Although X-radiography has been widely used in studies of damage in composite materials, micro-tomography has received less attention, due to the high cost and limited availability of equipment. However, micro-tomographic analysis is a powerful tool, allowing 3D images of sections through sandwich materials to be obtained without specimen preparation. Four examples from a recent study are presented here, which show how micro-tomography can provide unique information on the quality of honeycomb sandwich structures, information on damage mechanisms and internal details of through thickness reinforcement in pinned foam cores.

Foam core sandwich composite panels repeatedly slammed onto the body of calm water indicate greater accumulation and progression of damage accompanied by lower lifetimes as a function of increasing slamming energy or lower deadrise angle. E–N (slamming energy vs. lifetime) curves showed an exponentially decreasing trend with extensive scatter in the data. E–N curves also differed dramatically when compared with the conventional S–N fatigue life curve. A significant reduction observed in fatigue life of slammed specimens as compared with the nonslammed specimens was used as a basis for the development of a life assessment methodology. Catastrophic failure under slamming resulted from a major crack formation near the chine or the keel depending on the deadrise angle, however, during post-slamming fatigue, failure always occurred near the keel. Core tearing along the interface and core shear were observed to be the dominant modes of failure, while facesheet damage activity was largely absent prior to catastrophic failure.

The free vibration analysis of rectangular sandwich plates with a flexible core both with and without a uniformly distributed attached mass on the top facesheet is carried out using the finite element method (FEM) through ANSYS parametric design language, which is validated by several literatures through some numerical examples. The model uses the 8-node shell 99 element to model the composite laminates of the top and bottom facesheets of the sandwich plate and 20-node high-order solid 95 element in order to model the flexible PVC core. The validated finite element model is then used to study the parametric effects of geometry such as aspect ratio, length-to-thickness ratio, core thickness-to-plate thickness ratio, the size and the stiffness of the attached mass on the natural frequencies of the sandwich plate as well as the normal and shear stresses. The use of FEM also allows studying the effect of different boundary conditions for both the top and the bottom facesheets of sandwich plates with or without distributed attached mass. Numerical results that hitherto not reported in the literature have been presented in this article. The results presented in this investigation could be useful to acquire a better insight into the behavior of sandwich laminates carrying attached mass for engineering designers of sandwich structures. The results are presented and compared with the latest Numerical results found in literature.

Static indentation responses of foam sandwich beams and plates reinforced by fiber columns are investigated experimentally and theoretically. Based on the superposition principle, a new model is established for predicting the indentation response of sandwich beams. It should be pointed out that this model does not need to calculate the strain energy stored in the structure, which is usually difficult to be determined. For traditional foam sandwich beams and foam sandwich beams reinforced by fiber columns, the analytical predictions of indentation behaviors well agree with experimental measures. Furthermore, the analytical solution of indentation response of foam sandwich plate reinforced by fiber columns is derived by the principle of minimum energy and is compared well with experimental results.

The fracture response of a sandwich panel, with a center-cracked core made from an elastic-brittle square lattice, is explored by finite element simulations and simple analytical models. First, predictions are given for the unnotched strength of the sandwiched core and for the fracture toughness of the lattice under remote tension, remote compression, or remote shear. It is assumed that the lattice fails when the local stress in the cell wall attains the tensile or compressive strength of the solid, or when local buckling occurs. Failure maps are then constructed for a cracked sandwich panel, with axes given by a dimensionless crack length and dimensionless height of the sandwich core. The shear strength of the cracked sandwich panel is examined in detail. The precise form of the failure map for shear loading depends upon the tensile failure strain of the solid and upon the relative density of the lattice. The relevance of the shear failure map to lattices made from a wide range of engineering materials is illustrated through material-property charts. An extension of the method to cyclic loading is discussed.

In this article, we present a new finite element model for the analyzis of active sandwich laminated plates with a viscoelastic core and laminated anisotropic face layers, as well as piezoelectric sensor and actuator layers. The model is formulated using a mixed layerwise approach, by considering a higher order shear deformation theory to represent the displacement field of the viscoelastic core and a first-order shear deformation theory for the displacement field of the adjacent laminated anisotropic face layers and exterior piezoelectric layers. Control laws are implemented and the model is validated using reference solutions from the literature, and a benchmark application is proposed.

In this article, the optimization of the face sheet layers is carried out finding a spatially variable distribution of the stiffness properties that minimize the interlaminar stresses. The optimal orientation of the reinforcement fibers is the solution to the Euler–Lagrange equations representing the stationary conditions for the energy contributions under in-plane variation of plate stiffness coefficients. A refined multilayered plate model with a high-order, piecewise variation of displacements across the thickness is employed, that fulfill both the interfacial stress and displacement contact conditions, with the purpose to accurately and efficiently simulate the interlaminar stresses at the interfaces with the core.

Foam core sandwich composites with various facings composed of glass, carbon woven, carbon/Kevlar hybrid, and Kevlar fabrics were fabricated by using RTM process, as well as the through-thickness stitched sandwich samples with glass fabrics; and impact performance was studied at three energy levels. This article proposes that the damage extent of foam core sandwich construction may be characterized by the average damage angle, penetration depth, and maximal cracking width. The results show that the foam core samples with Kevlar facing are optimal for the peak load at load–time plots and the lowest impact damage extent at the same energy level. Compared to the unstitched samples, the average damage angle of stitched samples increases by 48%, the maximal cracking width and penetration depth of stitched samples decrease by 67% and 4% at 25J impact energy level.

The aim is the fabrication and mechanical testing of sandwich structures including a new core material known as fiber network sandwich materials. As fabrication norms for such a material do not exist as such, the primary goal is to reproduce successfully fiber network sandwich specimens. Enhanced vibration testing diagnoses the quality of the fabrication process. These sandwich materials possess low structural strength as proved by the static tests (compression, bending), but the vibration test results give high damping values, making the material suitable for vibro-acoustic applications where structural strength is of secondary importance e.g., internal paneling of a helicopter.

Core with a high load-bearing capacity can be advantageous in improving the overall performance of a sandwich panel. For periodic cellular metal (PCM) cores in shapes such as an octet, pyramid, or Kagome truss, higher load capacity could be achieved by increasing the relative density of the core. Namely, short and thick truss struts composing the truss PCM results in high load capacity. However, the limit inherent in the topology or fabrication process of truss cores has been an obstacle to designing high-strength sandwich panels with single-layered truss PCM cores. In this work, a truss PCM core with new topology, essentially a variation of a conventional pyramidal truss, is introduced. With the new type of core structure, it is possible to increase the relative density of the core more easily, and consequently achieve higher load capacity of the core. Analytic solutions for normal and shear strength are derived, and compared with those for a pyramidal core. Experimental results are presented to show the structural performance of the new core structure under out-of-plane compression and in-plane shear loading. The design flexibility of sandwich panels with the new core is demonstrated, and potential applications are discussed.

A design analysis of the mixed mode bending (MMB) sandwich specimen for face–core interface fracture characterization is presented. An analysis of the competing failure modes in the foam cored sandwich specimens is performed in order to achieve face–core debond fracture prior to other failure modes. The analysis facilitates selection of the appropriate geometry for the MMB sandwich specimen to promote debond failure. An experimental study is performed using MMB sandwich specimens with a H100 PVC foam core and E-glass–polyester faces. The results reveal that debond propagation is successfully achieved for the chosen geometries and mixed mode loading conditions.

The magnitude of the first natural frequency is often used as a criterion of the design efficiency of sandwich panels. The vibrations of fully clamped sandwich plate are analyzed using Galerkin method. The effects of the panel dimensions, elastic properties and densities of the facings, and core on the fundamental frequency have been studied using the numerical solution, analytical approach, and finite element analysis. It follows from the analysis that the first natural frequency can be calculated with sufficient accuracy on the basis of the analytical solution developed in this work.

Foam core sandwich composite panels were slammed onto the body of calm water as a function of slamming energy (161–779 J) and deadrise angle (0°–45°). Higher slamming energy and lower deadrise angle resulted in greater damage to the material. Discrete pressure data though critical in ship design, failed to yield any relevant damage information. Catastrophic failure was observed to occur beyond a threshold strain of 0.0035 mm/mm. With the introduction of a hydroelasticity function, quasi-static analysis accurately predicted strain behavior under slamming. Post-failure analysis of noncatastrophically damaged specimens indicated very little reduction in flexural capacity in spite of a measurable change in the acoustic emission activity. Core shear along the interface with the facesheets and local buckling of the facesheet and resin fragmentation were observed to be the dominant modes of failure under slamming.

The article concerns failure and fatigue phenomena associated with local effects occurring in the vicinity of junctions between different core materials in sandwich beams subjected to transverse shear loading. It is known from analytical and numerical modeling that these effects lead to stress concentrations at such junctions. However, their influence on the failure behavior is not fully understood, and there are indications that the available models overestimate the importance of the local effects. In the present article, typical sandwich beam configurations with glass fiber-reinforced plastic face sheets and core junctions between polymer foams of different densities and rigid aluminium were tested under quasi-static and fatigue loading conditions. The failure behavior was compared with results from finite element analyses using various failure criteria. It was found that for the transverse shear load case the inherent core shear stresses overrule the local effects in terms of being most critical with respect to causing failure for the most common sandwich configurations using low density semi-brittle polymer foam cores. A simple maximum shear stress criterion is appropriate for failure prediction (core shear failure) in those cases. However, if very strong core materials and thin face sheets are used, the stress concentrations induced by local effects can cause failure in the faces. Accordingly, the local face stresses can be of importance and should be taken into account for failure prediction.

A new crack arrester was proposed, in which a different material with higher stiffness is installed on the crack propagation path. The effect of this crack arrester was experimentally evaluated for interfacial crack propagation between a carbon fiber reinforced plastic surface skin and a foam core. The experimental results indicated that the crack arrester increased the critical load of the crack growth, and approximately five times larger apparent fracture toughness was obtained near the leading edge of the arrester by considering the energy release rate. It was also confirmed that the fabrication of the crack arrester had no detrimental effect on the intrinsic properties of the sandwich panel structures.

The remarkable damping over a broad temperature range and thermal insulation properties of cork make it a suitable material to be applied on integrated and surface damping treatments in sandwich structures, improving its dynamic behavior. Experimental analysis and numerical modeling of sandwich structures with cork compound layers is therefore essential for a better understanding of the cork compound influence on the dynamic properties of a layered structure. In this article, an evaluation study on the dynamic properties of a set of sandwich plates with cork compound cores inside two aluminium faces is performed. For this purpose, three test samples were assembled following the described configuration, using cork compounds with different properties (density, granulometry and thickness). To numerically simulate these layered plates, a partial layerwise plate finite element (FE), with a multilayer configuration, was developed and integrated in a MATLAB FE code. The constitutive relation of the cork compounds is included in the FE model by using the material complex modulus in a direct frequency analysis procedure. For the different cork compounds hereby considered, the extensional complex modulus was previously identified by using a specific experimental methodology which simulates a semidefinite two degrees of freedom system, where the cork compound test sample represents the complex stiffness. From the complex modulus data, both extensional storage modulus and loss factor of the cork compound were obtained. The experimental evaluation of the dynamic properties of the sandwich plates was performed carrying out an experimental modal analysis on each test specimen, being measured a set of frequency response functions (FRFs). Additionally, the developed layerwise plate element was validated through the comparison between the measured driving point FRFs and the FE method predicted ones.

The behavior of a repaired sandwich beam subjected to four-point bending is investigated numerically using the ABAQUS® software and specially developed interface elements including a cohesive mixed-mode damage model based on the indirect use of fracture mechanics. The two major repair configurations for sandwich structures, namely overlap and scarf repair, are studied. The interface elements, placed at the middle of the adhesive, parent laminate/adhesive and adhesive/patch interfaces, allow to obtain stress distributions at these locations as well as to simulate damage onset and growth. The influence of several geometrical parameters, such as overlap length and patch thickness for overlap repairs, and scarf angle for scarf repairs, is evaluated in terms of stress analysis and strength predictions. Conclusions were drawn about design guidelines of the sandwich composite repair.

The sandwich panels with viscoelastic cores, which represent the physical application of the viscoelastic integrated damping treatment concept, associate different materials, each one having a specific structural contribution, where the outside faces, usually made from a stiff material, guarantee the stiffness of the composite structure whereas the viscoelastic and soft core provides the damping capability. The application of soft cores, specially the thick ones, into sandwich plates produces an important decoupling effect, leading to a significant flexural stiffness reduction of the sandwich plate, as experimental and numerical results evidence. From this observation and pursuing a solution to minimize such effect, the partitioning of the core layer into multiple layers separated by thin constraining layers is hereby considered. Taking advantage of the application of the multiple viscoelastic layers in the sandwich core, it is also analyzed the potential use of different viscoelastic materials in order to spread out the efficient temperature range of the damping treatment. To verify and evaluate the effects of the multilayer and multimaterial viscoelastic cores in sandwich panels, an experimental and a numerical study were conducted on specimens representative of these design concepts. The results achieved from this study demonstrate the applicability of the two multiple layer configurations, evidencing the effect of the partitioning procedure of the core onto the reduction of the bending stiffness decay and the efficient temperature range enlargement when adopting viscoelastic materials with different transition temperatures.

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.

The purpose of this work is to use a meshless collocation method with multiquadric radial basis functions (RBFs) and optimal values of the shape parameter in the RBFs to analyze static deformations of sandwich and composite plates. Two shear deformation theories with the same number of degrees of freedom are tested (the third-order shear deformation theory of Reddy (TSDT) and a trigonometric layerwise theory). Although the TSDT proved to be most adequate for the analysis of composite plates, the same is not true for sandwich plates, specially in the case of large ratios of material properties between the core and face sheets. The trigonometric layerwise theory produced excellent results for both sandwich and composite plates. The multiquadric RBF method was introduced by Kansa [1,2] for solving boundary-value problems governed by partial differential equations. Here we show that this method with optimal values of the shape parameter gives deflections of sandwich and composite plates that agree very well with analytical solutions, for regular and irregular grids. An advantage of the meshless method is that it requires very little input data, thus the time required for preparing the data can be significantly reduced.

Based on the natural neighbor radial point interpolation method (NNRPIM), a 3D analysis of thick composite laminated plates is presented. The NNRPIM uses the natural neighbour concept in order to enforce nodal connectivity. Based on the Voronoï diagram small cells are created from the unstructured set of nodes discretizing the problem domain, the ‘influence-cells’, which are in fact influence domains entirely nodal dependent. The Delaunay triangles, the dual of the Voronoï cells, are used to create a node-dependent background mesh used in the numerical integration of the NNRPIM interpolation functions. The NNRPIM interpolation functions, used in the Galerkin weak form, are constructed in a process similar to that in the radial point interpolation method (RPIM) with some differences that modify the method performance. In the construction of the NNRPIM interpolation functions, no polynomial base is required and the used radial basis function (RBF) is the multiquadric RBF. The NNRPIM interpolation functions possess the delta Kronecker property, which simplifies the imposition of the natural and essential boundary conditions. In this work the 3D NNRPIM analysis is used to solve static and dynamic composite laminated plate problems. Thus, several benchmark examples are studied to demonstrate the effectiveness of the method.

A range of metallic lattice structures were manufactured using the selective laser melting (SLM) rapid prototyping technique. The lattices were based assemblies of repeating unit-cells with their strands oriented at 0°, ±45°, and 90° to the vertical when viewed from the front. Mechanical tests on the strands and the lattice blocks showed that these systems exhibit a high level of reproducibility in terms of their basic mechanical properties. An examination of the compression failure mechanisms showed that the [±45°] and [±45°, 90°] lattices failed in bending and stretching modes of failure, whereas the [0°, ±45°] lattices failed as a result of buckling of the vertical pillars. Sandwich structures were manufactured by binding woven carbon-fiber reinforced plastic to the lattice structures. Subsequent three-point bend tests on these structures identified the principal failure mechanisms under flexural loading conditions. Here, cell crushing, hinge rotation, and gross plastic deformation in the strands were observed directly under the point of loading. Low-velocity impact tests were conducted on the sandwich beams and a simple energy-balance model was used to understand how energy is absorbed by the sandwich structures. The model suggests that the majority of the incident energy of the projectile was absorbed in indentation effects, predominantly in the core material, directly under the steel indenter.

This article reports on work toward developing a practical methodology for the predictive modeling of the performance of thermoplastic composite (TPC) sandwich structures under impact loading. Explicit finite element analysis methods, using LS-DYNA® software, are described. Details of extensive materials characterization tests and material model parameter calibration for both the composite skin and polymer foam core are included. The simulations of deformation response, damage and failure of the sandwich structures is validated against experimental tests of the indentation and three-point bending of TPC sandwich beams. Good agreement between simulations and experiments has been achieved for indentation loading up to high degrees of core crush. The same is true for a significant part of the bending response including failure prediction. However, it has been necessary to introduce a principal strain fracture criterion to account for core shear and skin–core debonding failures at higher strains. For full predictive capability in this region, further experimental work is needed to obtain the necessary strain rate-dependent fracture data for the core and interface.

This study provides a detailed consideration of five manufacturing options that are used to produce aerospace sandwich panels used in secondary structure. The structural performance of each of the manufacturing options is considered along with a cost analysis. By considering the traditional preimpregnated (prepreg), autoclave-cured process, the sources of cost have been investigated, and it has been shown that by removing a portion of the large labor content and the autoclave cure, in favor of an oven-only cure, it would be possible to make significant savings. Monitoring the time to manufacture representative full-scale sandwich panels using the five manufacturing options has shown that by using a resin film infusion (RFI) oven cure, a 30% reduction in time to production is possible.<?tlsb> To make an initial assessment of the comparative structural performance of laminates produced using the five manufacturing options, this article also presents results of material quality, in-plane and out-of-plane loading tests. The results of these tests show that the laminates produced using RFI are comparable in quality and performance to laminates produced using the current aerospace industry standard prepreg/autoclave process.

This article describes an experimental investigation of characterization methods for ductile core materials. Full-field optical strain measurement methods are used to determine the strain distributions in standard testing methods such as block shear and four-point beam testing, particularly for highly ductile cores subjected to large deformations. The results show that the stress and strain fields in both block shear and sandwich beam tests are very different to those assumed by the testing standards. The test methods result in complex post yield states of stress in the core materials, meaning the core shear strength and ultimate shear strain should not be calculated by classical methods in the post yield region.

A finite element model based on an improved higher order zigzag plate theory developed by the authors is refined in this study and applied to bending and vibration response of soft core sandwich plates. The theory satisfies interlayer transverse shear stress continuity including transverse shear stress free condition at the plate top and bottom surfaces and transverse normal compressibility of the core. The in-plane displacements vary cubically through the entire thickness, while transverse displacement is assumed to vary quadratically within the core. In order to have a better computational benefit, a C⁰ finite element formulation is adopted. This is refined through satisfaction of certain constrains variationally using a penalty function approach. The performance of the model is demonstrated by comparing the present results with 3D elasticity solutions and other available results.

The scope of this study is to examine the failure response in an aluminum–glass–epoxy sandwich composite plate with two serial circular holes, which is subjected to a traction force by two serial bolts. To determine the effects of joint geometry and applied/without preload moments on the bearing strength and failure mode, parametric studies were carried out experimentally. The end distance to diameter (E/D) and width to diameter (W/D) ratios in the specimens were designed from 1–5 to 2–5, respectively, whereas the distance between two serial holes and diameter ratio were fixed as 4. Besides, each type of specimen were tested under various applied preload moments as 2, 3, 4, 5 Nm and without preload moments (0 Nm). According to the experimental results, the bearing strength and maximum failure load generally increase by increasing E/D and W/D ratios and of applied preload moments. Besides, failure modes strictly affected these parameters.

The new 3D models of the prestressed symmetrical and unsymmetrical ultrasonic sandwich transducers are described in this article. The sandwich transducers are modeled through the application of a 3D matrix model of piezoceramic rings and discs, recently proposed by the authors. Theoretical analyses prove the proposed models as simple and easy to calculate, particularly suitable for the investigation of coupled vibrations cases. Experiments show that the measured resonance and antiresonance frequencies are in good agreement with the predicted results. Compared with traditional 1D models and methods, the results obtained by the proposed models matched better with the experimental results.

Sandwich panels manufactured using thermoplastic fiber-metal laminates (FML) skins and an aluminum foam core were tested under quasi-static and low-velocity impact loading conditions. The quasi-static properties of the sandwich beams were evaluated using the three-point bend test geometry. Energy absorbing mechanisms such as buckling and interfacial delamination in the FML skin, as well as indentation, crushing, and densification in the aluminum foam have been observed to contribute to the excellent energy absorbing characteristics offered by these systems. The low-velocity impact behavior of the sandwich panels was evaluated using an instrumented dropping weight impact tower and modeled using an energy balance approach. A breakdown of the energy absorption revealed that these sandwich structures absorb much of the impact energy due to contact and bending effects. Finally, four-point bend testing after low-velocity impact revealed that these systems offer excellent residual flexural strength with relative values remaining close to 80% of the original strength after a 32 J impact.

The thermomechanical bending response of nonsymmetric sandwich plates of uniform thickness (constant depth) is studied. A power-law distribution for the mechanical characteristics is adopted to model the continuous variation of properties from those of one component to those of the other. The nonsymmetric sandwich plate faces are made of isotropic, two-constituent (ceramic–metal) material distribution through the thickness. The core layer is still homogeneous and made of an isotropic metal material. The modulus of elasticity, Poisson's ratio of the faces and the thermal expansion coefficient are assumed to vary according to a power law distribution in terms of the volume fractions of the constituents. Several kinds of nonsymmetric sandwich plates are presented. Field equations for functionally graded nonsymmetric sandwich plates whose deformations are governed by either the shear deformation theories or the classical theory are derived. Displacement and stress functions of the plate for different values of the power-law exponent and thickness-to-side ratios are presented. The results of the shear deformation theories are compared together. Numerical results for deflections and stresses of functionally graded metal–ceramic plates are investigated.