Repairing such damages resulted from over-height vehicle collisions is expensive, and it includes costs for bridge repair, and rerouting traffic as well as indirect economic and societal costs. In this study, in correlation with a series of impact tests for reinforced concrete beams with or without Impact-Laminate (I-Lam) panel protection by a wooden projectile, the numerical modeling using the finite element codes ABAQUS is conducted to simulate the over-height vehicle impact, and a good agreement with the available experimental data is obtained. Parametric studies are conducted to further investigate the effects of design parameters (e.g. the velocity of projectile, I-Lam parameters, and core layer sequence, etc.) on the effectiveness of the protection system. The observed phenomena from the parametric studies reflect a design philosophy aiming at improving protection efficiency and providing important information on design of I-Lam honeycomb sandwich collision protection systems for field application.
Composite materials are increasingly used in applications of civil infrastructure and building materials. The new generations of two-part thermoset polyurethane resin systems are desirable materials for infrastructure applications. This is due to high impact resistance, superior mechanical properties, and reduced volatile organic compounds when compared to the conventionally used resin systems such as vinyl ester and polyester. Glass fiber-reinforced two-part polyurethane composites and low-density polyurethane foam are used to design and manufacture composite structural insulation panels using vacuum assisted resin transfer molding process for temporary housing applications. Using these types of composite panels in building construction will result in cost-efficient, high-performance products due to inherent advantages in design flexibility. Use of core-filled composite structures offers additional benefits such as high strength, stiffness, lower structural weight, ease of installation and structure replacement, and higher buckling resistance than the conventional panels. Energy efficiency is known to be inherently better with the core-filled composite panel than in a metallic material. The panels can be designed to resist the required loads, and the study aims to evaluate the ability of lab scale tests and models to predict part quality in full-scale parts. Furthermore, it discusses the manufacturing challenges. Flexural tests and energy consumption evaluations were performed on these structural components. Finite element simulation results were used to validate the flexural experiment findings.
The dynamic response of circular sandwich panels with aluminium honeycomb and corrugated cores under projectile impact was investigated experimentally and numerically. Impulse loaded on the panel was controlled by projectile launching velocity and the deformation process of sandwich panels was recorded by a high-speed camera in the experiments. Typical deformation/failure modes of face-sheets and cores were obtained and analysed. The back face-sheet deflections and strain histories of face-sheets were measured and discussed. A parametric study was conducted by LS-DYNA 3D to analyse the effect of geometrical configuration on energy absorption mechanism and back face-sheet permanent deflection of circular sandwich panels. The results indicated that the impact resistance of the structure was sensitive to geometrical configuration. Increasing face-sheet thickness and core relative density significantly improved sandwich structure impact resistance. Increasing foil thickness improved the panel impact resistance more efficiently than decreasing wall side length. The results have important reference value to guide engineering application of the sandwich structure subjected to impact loading.
Two finite element formulations using different laminate plate theories are developed for the elastic-viscoelastic-elastic sandwich plates. A critical comparison and assessment between them are provided. The dynamic characteristics, namely, the natural frequencies and associated loss factors of the elastic-viscoelastic-elastic sandwich plates are calculated using the two finite element models. The two models are validated through the numerical example and experiment results. Comparisons of the accuracy and computational efficiency of the two finite element models are given. The results show that both of the two finite element models have good accuracy in predicting the natural frequencies and the loss factors with different efficiency. The works in this article have instructive significance in the calculation and application of the elastic-viscoelastic-elastic sandwich structures.
The interface slip will appear between the steel plates and concrete while the steel–concrete–steel composite beam under loading. It may influence the mechanical properties of the composite beam. In this paper, through theoretical analysis of the steel–concrete–steel composite beam, differential equation of interface slip is established at first. By simulating the real boundary, the formulas of interface slip are calculated under uniform and arbitrary concentrated load. Then, the axial force, the sectional curvature, and deformation of composite beams are obtained. In order to validate the reliability of the theoretical analysis, the deformation of 18 samples is calculated by using the deformation formulas of steel–concrete–steel composite beam. The results are in good agreement with the experimental consequences. Through an example, the mechanical properties of composite beams (axial force, sectional curvature, and deformation) are analyzed under interfacial slip. With the decreasing of interfacial slip, axial force of upper plate increases, and sectional curvature and deflection decrease. For lower steel plate, the interfacial slip has smaller effect.
In this paper, the free vibration and buckling analyses of the cylindrical sandwich panel with magneto-rheological fluid layer for simply supported boundary conditions was performed based on an improved higher order sandwich panel theory. This paper deals with investigation of the effects of magnetic field, geometrical parameters such as the core thickness to the panel thickness ratio, MR layer thickness to the panel thickness ratio and the fiber angle on the natural frequencies, loss factors and buckling loads corresponding to the first four mode shapes. In order to validate the results obtained from the present study, the cylindrical sandwich panel was simulated and analyzed in finite element software ABAQUS. A good agreement was observed between the results of present method and those extracted from simulation.
Sandwich panels are made of two materials that are relatively weak in their separated state, but are improved when they are constructed together in a sandwich panel. Sandwich panels can be used for almost any section of a building including roofs, walls and floors. These building components are regularly required to provide insulation properties, weatherproofing properties and durability in addition to providing structural load bearing characteristics. Polystyrene/cement mixed cores and thin cement sheet facings sandwich panels are Australian products made of cement-polystyrene beaded mixture encapsulated between two thick cement board sheets. The structural properties of sandwich panels constructed of polystyrene/cement cores and thin cement sheet facings are relatively unknown. Therefore, in this study, to understand the mechanical behaviour and properties of those sandwich panels, a series of experimental tests have been performed and the outcomes have been explained and discussed. Based on the results of this study, values for modulus of elasticity and ultimate strength of the sandwich panels in dry and saturated conditions have been determined and proposed for practical applications.
Natural biodegradable sandwich materials were fabricated and mechanical characterization was performed under flexural and impact loading conditions. The sandwich structure consists of hardwood as face plates, mushroom foam as a core, and natural glue as an adhesive. Two different hardwood layers were used to investigate the effect of thickness and the stiffness on the flexural and impact properties. Under impact loads, two different projectile velocities were employed to understand their effect on energy absorption of the sandwich materials. It was found that the sandwich made using high stiffness wood panels showed improved flexural properties as well as maximum energy absorption at higher impact velocities. The damage of front and rear sides of impact specimens was analyzed to understand the effect of impact velocity on damage zone. The damage area on the rear side of the specimen decreased significantly with high stiffness wood face plates at lower projectile velocity. The increase is energy absorption is better supported by the increase in damage area on both front and rear sides of sandwich materials at higher projectile velocity. Both flexural properties and energy absorption demonstrated great promise to use these sandwich structures for structural load bearing applications.
For sandwich beams with second-order hierarchical corrugated truss core under three-point bending, a correction factor of shear deflection was firstly proposed to improve the prediction accuracy of the bending analysis, which was verified by finite element analysis and compared with the original formula. Then, the failure modes of the sandwich beam under bending were analyzed, including four competing modes of the large struts (i.e. plastic yielding, buckling, wrinkling of facesheet, shear buckling) and two competing modes of the small struts (i.e. plastic yielding, buckling). Subsequently, the analytical expressions of critical load for each failure mode were derived. On this basis, the failure mechanism maps were constructed. Finally, several typical points from the map were selected and verified by finite element analysis, and a good agreement of predicted failure modes was observed.
Sandwich panels with auxetic lattice cores confined between metallic facets are proposed for localised impact resistance applications. Their performance under localised impact is numerically studied, considering the rate-dependent effects. The behaviour of the composite structure material at high strain rates is modelled with the Johnson-Cook model. Parametric analyses are conducted to assess the performance of different designs of the hybrid composite structures. The results are compared with monolithic panels of equivalent areal mass and material in terms of deformations and plastic energy dissipation. Design parameters considered for the parametric analyses include the auxetic unit cell effective Poisson’s ratio, thickness of the facet, material properties and radius of the unit cell’s struts. Significant reduction in computational time is achieved by modelling a quarter of the panel, with shell elements for facets and beam elements for the auxetic core. With projectile impacts up to 200 m/s, the auxetic composite panels are found to be able to absorb a similar amount of energy through plastic deformation, while the maximum back facet displacements are reduced up to 56% due to localised densification and plastic deformation of the auxetic core.
In this paper, a set of numerical and experimental studies are performed to improve mechanical and vibrational properties of carbon nanotubes-reinforced composites. First, at a design concept level, linear distribution patterns of multi-walled carbon nanotubes through the thickness of a typical beam is adopted to investigate its fundamental natural frequency for a given weight percent of multi-walled carbon nanotubes. Both Timoshenko and Euler-Bernoulli beam theories are used in the derivation of the governing equations. The finite element method is employed to obtain a numerical approximation of the motion equation. Next, based on the introduced distribution patterns, laminated multi-walled carbon nanotubes-reinforced polystyrene-amine composite beams are fabricated. Static and experimental modal tests are performed to measure the effective stiffness and fundamental natural frequencies of the fabricated composite beams. Also, in order to generate realistic model to investigate the material properties of fabricated composite beams, the actual tensile specimens of multi-walled carbon nanotubes/polystyrene-amine composites are successfully fabricated and the tensile behaviors of both pure matrix and composites are investigated. To better interfacial bonding between carbon nanotubes and polymer, a chemical treatment is performed on carbon nanotubes. It is seen that the addition of a few wt. % of multi-walled carbon nanotubes make considerable increase in the Young's modulus and the tensile strength of the composite. It is observed from the free vibration tests that the uniform distribution of multi-walled carbon nanotubes results in an increase of 9.5% in the fundamental natural frequency of the polymer cantilever beam, whereas using the symmetric multi-walled carbon nanotube distribution increased its fundamental natural frequency by 17.32%.
An approach is introduced for determining accurate two-dimensional equivalent laminated models of sandwich laminates with honeycomb core and composite facesheets by optimization involving modal behavior. The approach relies on minimizing the objective function which is defined as the sum of the square of the differences between the natural frequencies of the honeycomb sandwich laminate estimated by the finite element analysis of the 3D detailed model with the actual honeycomb core geometry and by the 2D equivalent laminated model with the honeycomb core replaced by the equivalent 2D orthotropic material model. Equivalent elastic constants of the 2D orthotropic model of the honeycomb core are defined as the design variables of the optimization problem, and a finite element solver and genetic algorithm-based optimizer are coupled to perform the optimization task. Results show that with the optimization-based approach, very accurate 2D equivalent models of honeycomb sandwich laminates are obtained compared to equivalent models obtained by replacing the honeycomb core with elastic constants of the 2D orthotropic material model obtained utilizing analytical models available in the literature.
The constitutive behavior of a continuum equivalent to cellular core are calculated using detailed finite element models of representative unit cell for hexagonal, square and triangular core shapes. The variation of the constitutive behavior of the homogeneous structure equivalent to the cellular core with respect to the core density is studied. Formulas for direct calculation of the constitutive behavior are generated for cell size range 1/8'' to 3/8'' and core density range 1.0 pcf to 8.1 pcf. The calculated properties are tested using two-dimensional and three-dimensional finite element modeling techniques of sandwich panels. Error analysis is then performed to identify the effect of the panel size to cell size ratio as well as the facesheet thickness to core thickness ratio on the error. Results show linear variation of mechanical properties with core density except for panel-wise in-plane shear modulus which is too low and can be neglected. The facesheet thickness to core thickness ratio was found to have low impact on the error while the panel size to cell size ratio was found to have a significant impact on the error level. A minimum panel size to cell size ratio of 60 was found to be necessary to maintain error within 5%.
Radar domes are the cover structures over the radar antenna systems to provide environmental protection to the sensitive parts such as electronics, steering hardware, wave guide, additional equipments, and antenna itself. Composite dielectric materials are preferred solutions for radome construction because of their negligible effects on electromagnetic transmission of enclosed antenna. Very recently, the effect of interlaminar stress distribution and radome geometry over the transmission capabilities is reported by several researchers. The aim of this study is to present an efficient solution methodology with minimized mathematical effort for the analytical solution of sandwich composite dielectric materials for radome structures with one and double core layers. Analytical solution methodology for the analyses of stresses and deformations is based on Third-Order Shear Deformation Theory (TSDT). Double Fourier series which are specialized for boundary discontinuity are used to solve highly coupled linear partial differential equations. Numerical solutions for the designed spherical radome geometry with one and double core layers are presented for laminated sandwich shells to provide benchmark results for the pre-design activities of radome structures.
Experimental and numerical studies of sustainable sandwich bio-composites were presented. The bio-composites were developed using plant-based materials. The laminated face sheets comprised woven hemp fabric and a tree sap–based epoxy, while the core comprised castor oil-based polyurethane foam reinforced with waste rice hulls ashes. Uniaxial compression, tension, and three-point flexural bending tests were performed. Finite element models were developed. Good agreements between experimental and numerically simulated flexural responses were found. From numerical simulation results, the sandwich bio-composites were found to be a structurally acceptable replacement for standard gypsum drywall. Further experimental works are needed to validate the applications.
This article investigates the energy-absorbing behaviour of lightweight foam structures reinforced with aluminium and steel cylindrical tubes. Initial testing focuses on establishing the influence of the inner diameter to thickness ratio (D/t) of the metal tubes on their specific energy-absorption characteristics under quasistatic compression and low velocity impact loading. Following this, individual metal tubes are embedded in a range of crosslinked PVC foams, and the specific energy-absorption characteristics of these reinforced systems are determined. The effect of increasing the number of tubes on the energy-absorbing response of the tube-reinforced structures is also studied. The crushing responses of both aluminium and steel structures are then predicted using the finite element analysis package Abaqus/Explicit, and the predictions of the load–displacement responses and the associated failure modes are compared to experimental results. Agreement between the numerical predictions and the experimental data is good across the range of structures investigated, with the model accurately predicting the compression response and failure characteristics observed in the structures. It has been shown that the stiffness of the foam does not significantly alter the energy-absorbing behaviour of the stiffer metal tubes, suggesting that the density of the foam should be as low as possible, whilst maintaining the structural integrity of the part.
This paper presents an analytical approach to investigate the nonlinear dynamic response and vibration of shear deformable imperfect eccentrically stiffened sandwich plate with functionally graded material (FGM) on elastic foundation using both of the first-order shear deformation plate theory and stress function with full motion equations (not using Volmir's assumptions). The thick sandwich plates are assumed to rest on elastic foundation and subjected to mechanical loads in thermal environment. Numerical results for dynamic response of the eccentrically stiffened thick sandwich plates are obtained by Runge–Kutta method. The results show the influences of geometrical parameters, material properties, imperfections, the elastic foundations, eccentric stiffeners, mechanical loads and temperature on the nonlinear dynamic response and nonlinear vibration of functionally graded sandwich plates. The numerical results in this paper are compared with the results reported in other publications.
The crushing of single- and double-layer zig-zag trapezoidal corrugated core sandwiches was investigated experimentally and numerically at quasi-static and dynamic rates. The buckling stress of sandwiches increased when the rate increased from quasi-static to dynamic. The increased buckling stresses were ascribed to the micro-inertial effects, which altered the buckling mode of the core from three plastic hinges to higher number of plastic hinge formations. The initial buckling stress was numerically shown to be imperfection sensitive when the imperfection size was comparable with the buckling length. The numerical buckling stresses of zig-zag and straight corrugated cores were similar, while higher inertial effects were found in triangular corrugated core.
Using harmonic differential quadrature method, an approach to analyze sandwich cylindrical shell panels with any sort of boundary conditions under a generally distributed static loading, undergoing elasto-plastic deformation is proposed. The faces of the sandwich shell panel are made of some isotropic materials with linear work hardening behavior while the core is assumed to be an isotropic material experiencing only elastic behavior. The faces are modeled as thin cylindrical shells obeying the Kirchhoff–Love assumptions. For the core material, it is assumed to be thick and the in-plane stresses are negligible. Upon application of an inner and outer general lateral loading, the governing equations are derived using the principle of virtual displacements. Using an iterative approach, named elasto-plastic harmonic differential quadrature method (EP-HDQM), the equations are solved. The obtained results are compared with the results from finite element software Ansys for different sandwich shell panel configurations. Then, the effects of changing different parameters on the stress and displacement components of sandwich cylindrical shell panels in different elasto-plastic conditions are investigated.
An accurate discrete model is presented here for the analysis of simply supported web-core sandwich plates. In this model, the face plates are analysed using the equations of 3D elasticity and for the webs a plane stress idealization is used to model the kinematics of transverse bending while simple one-dimensional classical models are employed for lateral bending and torsion. Thus, this model accounts for the non-classical effects of transverse shear deformation and transverse thickness-stretch in the face-plates and the webs. It is shown that this model is capable of accurately capturing the effects of secondary local bending of the face-plates between the webs on the displacement and stress fields. Results obtained by this rigorous approach are used to highlight the errors of the commonly used model based on the classical hairbrush hypothesis for the face-plates.
The vacuum infusion is a process usually applied to manufacture large structures of composite materials, such as wind turbine blades. The specific stiffness and weight ratio required by these structures can be achieved by manufacturing sandwich composites. The forecast by numerical simulation of the resin infusion flow is an indispensable tool to design and optimize the manufacturing process of composite. Present work analyzes by numerical simulation the mold filling process of a sandwich composites, performed by fiberglass plies with different fiber orientations and a perforated core. The flow through a single perforation of the core is analyzed and the influence of the permeability values of fiberglass on the volume flow through core perforations is determined. In order to reduce the computing costs, a transfer function to simulate the flow through the perforations is developed and integrated in the numerical code by computational subroutines. A 3D numerical modeling of a sandwich composite, in which the flow through the core perforations is simulated via computational subroutines, is carried out and experimentally validated.
This work deals with the durability assessment, after ageing, of wood-based sandwich panels, carried out according to the European guideline ETAG 016, Annex C7 Climatic testing cycles. ETAG 016-2 is a guideline to assess the wood-based sandwich panels for roofs. It has tests and criteria for evaluating the results of these tests. As regards durability and performance under the weather, it proposes to choose one of three different climatic cycles. The choice depends on the material of the core. Each climate cycle has assessment criteria to measure whether the cycle has been successfully overcome or not. This assessment is related to the standard EN 14509, paragraph 5.2.3.1 that contains the same climatic test cycles. In this work we subjected several types of non-metallic sandwich panels to this test and criteria. Then, a critical review of these acceptance criteria was done. As a result, some recommendations are proposed regarding the acceptance criteria and testing procedure.
With the development of technology, the use of glass fibres and sandwich structures in a wide variety of engineering applications has had important growth in recent years because of their low self-weight and high strength when compared to conventional materials. Moreover, natural materials can be used as the core material in sandwich structures instead of conventional materials. Since synthetic materials cause a reduction of carbon release during material formation, these kinds of materials are renewable and recyclable. Therefore, in sandwich structures, renewability and recycling can be provided. In this study, dynamic properties of natural material-based sandwich composites were investigated experimentally and numerically. The effects of the thickness of the core and fibre orientation and number of layers on frequency and damping were analysed. It was observed that if the core thickness of the structure is properly optimised, this sandwich structure demonstrates better dynamic properties. Thus, sandwich materials from natural origin may offer more environmental friendly solutions compared to other materials.
Marine grade hybrid sandwich panels are designed in accordance with BS EN ISO 12215-5:2008 using a wet lay-up and cured under vacuum pressure. The high fibre content composite laminate skins use marine grade orthophthalic polyester, POLYLITE® 440-M850, resin and chopped strand mat (CSM)/woven E-glass; the core is DIVINYCELL® H100 closed cell linear polyvinyl chloride (PVC) foam. Impact damage testing followed American Society for Testing and Materials (ASTM) D7766-11 procedure C and ASTM D7136/D7136M-05. Impact damage was sustained by the default hemispherical indentor and further ‘standard’ geometrical indentor rocks – conical, pyramid and cylindrical. The investigation reviews the current state of affairs in impact sandwich testing procedures. A 50–80% through thickness penetration criteria is proposed following noted shortcomings in the standard originally intended for laminates. It is shown that the panel overall flexural rigidity to thickness ratio better describes the transition between ‘thin’ to ‘thick’ impact response; force measurements indicate that strain rate effects need to be considered; dent indentation can only be assessed through data acquistion; destructive damage observation sectioned specimen observations describe the impact damage sustained under the various indentor conditions, as well as the roles played by both the face composite skins and the core material.
High fibre content composite laminate skins use marine grade orthophthalic polyester, POLYLITE® 440-M850 resin and chopped strand mat/woven E-glass for the thin outer face skins and DIVINYCELL® H100 closed-cell linear PVC foam as core. Marine grade hybrid sandwich panels are designed in accordance with BS EN ISO 12215-5:2008 for small craft hull construction, using a wet lay-up and cured under vacuum pressure. Impact damage testing followed ASTM D7766-11 and ASTM D6264-98 procedures A and B for rigidly supported and simply supported sandwich panels. A review of the current state of standard testing procedures of marine sandwich panels is described. Testing using the default hemisphere indentor also included other standard rock geometries – conical, square-based pyramid and flat-faced cylindrical. The damage incurred under each variation of indentation impact is described, in terms of force, absorbed energy and indentation displacement. New contact laws are suggested for the different rock geometries. Destructive sectioning of the panels provides the visual damage incurred through the thin face skins and core and the roles played by all members comprising the sandwich panels.
Low weight and high load capacity are remarkable advantages of sandwich panels which make them more considerable by structure designers. In this paper, multi-objective optimization of sandwich panels with corrugated core is carried out using modified Non-dominated Sorting Genetic Algorithm (NSGAII) considering two objective functions: the structure’s weight and deflection. Deflection of the panel as one of the objective functions is calculated using finite element analysis by commercial software ANSYS. To employ this FE model in the multi-objective optimization process, the software products ANSYS and MATLAB have been coupled together during the run time. Finally, nearest to ideal point (NIP) method and technique for ordering preferences by similarity to ideal solution (TOPSIS) method are used to find the some trade-off optimum design points from all non-dominated optimum design points represented by the Pareto fronts.
The honeycomb sandwiches are widely used in the transportation engineering for the realization of lightweight and crashworthy structures. However their application requires a better understanding of their impact response. Aims of this paper are the numerical investigation of aluminium honeycomb sandwiches subjected to low-velocity impact tests and the validation of finite element (FE) results. Before and after the low-velocity impact tests at different velocities, three dimensional (3D) reconstructions of the honeycomb panels have been carried out by a computed tomographic system in order to acquire exactly the dimension and the shape of the damage and to obtain information about geometry and cells defects. The FE models have been computed from CT data of the undamaged panels. The direct comparison has been done by superimposing the deformed images obtained from FE analyses and from 3D CT space reconstructions. The numerical model was also validated comparing the FE results with experimental data.
Polymer–matrix composites are very popular due to their low cost and simple fabrication methods. While techniques and methodologies for composites design are emerging, the knowledge and understanding of machining issues lag far behind. The objective of the work is to analyze the influence of machining parameters on the material characteristics of hybrid-composite pipes. Turning operation was carried out as per the design of experiments by central composite design in hybrid polymer composite pipe made of carbon fiber and Aramid fiber. Based on the experimental results, regression analysis was conducted to determine the input–output relationships of the process. A mathematical model is developed, and the responses are predicted. The effects of each process parameter on the response were analyzed using response surface methodology. The process parameters were then optimized using genetic algorithm to yield minimum cutting force and minimum surface roughness (Ra).
The subject of the paper is about sandwich beam-column with a metal foam core. The global buckling phenomenon of an axially loaded member is investigated. Two analytical models of the beam-column are presented. In the first model, a classical one, the mechanical properties of the core are assumed to be linear. In the second model, the non-linear behaviour of the core is taken into account. Solutions of these two models are presented and the formulae for the critical loads are determined. The numerical finite element models of the beam-column are also elaborated and the linear buckling and the non-linear analyses are performed. The results of experimental tests on the aluminium sandwich beam-columns with aluminium foam core are described. Analytical, numerical finite element model and experimental values of the critical loads are compared.
Effects of random skin/core weld defects on out-of-plane tensile, compressive and in-plane shear behaviors of hexagonal cell aluminum honeycomb sandwich panels are investigated using ABAQUS/Explicit code. Firstly, a meso-structure model of the sandwich panels is constructed, which consists of the upper and lower skins, the honeycomb core and the skin/core welds. Defects are introduced by removing of skin/core welds, and the defect ratios are 1%, 2%, 4%, 8%, 16% and 32%. Next, the out-of-plane tensile, compressive and in-plane shear response and crushing of the sandwich panels under quasi-static loadings are calculated when different defect ratios are considered, respectively. Finally, the sensitivities of mechanical properties and failure behaviors of these defected sandwich panels are compared with that of an intact honeycomb sandwich panel. The numerical results indicate that defects introduced by randomly removing skin/core welds caused some mechanical properties a sharp decrease in the sandwich panels (e.g. a 1% defects caused more than 50% reduction in the out-of-plane tensile failure strain), while some properties exhibit a gradual decay, i.e. the compressive load plateau.
Foam core sandwich composites were subjected to creep to failure and cyclic (loading/unloading) creep in seawater in order to mimic an actual ship hull service lifetime scenario. In spite of minute water absorption observed in the creep to failure tests, about 15% higher deflection and over 50% reduction in the overall lifetime were observed in specimens subjected to seawater as compared with tests performed in air; which is explained in terms of various possible paths to interface cell wall collapse. In the cyclic creep testing performed in seawater, the specimens were loaded for 24 h while the unloading (relaxation) times were varied from 24 to 6 h. Significantly reduced life and extensive damage were observed under cyclic creep as compared with creep to failure specimens. Curiously, lifetime and number of cycles to failure were found to decrease as a function of increasing relaxation periods. Modes of failure were predominantly indentation and core compression.
The failure modes of open weave (latticed) sandwich skins under biaxial compression load are analysed. Due to the latticed structure of these sandwich face sheets, they are able to transfer loads in both directions. Tests show a distinct non-linear correlation between load and deformation. This non-linear load-deflection-behavior indicates that the failure may be induced by geometrical imperfections. Based on this assumption and measured geometrical imperfections, a mathematical model for the latticed face sheet consisting of beams under compression is derived. Several beams are combined within one or two cells of the honeycomb core to describe the failure modes. Furthermore, biaxial loading is analysed by using the finite element method. Experimental results are in good agreement with failure loads derived by analytical and by finite element method analyses as well.
An improved third-order shear deformation theory is employed to investigate free and forced vibration responses of functionally graded plates. A power law distribution is used to describe the variation of material compositions across the plate thickness. The governing equations for vibration analysis obtained using an energy approach are then solved using the Ritz method. Two types of solutions, temperature independent and dependent material properties, are considered. Many effects of the volume fraction index, temperature, material pairs, thickness, plate aspect ratio, etc., which have significant impact on dynamic behaviour of the plates, are considered in the numerical illustrations of free and forced vibration results. At high temperatures, it is observed that the maximum deflections of the functionally graded plates subjected to the dynamic loading increase with the increase of frequency ratio and temperature.
Polymer foams are frequently used as core materials in sandwich structures for applications such as aerospace, naval, and wind industry. It is known that the core material contributes to the overall mechanical properties of these sandwich structures up to a remarkable extent. In addition, due to the curvature and geometrical complexities of several applications, these cores are available with special cuts/grooves (finishing options) to provide bendability and better resin infusion during processing. The goal of this study is to investigate the mechanical performance of synthetic polymer foams as core materials for sandwich structures. Balsa wood, which is the most common traditional structural core, was used as reference material. Glass fiber reinforced epoxy was employed as face sheets. End-grain Balsa wood, commercially available polyvinyl chloride and polyethylene terephthalate, and experimental grades of polyurethane foams were chosen as alternative core materials. Quasi-static flexural tests were carried out using a four-point bending setup according to ASTM C 393. Sandwich properties such as stiffness, core shear strength, and energy absorption were determined and compared. The contour cuts filled with resin show a reinforcing effect against transverse shear stresses leading to higher shear strengths. Digital image correlation was used to study the yielding and permanent strain of the foam core sandwich beams.
Eco-Core is syntactic foam made by high volume percent of flyash and a small amount of phenolic resin. Because of very low volatile content in the mixture, it has demonstrated to be a fire resistant material for composite sandwich structural applications. Its superior mechanical and fire safety properties were established previously. Its fatigue performance under compression, shear and flexure loading are being investigated. Objective of this paper is to establish shear stress–life relationship and failure modes of sandwich beams. The Eco-Core sandwich specimens made of FGI 1854 glass/vinyl ester face sheet were designed to fail in core shear and were tested by shear fatigue loading at a frequency of 2 Hz and load ratio of R = 0.1. The Eco-Core density was about 0.5 g/cm3. The fatigue test was conducted at maximum shear stress (max) values of 0.7c to 0.9c using four-point bend load specimen, where c is the static shear strength of the core. Shear fatigue failure of the core material was found to be three types: shear crack on-set, crack propagation and ultimate shear failure (represented by shear crack linking to top and bottom face sheets and finally interface delamination). These failures were found to be represented by 2, 5 and 7% change in compliance. The fatigue stress–life (S–N) relationship was found to follow the power law equation, max/c = AoNα. Constants of the equation were established for all three modes of failure. Based on 1 million cycles limit, the endurance limit was determined to be 0.66c, 0.68c and 0.69c, respectively, for damage onset, propagation and ultimate failure. The typical shear fatigue failure was a 45° crack in the core, followed by crack propagation to face sheet and finally interfacial delamination between face sheet and core.
This paper reviews the most significant works in literature about the acoustic behaviour of sandwich panels, starting from the first examples of multi-layered structures, comprising a series of different layers enclosing an air-gap, to the actual configurations in which the opportunities of emerging manufacturing technologies are considered in the design stages. The focus is on presenting an exhaustive list of dedicated and validated models, which are able to predict the sound transmission through sandwich panels according to their specific configuration. Some experimental works, aimed to the model correlations, are reviewed, too.
Multi-component sandwich panels have emerged in recent years for new engineering and structural uses. These structures have been modeled as large deflections and multi-component sandwich beams subjected to pure bending with its ends with monolithic section. In the case of curved beams subjected to pure bending, a linear distribution has been considered. Different end-section reductions and sandwich–monolithic transitions have been analyzed with a finite element analysis model developed. Finally, the results of the finite element model are validated using an experimental test.