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This paper presents an analysis for the dynamic stability of sandwich beams/wide plates subjected to periodic axial loads. The formulation of the problem is done by use of the Extended High-order Sandwich Panel Theory (EHSAPT). The equations of motion are derived from Hamilton's principle and are expressed in terms of seven generalized displacements. A sandwich panel with simply supported edges is studied as an example, and the equations of motion for a given harmonic n are further derived. Two time-variations for the axial forces, namely, harmonic axial forces and step-wise periodic axial forces, are considered in this work. By considering the features of these two periodic load profiles, Floquet theory and Bolotin's method are adopted to perform the dynamic stability analysis. Sandwich panels with different face-to-core thickness ratios are studied. Stability maps for varying frequency and amplitude of the forces are presented. Numerical examples show that when a sandwich panel is subjected to periodic loads, it is possible that it can experience dynamic instability even when the dynamic loads are much lower than the static critical loads. [ABSTRACT FROM AUTHOR]

Honeycomb panels manufactured from wood and paper materials promote a high-value utilization of wood resources. However, the poor mechanical properties and uncommon thicknesses (32 and 40 mm) of paper honeycomb panels limit their application and promotion in the panel furniture industry. Hence, numerous attempts have been made to develop high-strength and low-density wood sandwich composites for the frames and partitions of cabinet furniture. This paper describes a method for manufacturing lightweight sandwich panels (18 mm thickness) with auxetic cores, made of fast-growing poplar. The mechanical properties of the panels and the negative Poisson's ratio of the perforated sheet cores were determined by three-point bending and uniaxial compression tests. The lower density, higher modulus of elasticity, and bending strength suggest that the developed panel is a good substitute for traditional wood-based panels. A numerical model for panels in bending tests is proposed and validated against the experimental analysis results. These investigations facilitate further research into optimizing the sandwich panel structure and its applicability in the architectural and furniture industry. Graphical Abstract [ABSTRACT FROM AUTHOR]

A homogenization method for the vibration analysis of periodic sandwich panels is presented. Periodic boundary conditions are applied to a representative volume element (RVE) of the structures. The complex responses of the RVE under eight sinusoidal excitations are calculated and the complex stiffness matrix of the sandwich panels is obtained by the direct-solution steady-state dynamic analysis (DSDA) using the finite element method (FEM) in the frequency domain. Based on equivalent single-layer (ESL) theory and finite element-modal strain energy (FE-MSE) method, the natural frequencies, the mode shapes and the loss factors of the structures are obtained by shell models. The effectiveness of the homogenization method is validated by using pyramidal lattice truss sandwich panels made of carbon fiber reinforced plastics (CFRP). The results obtained by the homogenization method are compared with those obtained by experiments and solid models. The effects of the number of cells and damping layers are discussed. [ABSTRACT FROM AUTHOR]

In this study, sandwich panels are presented with polylactic acid (PLA) core made through the FDM method. The face sheets of the panels are made out of aluminum 3105 alloy. The mechanical properties of the proposed samples were investigated under flexural quasi-static load. Different panels were tested by a three-point flexural test to determine their collapse properties; moreover, cubic specimens were exposed to quasi-static compression load. The results confirmed the effective role of layering type in the FDM method on the collapse properties of the sandwich panel. Results showed that type of the main pattern extension can affect the collapse properties. A comparison of the cubic samples under three different quasi-static compressive loading indicated that the collapse properties and absorbed energy of the samples depend on the loading direction. The proposed lightweight structures absorb high energy in comparison with ordinary one which was investigated in this paper; thus they can be an ideal structure for industries. Numerical modeling was another part of the study which was done by LS-DYNA and good agreement between numerical and experimental results was observed. [ABSTRACT FROM AUTHOR]

The object of this study is to investigate the influence of different fabrics' configurations, of faced polyester-sisal fiber composites, on the mechanical behavior of a vacuum infused polyethylene terephthalate-foam core sandwich panel. Sisal-unidirectional (yarn and combed yarn fabrics) and plain weave fabrics were fabricated and the sandwiches' performances were compared to faced polyester-unidirectional glass fabric. Comparisons about the morphological aspects and relations with mechanical behavior (flatwise and edgewise compression and 3-point bending) were performed. Different face sheets' thickness was used for this purpose, aiming to correlate the results found for those sisal and glass sandwich panels. The main results showed that faced sisal fabrics/polyester present competitive performance and sometimes higher than those glass/polyester, due to differences on failure mechanisms of the panels. [ABSTRACT FROM AUTHOR]

Hybridization of sandwich panels and their different components have drawn huge attention due to the significant improvement in their attributes. Hybrid core of 'Polyester Pin-reinforced Foam filled Honeycomb Sandwich panels' (PFHS) were fabricated and compared with unreinforced 'Foam filled Honeycomb Sandwich panels' (FHS) in terms of low velocity impact and Compression After Impact (CAI) performance. The impact damage area was calculated by employing MATLAB image processing technique. Incorporating through thickness pins for connecting faces and core is an effectual way to improve interfacial bonding, specific bending stiffness and also imparts out of plane properties for sandwich panels. The low velocity impact tests performed on the sandwich panels revealed that the polyester pin reinforcement in foam filled honeycomb sandwich panel improved the load bearing capacity, total absorbed energy and reduced the impact damage area significantly. In CAI test, debond, wrinkling of face sheet, and buckling of face sheet and core are the major modes of failure. The addition of the pins enhanced the compressive strength for all the impact energy levels. [ABSTRACT FROM AUTHOR]

This paper experimentally and numerically investigates the free vibration, quasi-static compressive and split Hopkinson pressure bar impact responses of hybrid composite pyramidal truss sandwich panels. Such sandwich panels made of carbon fibre composite face sheets and aluminium alloy pyramidal truss cores are fabricated using an interlocking and adhesive bonding approach. Modal tests and quasi-static compression tests are conducted. A good consistency for natural frequencies, modal shapes and static stress–strain curves of the specimen with the same specification is obtained, which ensures the good repeatability of the present specimens. Considering the effect of strain rate, a series of split Hopkinson pressure bar tests combined with numerical simulations is carried out to investigate their dynamic compression responses. A good agreement between simulation results and experimental data is observed, which shows that the adopted split Hopkinson pressure bar testing device and the modified Johnson-Cook model are reasonable and reliable. Results show that the dynamic compression modulus and strength of specimen are strongly influenced by the relative density of the truss cores and much higher than the corresponding static compression modulus and strength. Furthermore, it is also revealed that all the specimens have excellent energy absorption performance, which may have greatly advantage in shock isolation application. [ABSTRACT FROM AUTHOR]

Balsa wood is increasingly considered as a core material in sandwich panels for applications in several industries. However, there is still very limited information about its mechanical behaviour at elevated temperature. This article presents an experimental and analytical study about the effects of elevated temperature on the shear behaviour of end-grain balsa with nominal density of 109 kg/m3. The Iosipescu/V-notch test method was used to investigate the shear behaviour of balsa specimens extracted along the two material directions that are relevant for core materials subjected to shear: (i) transverse to the wood grain and (ii) parallel to the wood grain. The shear tests were conducted under steady-state conditions for temperatures between room temperature and 240°C. For the various temperatures, the stress vs. strain behaviour in shear was non-linear, with such non-linearity becoming more accentuated at higher temperatures. The shear modulus and strength presented a linear and comparable reduction with increasing temperature, and the residual properties at 240°C dropped to less than 20% of the room temperature properties. In the final part of the article, four alternative analytical models available in the literature were assessed regarding their ability to describe the reduction with temperature of the shear properties of balsa. All models were able to successfully fit the test data. [ABSTRACT FROM AUTHOR]

Sandwich structures comprising glass-fibre reinforced polymer faces and low-density core constitute an efficient and versatile constructive system for civil and ocean engineering structures. However, being multilayered with relatively soft core materials, they are particularly susceptible to damage under concentrated loads. Whilst numerous studies exist on the indentation and impact behaviour of sandwich composites, the great majority considers the thin-skinned laminates used by the aeronautical industry. To mitigate the lack of studies on the significantly thicker and more robust civil and ocean engineering sandwich laminates, the quasi- static indentation and low-velocity impact behaviour of such panels is experimentally studied. Three types of core materials (polyurethane and polyethylene terephthalate foams and end-grain balsa) and five different indenters, of varying shape (hemispherical versus flat) and diameter (10, 20 and 30 mm), are considered. Flat and larger indenters required higher loads and energies for first damage and perforation. The first damage and peak resistance values of the polyethylene terephthalate panels were, respectively, 15 and 8% higher than in the polyurethane panels; for the balsa panels, such figures were 20 and 10%. The polyurethane panels showed the highest energy absorption capacity. Predictions of first damage resistance given by two analytical models (for flat and hemispherical indenters) were assessed against the gathered experimental data. The obtained predictions were reasonably accurate, but indicate a need for further calibration, mainly concerning the effects of core material. [ABSTRACT FROM AUTHOR]

Sandwich panels can be subjected to significant changes in ambient temperature, which develop and sustain over certain time periods and lead to creep of the core material, and consequently to changes in the internal stresses and deformations with time. This paper deals with this issue with focus on the geometrically nonlinear aspects of structural behaviour. A theoretical model is developed, which combines the concepts of the principle of superposition of viscoelasticity, with the high-order sandwich theory (HSAPT), and the temperature dependency of the viscoelastic material properties. The nonlinear HSAPT formulation accounts for the deformability of the core in shear and through its thickness and it is based on large displacement kinematics of the face sheets. The convolution integral of viscoelasticity is converted into a rheological generalized Maxwell model after the expansion of the relaxation moduli into Prony series with temperature-dependence terms, which enables the solution of the governing equations through an incremental step-by-step time analysis without the need to store the response history. The capabilities of the model are demonstrated through numerical examples. It is shown that the creep of the core material can lead to bifurcation buckling of the sandwich panel under sustained temperatures that are smaller than the critical temperature obtained under an instantaneous increase of temperature. [ABSTRACT FROM AUTHOR]

In this paper, a numerical model is developed to simulate the ballistic impact of a projectile on a sandwich panel with honeycomb core and composite skin. To this end, a suitable material model for the aluminum honeycomb core is used taking the strain-rate dependent properties into account. To validate the ballistic impact of the projectile on the honeycomb core, numerical results are compared with the experimental results available in literature and ballistic limit velocities are predicted with good accuracy. Moreover, to achieve composite skin material model, a VUMAT subroutine including damage initiation based on Hashin's seven failure criteria and damage evolution based on MLT approach modulus degradation is used. To validate the composite material model VUMAT subroutine, the ballistic limit velocities of the projectile impact on the composite laminates are predicted similar to the numerical results presented by other researchers. Next, the numerical model of the sandwich panel ballistic impact at different velocities is compared with the available experimental results in literature, and energy absorption capacity of the sandwich panel is predicted accurately. In addition, the numerical model simulated the sandwich panel damage mechanisms in different stages similar to empirical observations. Also, the composite skin damages are investigated based on different criteria damage contours. [ABSTRACT FROM AUTHOR]

Failure and damage of crushable materials employed as core for the sandwich structures reduces serviceability and energy absorption capacity of the components especially under bending load so that many beneficial properties seem to be achieved by application of noncrushable lightweight materials instead of crushable foams as core for the sandwich structures. In this paper, an elastomeric foam is employed as core for two aspect ratios of the composite sandwich panels and the enhancement of the load-carrying capacity in the elastomeric foam-cored sandwich panels is investigated in comparison to which is measured about the individual composite panels applied as skins. Both experimental and finite element simulation programs are included in the research. The load-carrying performance of the elastomeric foam-cored sandwich panels is considered dependent on two main features of the constituent materials as hyperelastic behavior of the foam core and progressive damage of the composite skins which are simulated in the finite element models in order to describe the failure mechanism in the panels. Collapse of the elastomeric foam-cored sandwich panels is considered due to connection of some failure lines in the composite skins; however, the foam core remains undamaged. The elastomeric foam core can transfer the load from the top composite skin to the bottom one so that a great energy absorption capacity is provided for these panels. The elastomeric foam after failure of the composite skins can mobilize the residual strength of the laminates to endure against large deformations prior to final collapse. By application of the composite laminates in sandwich form with elastomeric foam core, the maximum load carrying and energy absorption capacity of the composite laminates increased about 60 and 110%, respectively. The results show more favorite failure behavior for the elastomeric foam-cored sandwich panels in comparison to which is expected usually for the crushable foam-cored sandwich panels which may be concerned in many industrial applications. [ABSTRACT FROM AUTHOR]

Hybridization of face, core and their combination is of great interest to develop high performance composite sandwich panels. In this regards, hybrid core of 'polyester pin-reinforced foam filled honeycomb sandwich panels' was fabricated and compared with unreinforced 'foam filled honeycomb sandwich panels' in terms of compressive and low velocity impact performances. MATLAB image processing techniques was applied to determine the impact damage area. Incorporating reinforcing pins for connecting faces and core is an effectual way to improve interfacial bonding, also imparts through thickness properties for sandwich panels. Compression tests performed on the sandwich panels revealed that the polyester pin reinforcement in foam filled honeycomb sandwich panel enhanced the load bearing capacity considerably. The low velocity impact properties such as load at initial damage, total absorbed energy were greatly improved and impact damage area reduced significantly by the addition of the pins. [ABSTRACT FROM AUTHOR]

Extensive studies have been previously carried out on the effects of various types of local damage on the performance of sandwich panels used in the hull structures of naval ships. More recently, the approach was adapted for application on board a specific ship series. Strength reduction data were obtained for a set of sandwich materials that were representative for the vessels in question. The face sheet materials were glass fibre-reinforced plastics with non-crimp fabrics and two different types of vinylester resin. The core materials were PVC foams. Tests were performed on laminate specimens with and without circular holes under tensile loading and on sandwich face sheets with holes, cracks and impact damage under compressive loading. The strength reductions caused by impacts with sharp and blunt objects were compared with those caused by machined cracks and circular holes, respectively, and with Whitney and Nuismer's point stress and average stress models for infinitely large laminates with cracks and holes. It was found that strength reductions due to impact damage can be estimated using tests on specimens with machined cracks and holes, and also with the average stress models if appropriate values of characteristic length are assumed. Special attention is paid to the need to take account of the geometry and the finite size of tested specimens. [ABSTRACT FROM AUTHOR]

The response and the debonding mechanisms in axially compressed sandwich panels with an interfacial delamination are investigated using a nonlinear model. The mathematical model combines the extended high-order sandwich panel theory with a cohesive interface modeling. It includes the first-order shear deformation kinematic assumptions for the face sheets and high-order small deformations kinematic assumptions that account for out-of-plane compressibility for the core. The interfaces bond the face sheets and the core by means of traction–displacement gap laws. These interfacial laws can describe a diversity of physical conditions. In particular, interfacial debonding nucleation and propagation are described using cohesive laws that introduce the interfacial nonlinearity into the model. Geometrical nonlinearity of the face sheets is introduced in order to capture the instability associated with the buckling of the delaminated face sheet. The cohesive interfaces and others parameters are calibrated to match experimental results taken from the literature for a sandwich specimen subjected to an end-shortening compression. The instabilities due to the in-plane compression, together with the existence of delaminated regions and their tendency to grow, prompt buckling of the delaminated face sheet as well as nucleation and propagation of the interfacial debonding. The theoretical quantification of this complex mechanism compares well with the experimental results in terms of the physical response, the nucleation and propagation of the interfacial crack, and the evolution of local/global geometrical instabilities. In addition, the analysis explores debonding mechanisms that are beyond the capabilities of the experimental technique. Finally, the sensitivity of the response and the associated geometrical and interfacial instabilities to the boundary conditions are investigated. [ABSTRACT FROM AUTHOR]

In this paper, the load-carrying capacity and failure mechanisms of sandwich beams and panels with elastomeric foam core and composite laminate face sheets are investigated. For this purpose, the flexural behavior of laminated composite beams and panels (applied as face sheets) is firstly investigated under three-point bending and central concentrated loads, respectively. Then, the same examination is conducted for the sandwich beams and panels, in which the proposed elastomeric foam is utilized as the core material. It is shown that the failure mechanisms which are associated to the core in the sandwich structures with crushable foams are not considered in the examined sandwich structures. The collapse of the sandwich specimens, examined here, is observed due to the failure of the skins in some steps. By multi-step collapse of these specimens via separately failure of the top and bottom skins, a considerable amount of energy is absorbed between these steps. Due to non-brittle behavior of the core material under loading, a large compression resistance is observed after failure of the top skin which led to the recovery of the load-carrying capacity in the sandwich beams. A similar behavior for the sandwich panels led to the increase of the ultimate strength after appearance of the failure lines on the top skin. The general outcomes of this investigation promise a good influence for the application of elastomeric foam as core material for sandwich structures. [ABSTRACT FROM AUTHOR]

Metallic thermal protection systems are used to protect the airframe and pay load from aerodynamic and aerothermal heating in hypersonic cruise vehicles that are powered with advanced scramjet engines. Metallic thermal protection systems is a composite structure that contains honeycomb sandwich panels at the top and bottom and a variety of thermal insulating materials placed in between them. Several design factors influence the structural and thermal performance of the honeycomb sandwich panels. Panel bending stiffness is one important structural property that is generally estimated using a destructive 3-point or 4-point bending test. In this study, a numerical model based on the impulse excitation nondestructive evaluation technique has been developed to estimate the effect of various design parameters that affect the bending stiffness of the honeycomb sandwich panels. The results obtained are analyzed using standard statistical procedures. A major advantage of this method lies in evaluating the panel stiffness at the design stage without resorting to actual fabrication of the panels for destructive testing. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The paper investigates the effects of creep of the core material on the geometrically nonlinear behavior of sandwich panels with debond at one of the core–face interfaces within the framework of the high-order sandwich panel theory (HSAPT). The theoretical model accounts for the viscoelasticity of the core via an incremental exponential law that corresponds to the rheological generalized Maxwell model obtained from the expansion of the relaxation moduli into Prony series. The HSAPT structural modeling accounts for the deformability of the core in shear and through its thickness, as well as for the large displacements of the facesheets. An incremental time-stepping analysis is conducted, which accounts for the time variations of the stresses, deformations, and the geometrically nonlinear effects. The incremental governing equations and the stresses and deformations fields through the thickness of the core are derived, and the capabilities of the model are studied numerically. The results show that creep of the core leads to significant variations in the deformations and stresses over time, which when combined with the geometrically nonlinear effects of the debonded facesheet, can have an important influence on the performance of sandwich panels under sustained loads. It is shown that stress relaxation due to creep and/or contact may restrain the buckling of the debonded facesheet in some cases. [ABSTRACT FROM AUTHOR]

A numerical model for the quasi-static indentation and compression-after-impact behaviour of a composite sandwich panel is presented, using cohesive surfaces for interlaminar damage prediction. Intra-laminar damage and core crushing is also included. The models show generally good agreement with experimental results for residual strength, performing best when two cohesive surfaces are used in the impacted skin, but tend to over-estimate the undamaged panel strength. Damage extent predictions from the indentation phase of the analysis are often quite poor, but do not necessarily correlate with the accuracy of the strength estimates. The model provides a promising basis for further development. [ABSTRACT FROM AUTHOR]

This paper presents the experimental results pertaining to the deformation, failure mode, failure mechanism and residual strength for the compression-after-impact tests of woven polymer-based foam-core sandwich panels. Sandwich panels with impact-type damage, inflicted via low-velocity impact, were tested to failure in compression. The optical strain measurement system ARAMIS was used to obtain the complete non-uniform strain and displacement fields of the damaged face-sheet. Consequently, damage in the face-sheets was characterized by fractography. It has been found that the dominant damage mechanism is kink band in the impacted face-sheets due to fiber plastic micro-buckling. Besides, the effects of impactor diameter, impact energy, face-sheet thickness and foam core thickness on the residual strength under in-plane compressive loading were studied. This study will be useful for characterizing and predicting the residual compressive strength and understanding the failure mechanism of woven polymer-based foam-core sandwich panel. [ABSTRACT FROM AUTHOR]

Asymmetric sandwich panels with skins of differing thickness are subjected to various degrees of damage via quasi-static indentation before compressive loading to failure. These are compared with panels with skins of equal thickness. The experiments show that the asymmetric panels experience an improvement in strength with small amounts of indentation compared with undamaged asymmetric panels, and for more severe damage, show greater residual strength than the symmetric panels. The two configurations are numerically modelled using Abaqus, including inter- and intra-laminar damage, and core crushing. The strength predictions from the models agree well with the experiments. [ABSTRACT FROM AUTHOR]

A series of drop weight tests were conducted to evaluate the dynamic flatwise compression strength and flexural strength of sandwich panels with a novel core structure. This sandwich core material, known as Norcore, consists of interconnected cells in a unique configuration of truncated pyramid with sloping cell walls. Core materials made of thermoplastic including virgin Lexan, polycarbonate, polycarbonate regrind, high-impact polystyrene, and acrylonitrile butadiene styrene were tested. The test results showed that these sandwich panels have good strength as well as energy absorption capacities. [ABSTRACT FROM AUTHOR]

The results of blast tests on sandwich panels with honeycomb cores are reported. Two core heights (13 mm and 25 mm) and two face sheet materials (glass fiber epoxy composite and aluminum alloy) were investigated. Increasing the core thickness reduced the permanent displacements exhibited by the sandwich panels. The panels with composite face sheets also exhibited smaller residual displacements than the aluminum face sheet counterparts. Damage took the form of core crushing, core shearing, debonding of the face sheet from the core, permanent displacement, cracking of the composite face sheets, and tearing. Higher damage levels were observed at elevated impulse levels. Load localization was found to concentrate the damage to the central portion of the panel, preventing the whole panel being employed in resisting the blast. [ABSTRACT FROM AUTHOR]

A new idea for fabricating a truss periodic cellular metal (PCM) is described. Namely, an octet-like truss PCM named 'M-octet' is fabricated by folding expanded metals. Its strengths under compression and shear load estimated by elementary mechanics are compared with experimental results and those of other competing cellular metals. When employed as a core in a sandwich panel, the load capacity subjected to bending load is predicted by an energy-based approach for various failure modes. For a given core height, a failure mode map is constructed with the nondimensional parameters such as geometric variables, material parameter (yield strain), load index, and weight index. Based on the failure mode map, three designs are chosen for maximum load per weight ratio. The predicted failure loads for the three different geometric designs are compared with the values measured through the experiments which are performed with sandwich specimens with the tetrahedral truss cores made of a wrought steel SS41. The measured equivalent normal yield and shear yield stresses agree fairly well with those predicted by the analytic solutions. In the three-point bending tests, the design with the thickest core truss and face sheets shows the peak load point delayed the most, which gives benefits in terms of the energy absorption and deformation stability after the peak load point. [ABSTRACT FROM AUTHOR]

A high-order theoretical approach for the free-vibration analysis of delaminated unidirectional sandwich panels with a compressible core is presented. The analytical approach accounts for the flexibility of the core in the vertical direction and the resulting high-order displacement, acceleration, and velocity fields within the core. The existence of delaminated regions along the sandwich panel and the various interfacial stress transfer conditions at the delaminated core-face interface are considered and studied. The analytical model developed in the article is applicable to any type of boundary conditions including different support conditions at the same section. The derivation of the model uses Hamilton's variational principle as well as the beam and lamination theories for the face-sheets and two-dimensional elasticity theory for the modeling of the core. The free-vibration problem is formulated and solved assuming a harmonic behavior in time and using the multiple shooting method along with the Newton-Raphson scheme for the solution in space. Numerical results that emphasize the influence of the boundary conditions, the effect of the delaminated regions, the influence of the contact conditions within the debonded region, and the high-order effects through the depth of the core are presented and discussed. The model and the solution procedure are verified through a comparison with finite elements analyses and with an analytical solution that is based on the Modified Galerkin method. The article closes with a summary and conclusions. [ABSTRACT FROM AUTHOR]

This study investigates several available sandwich beam theories for their suitability of application to one-dimensional sandwich plates with functionally graded core. Two equivalent single-layer theories based on assumed displacements, a higher-order theory, and the Fourier-Galerkin method are compared. The results are also compared with the finite element analysis. The core of the sandwich panel is functionally graded such that the density, and hence its stiffness, vary through the thickness. The variation of core Young's modulus is represented by a differentiable function in the thickness coordinate, but the Poisson's ratio is kept constant. A very good agreement is found among the Fourier-Galerkin method, the higher-order theory, and the finite element analysis. [ABSTRACT FROM AUTHOR]

This article presents a technique for the production of graded sandwich cores, and a study of the effect of lengthwise grading of core properties for the facilitation of transverse loading. A significant failure mode of sandwich panels is caused by indentation of the foam core in regions at which local forces are introduced into the sandwich panel. The aim of this work is to synthesize a microstructure that can diminish this failure mode. By applying a rapid prototyping technique a periodic, open beam structure is synthesized. Additionally, the structure has been graded in the longitudinal direction by alternating over the beam diameter. The structure obtained is joined to aluminium face sheets to form a sandwich beam. The panel has been subjected to a three point bending test and the results have been compared to a finite element model, and further indentation tests show an increased transverse stiffness in the strengthened part of the beam. [ABSTRACT FROM AUTHOR]

In this study a tool for assessing residual strength of debond damaged laterally loaded sandwich panels is developed. The analysis tool consists of a parametric finite element model and a fracture mechanics calculation procedure to determine the residual strength. The parametric approach allows variation of all geometric and material entities. The fracture mechanics calculation uses crack flank displacements obtained from the finite element analysis solution and experimentally measured mixed-mode fracture toughness values to determine the ultimate failure load. The analysis tool is validated with a number of different ship type panels. Debond criticality is evaluated by using the developed tool and by comparing the test results from panel experiments. The comparison shows that the analysis tool predicts both failure load and failure mode well. The tool can be used to determine the residual strength of different damage cases and has a considerable potential for further development. [ABSTRACT FROM AUTHOR]

The article presents the results of an investigation on the role of the localized effects in the geometrically nonlinear response of modern sandwich panels made of a 'soft' core when subjected to distributed loads. The adopted nonlinear analysis approach incorporates the effects of the vertical flexibility of the core, and it is based on the approach of the high-order sandwich panel theory (HSAPT). The nonlinear governing equations are solved using the multipoint shooting method along with parametric and arc-length continuation procedures. The nonlinear response of the distributed loads is described in terms of deflections and stress resultants in the face sheets, as well as in terms of the interfacial stress components at the upper and lower face-core interfaces. In addition, the equilibrium curves of the load versus the extreme absolute values of the aforementioned structural quantities are presented. The numerical study investigates the localized effects involved in the case of a panel made of a core with uniform properties and a core that is reinforced at its mid region when subjected to a uniformly distributed loading. One of the findings of this research is that a typical modern panel exhibits a limit point response even when subjected to uniformly distributed loads that cause bending. The material discontinuity of the core as well as the buckling of the compressed face sheet leads to localized effects that significantly affect the nonlinear response. [ABSTRACT FROM AUTHOR]

Axisymmetric and non-axisymmetric modes of vibration of circular sandwich panels are investigated by applying first-order shear theory (Mindlin-Reissner Plate). Solutions are obtained for boundary conditions with free, simply supported, and clamped edges. Numerical finite element method (FEM) simulation using ABAQUS is applied to validate the analytical modeling. Both analytical and numerical calculations are compared with experimental measurements of the first few modes of vibration; the frequency differences are less than 4%. [ABSTRACT FROM AUTHOR]

Damage characteristics of composite-skinned honeycomb sandwich panels in bending are investigated with both hemispherical (HS) and fiat-ended (FE) indenters. The thickness of the cross-ply skins varies from 8 to 16 plies, whereas the density of the 12.7-mm thick aluminum honeycomb core varies from 50 to 70 kg/m³. Clamped panels with a 100-mm testing area are loaded quasi-statically either in bending or on a rigid base. The effects of varying these parameters on damage mechanisms are examined through response curves as well as cross sections of selected specimens. Special emphasis is placed on their potential change induced by the variation of skin thickness and core density with a specific indenter. Damage mechanisms are identified as core crush, top-skin delamination, and fracture or shear-out. The threshold and ultimate loads as well as the initial slope increase significantly either on increase of skin thickness or change of the nose shape of indenter from a hemisphere to a flat-end. The increase in the post-initial-damage slope is small and can be attributed to membrane stretching of the damaged top skin. Increasing the core density affects substantially not only the threshold load, but also the initial slope associated with the FE indenter. Changing the nose shape of the indenter has an overriding effect on the nature of damage mechanisms. In particular, top-skin delaminations occur after core crush. The panel deflection contributes to 20–53% sandwich deformation. The bottom skin in all the tests remains intact. [ABSTRACT FROM AUTHOR]

An experimental study of the in-plane compression failure of sandwich panels consisting of glass/epoxy face sheets over a range of PVC foam cores (H45, H100, and H200) and a balsa wood core containing one or two circular or square interfacial debonds is conducted. For the great majority of the specimens, failure occurred by local buckling of the debonded lace sheet followed by rapid debond growth towards the panel edges, perpendicular to the applied load. Panels with the largest debond (10cm diameter or 9cm length) displayed some post-buckling strength. Examination of the face and core failure surfaces after total separation showed that the tendency for interface (core/resin) failure increases with increasing core density. It was found that the compression strength strongly decreases with decreasing core stiffness and increasing debond size. The compression strength of panels with H45 core decreased with reduced core thickness. Failure loads for panels with symmetric debonds at both face/core interfaces were practically the same as those for panels containing a single debond. Panels with square debonds failed at lower loads than those with circular debonds of the same area. Circular debonds of 50 mm diameter and square debonds of 45 mm length set the threshold for debond failure of the panels considered here. [ABSTRACT FROM AUTHOR]

Face/core debond-damaged sandwich panels exposed to non-uniform compression loads are studied. The panel geometry is rectangular with a centrally located circular debond. The study primarily includes experimental methods, but simple finite element calculations are also applied. The complexity of applying a controlled non-uniform compressive load to the test panels requires a strong focus on the development of a suitable testrig. This is done by the extensive use of product development methods. The experimental results based on full-scale testing of 10 GFRP/foam core panels with prefabricated debonds show a considerable strength reduction with increasing debond diameter, with failure mechanisms varying between fast debond propagation and wrinkling-introduced face compression failure for large and small debonds, respectively. Residual strength predictions are based on intact panel testing, and a comparison between a simple numerical model and the experimental results shows fair agreement. [ABSTRACT FROM AUTHOR]

This article describes the development, validation and application of a FEM based numerical model for prediction of residual strength of damaged sandwich panels. The core of the theoretical method is a newly developed procedure for prediction of the propagation of a face-core debond. As demonstrated, the method can predict the maximum load carrying capacity of real-life panels with debond damages, where the failure is governed by face-sheet buckling followed by debond growth. The developed theoretical procedure is an extension of the as Crack Surface Displacement method, here denoted as the Crack Surface Displacement Extrapolation method. The method is first developed in 2D and then extended to 3D by use of a number of realistic assumptions for the considered configurations. Comparison of the theoretical predictions to a series of large-scale experiments, described in [17], shows that the model is indeed able to predict the failure modes and the residual strength of damaged panels with accuracy sufficient for practical applications. This opens up for a number of important engineering applications, for example risk-based inspection and repair schemes. [ABSTRACT FROM AUTHOR]

Based on sandwich panel penetration experiments and the observed damage patterns, as described in Part I of this article, the individual energy contributions are estimated from simple physical models. It is shown that the most important contributions are from membrane-state fiber stretching, core compression, and friction between core material and impactor. Lesser contributions are from delaminations, core fracture, and debonding between core and back face-sheet. Addition of the energy absorption contributions generally give smaller total absorption than measured; the most likely reasons for this discrepancy are outlined in the conclusion. [ABSTRACT FROM AUTHOR]

Part I of this article describes a series of tests of localized, penetrating impact on sandwich panels. The sandwich panel size was 500 × 500 mm² with a thickness of 46 mm. Three types of panel configuration were tested. Penetration was carried out in quasi-static conditions and at approximately 70 and 93 m/s. Impactor mass was 1 kg with an impactor diameter of 50 mm and with three different impactor tip geometries. For each of the 27 combinations, the total energy absorption was measured, and the damage patterns are described and quantified. [ABSTRACT FROM AUTHOR]

The problem of face wrinkling in sandwich structures was identified as an important failure mode and first analyzed in 1940 by Gough et al., who used a Winkler-type elastic foundation to model the core. This work was followed by experiments and further analysis by many others. Until 1966 all of the research was devoted to uniaxial compression loading. However, in many applications, such as naval and aircraft structures, panels are subjected to biaxial loading. Such loadings were first analyzed by Plantema in 1966 for the case of sandwich constructed of isotropic materials. Sullins et al. suggested an interaction equation that can be used as a criterion of wrinkling under compression in the principal directions. This equation is formulated in terms of the ratios of the principal compressive stresses to the corresponding wrinkling stresses. More recently Fagerberg and Vonach and Rammerstorfer considered the case of sandwich with orthotropic facings. The present work attacks the subject problem using three different models for the core. One model uses a simple Winkler elastic foundation approach. The second model, following Hoff and Mautner [Hoff, N.J. and Mautner, S.E. (1945). The Buckling of Sandwich-Type Panels, Journal of the Aeronautical Sciences, 12: 285-297.], uses a linear model for the decay in deformation from the core-facing interface, while the third model, following Plantema [Plantema, F.J. (1966). Sandwich Construction, John Wiley & Sons, Inc., New York.], uses an exponential decay. [ABSTRACT FROM AUTHOR]

The bending behavior of unidirectional singly curved sandwich panels with a "soft" core and delaminated (debonded) zones or slipping layers is described herein. The delaminated zone is assumed to exist through the width of the panels, and is in the form of an interfacial debonded zone at one of the face-core interfaces, thus subdividing the length of the panel into three zones, i.e. two fully bonded zones and one that is debonded (delaminated). The formulation uses an interracial spring description, i.e. distributed shear and radial normal springs at the face-core interfaces, to model the effects of the slipping layers and the various delaminated (debonded) zones. The formulation is based on a closed-form high-order approach (HSAPT) that incorporates the high-order effects of the "soft" core. The variational approach is used to derive the governing equations along with the general explicit boundary and continuity conditions. A typical curved sandwich panel with an inner or an edge delamination, with or without contact, subjected to a distributed load and pinned supports is discussed. A comparison between the various delamination types and the fully bonded case is presented in terms of the displacements, stresses and internal stress resultants of the faces and in the vicinity of the edges of the delaminated zones. Finally, the effect of slipping interfacial layers, with or without contact when subjected to a distributed load, is considered and compared with the behavior of a fully bonded sandwich panel. From the numerical studies it is found that the membrane action of the face sheets plays a major role in the overall structural resistance of the sandwich panel compared to that of flat panels. However, in the near vicinity of the edges of the debonded zone, the membrane action does not prevent the interfacial stresses to reach very large values in the case of delamination without contact. For all of the delamination types, contact exerts a favorable effect, in the sense that the induced interfacial stresses are much smaller when compared to cases of delamination without contact. [ABSTRACT FROM AUTHOR]