Sandwich Structures

A full mechanical characterisation of three types of 3-D printed lattice cores was performed to evaluate the feasibility of using additive manufacturing (AM) of lightweight polymer-based sandwich panels for structural applications. Effects of the shape of three selected lattice structures on the compression, shear and bending strength has been experimentally investigated. The specimens tested were manufactured with an open source fused filament fabrication-based 3-D printer. These sandwich structures considered had skins made of polypropylene (PP)-flax bonded to the polylactic acid (PLA) lattice structure core using bi-component epoxy adhesive. The PP-flax and the PLA core structures were tested separately as well as bonded together to evaluate the structural performance as sandwich panels. The compression tests were carried out to assess the in-plane and out of plane stiffness and strength by selecting a representative number of cells. Shear band and plastic hinges were observed during the in-plane tests. The shear and three-point bending tests were performed according to the standard to ensure repeatability. The work has provided an insight into the failure modes of the different shapes, and the force-displacement history curves were linked to the progressive failure mechanisms experienced by the structures. Overall, the results of the three truss-like lattice biopolymer non-stochastic structures investigated have indicated that they are well suited to be used for potential impact applications because of their high-shear and out of the plane compression strength. These results demonstrate the feasibility of AM technology in manufacturing of lightweight polymer-based sandwich panels for potential structural uses.

Numerous research is going on for better design to improve performance of load carrying platform. Sandwich panels is one solution of this. Various types of designs are used for different applications. They are used in industries like aerospace, marine etc. They can also be used for general application like crane platform etc. They are mainly used to reduce weight of the element so as to make overall weight less. Various materials can be used to make such panel depending upon the requirement of weight reduction and cost effectiveness. Aim of this is to know variation of equivalent stress and deformation with the change of loading condition like distributed load and concentrated load for simply supported and fixed supported panels by ANSYS analysis. It is observed that concentrated loading gives more stress and deflection as compared to distributed loading. Simply supported panel gives less max stress as compared to fixed support.

Sandwich materials are the most important applications of technological composites. Composite material is a structure formed by combining macroscopic meaning of more than one base material for a specific purpose. Sandwich materials are again in accordance with this definition and different structures are combined without dissolving in order to provide various mechanical properties desired. In this study, stress distributions of two types of support, flat and radial, were investigated. In the results obtained, the ansys was analyzed in the package program by comparing the geometry of the plate and the differences of the support shapes and the cases where the support shape is the same and the plate geometry is different.

► SUMMARY:

♦ The Resilient Composite Systems (R.C.S.) could be counted as "The Methodically Reinforced Nonlinear Porous Materials", also having the high specific modulus of resilience in flexure. They are the compound materials, with some particular structural properties, in which, contrary to the basic geometrical assumption of the flexure theory in Solid Mechanics, the strain changes in the beam height during bending is typically "Nonlinear". The Resilient Composite Systems are made by expediently creating the disseminated suitable hollow pores and/or by distributing the appropriate lightweight aggregates throughout the methodically reinforced, fibered conjoined matrix so that; "the strain changes in the beam height during bending" is typically "nonlinear". Thereby, by applying the mentioned method to make the said composite systems, "considerably increasing the modulus of resilience and the bearing capacity in bending" together with "the significant decrease of the weight" and "the removal of the beam fracture possibility of primary compressive type" have been feasible. Through making these particular consistently functioning systems, the above-stated paradoxical virtues have been concomitantly fulfilled in "one functioning unit" altogether.

♦ The RCS (with the mentioned general structural properties and specific functional criteria) whose cement materials include the "C-S-H (Calcium Silicate hydrate) crystals" have been termed "Elastic Composite, Reinforced Lightweight Concrete (E.C.R.L.C.)".

♦ The RCS and ECRLC as a kind of RCS could be used in many cases. For instance, "Constructing Lightweightly and Consistently", also with practically utilizing the RCS and ECRLC in particular, can be counted as the pivotal and practical tactic to effectively increase the resistance & safety of the constructions against "earthquake" and lateral forces, in the large extent. For example, using some lightweight and insulating, "non-brittle", reinforced sandwich panels or 3D-panels, "with the high modulus of resilience and appropriate behavior against the bending loads and impacts", for construction could be taken into consideration according to the case... [It is worth remarking that; contrary to the ECRLC, the usual reinforced lightweight concretes, especially in the very low densities, are dramatically brittle; they do not have the appropriate behavior and resistance against the high bending loads and impacts.]
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● "Resilient Composite Systems (RCS)":

♦ "Resilient Composite Systems" (RCS) are the compound materials, having some particular structural properties, in which, contrary to the basic geometrical assumption of the flexure theory in Solid Mechanics, the strain changes in the beam height during bending is typically "Nonlinear". The RCS could be counted as "The Methodically Reinforced Nonlinear Porous Materials", also having the high specific modulus of resilience in flexure. (Elastic Composite, Reinforced Lightweight Concrete; ECRLC is a type of RCS with the mentioned specifics.)

♦ Generally, the Resilient Composite Systems comprise these components as the main, necessary elements:
• 1) Mesh (Lattice);
• 2) Fibers or strands;
• 3) Conjoined matrix, having the disseminated suitable pores and/or the disseminated appropriate lightweight aggregates beads or particles. [Here, the general term of "lightweight aggregate" has a broad meaning, also including the polymeric and non-polymeric beads or particles.]

♦ The Resilient Composite Systems are made by creating the disseminated suitable hollow pores and/or by distributing the appropriate lightweight aggregates throughout the reinforced, fibered conjoined matrix so that; "the strain changes in the beam height during bending" is typically "nonlinear". Thereby, by applying the mentioned method to make the said particular composite systems, "considerably increasing the modulus of resilience and the bearing capacity in bending" together with "the significant decrease of the weight" and "the removal of the beam fracture possibility of primary compressive type" have been feasible. Through making these particular consistently (unitedly, integratedly) functioning systems, the stated paradoxical virtues have been concomitantly fulfilled in one functioning unit altogether.

♦ Generally, in these consistently functioning units, the amount and the use manner of the mentioned components in the organized system are so that; the reciprocal interactions among the components finally lead to the "typically nonlinear strain changes in the beam height during bending" (as the "basic functional character" of these systems with the specific testable criteria and indices) and the functional specifications fulfillment of the system.

♦ In the RCS in general, the main strategy to raise the modulus of resilience in bending is "increasing the strain capability of the system in bending" within the elastic limit.

♦ Here, the main tactic to realize the stated strategy is: "creating the suitable hollow pores and/or using the appropriate lightweight aggregates, all disseminated throughout the methodically reinforced conjoined matrix", to provide the possibility for occurring of the expedient internal deformities in the matrix during the bending course, which could lead to the more appropriate distribution of the stresses and the strains throughout the system and the more strain capability of the beam in flexure. On the other hand, only creating the hollow pores and/or using the lightweight aggregates in the matrix, by itself, not only cannot lead to the mentioned goals but also brings about weakening of the matrix and its fragility. Hence, concomitantly, the matrix should be well supported and strengthened. Here, this essentially ameliorating and strengthening the matrix are performed by giving attention to "the internal consistency of the matrix" and also via "employing the expedient reinforcements in two complementary levels": 1) Using the fibers to the better distribution of the tensile stresses and strains in the matrix, and to increase the matrix endurance and the modulus of resilience in tension and bending; 2) Using the mesh or lattice to better distribution of the tensile stresses and strains in the system, and to increase the system endurance and the modulus of resilience in tension and bending.

♦ In these systems, the presence of the mentioned hollow pores and/or lightweight aggregates disseminated throughout the conjoined matrix (which has been ameliorated through making "an integrated, reticular structure") provides the possibility for occurring of the expedient internal deformities in the matrix during the bending course. By the way, this can lead to the less accumulation of the internal stresses in the certain points of the matrix during bending, the better absorption and control of the stresses, and providing the more strain capability of the beam, especially within the elastic limit.

♦ The occurrence of the remarked internal deformities in the said methodically reinforced matrix during the bending course also means; the occurrence of the deformities in the said hollow pores and/or lightweight aggregates well disseminated throughout the conjoined matrix, in two different forms. Indeed, we have the internal deformities in the fibered lightweight matrix of the system throughout the bending course, in two main different forms, leading to: A) The comparative increase of the thickness (height) of the in-compressing layers (particularly in the upper parts of the beam) and the conversion of some internal compressive stresses to the internal tensile stresses (on the axis perpendicular to the mentioned internal compressive tensions) in the in-compressing layers; B) The comparative decrease of the thickness (height) of the in-tension layers (particularly in the lower parts of the beam) and the conversion of some internal tensile stresses to the internal compressive stresses (on the axis perpendicular to the mentioned internal tensile tensions) in the in-tension layers.

♦ In the under-bending sections of the "Resilient Composite Systems", the deformities occurring in the "conjoined layers perpendicular to the applied load direction" during the bending course are so that; "the initially plane sections perpendicular to the beam axis" typically shift from "the plane status" to "the curve status" during the bending course. Thereby, the basic geometrical assumption of the flexure theory in Solid Mechanics ("linearly" being of the strain changes in the beam height during bending) and the respective trigonometric equations & equalities are mainly overshadowed in these systems.

♦ This way, through occurring of the remarked internal deformities in the strengthened matrix during the flexure course, the stresses are more distributed and absorbed, and the rise rate of the internal stresses in the matrix ... reduces.  ...
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► [sites.google.com/site/newstructure1]

Safety at sea is the vivid motivator for naval architects to design structures absorbing higher amounts of energy in case of collisions or grounding. This paper brings a qualitative assessment of 10 different steel sandwich alternatives to identify potential novel crashworthy side shell structures. First, the design alternatives are evaluated for crashworthiness under consistent conditions in a numerically simulated test pro-cedure, followed by the estimation of production costs. On the basis of performance over these two attributes one design is selected for second stage for further study in more realistic conditions. Namely, it is integrated in two variants into a section of an inland waterway tanker which is collided with a rigid bow. The results are compared with the performance of a conventional double side. One variant of a novel structure offered 40 % higher capacity to absorb collision energy.

A four-variable refined plate theory is developed to investigate the thermomechanical bending of two types of functionally graded material (FGM) sandwich plates. The sandwich core of one type is isotropic with the FGM face sheets whereas in the second type, the sandwich core is FGM with the face sheets isotropic and homogeneous. The governing equations are deduced based on the principle of virtual work and the number of unknown functions involved is reduced to only four. The Navier-type exact solutions for bending analysis of simply supported FGM sandwich plates subjected to thermomechanical loads are presented. Deflections and stresses of two types of FGM sandwich plates are investigated. The accuracy and efficiency of the present theory is validated by comparing it with various available solutions in the literature. The influences of volume fraction distribution, aspect ratio and thermal load on plate ther-momechanical bending characteristics are studied in detail.

In this paper, thermomechanical bending analysis of functionally graded material (FGM) sandwich plates is performed by using a four-variable refined plate theory. A new type of FGM sandwich plates, namely, both FGM face sheets and FGM hard core are considered. Containing only four unknown functions, the governing equations are deduced based on the principle of virtual work and then these equations are solved via Navier approach. Analytical solutions are obtained to predict the deflections and stresses of simply supported FGM sandwich plates. Comparative studies for a degradation model (functionally graded face sheets and homogeneous core) are conducted to verify the accuracy and efficiency of the present approach. The influences of volume fraction distribution, geometrical parameters and thermal load on thermomechanical bending behavior of the FGM sandwich plates are studied.

Aluminium Foam Sandwich (AFS) panels are becoming always more attractive in transportation applications thanks to the excellent combination of mechanical properties, high strength and stiffness, with functional ones, thermo-acoustic isolation and vibration damping. These properties strongly depend on the density of the foam, the morphology of the pores, the type (open or closed cells) and the size of the gas bubbles enclosed in the solid material. In this paper, the vibrational performances of two classes of sandwich panels with an Alulight® foam core are studied. Experimental tests, in terms of frequency response function and modal analysis, are performed in order to investigate the effect of different percentage of porosity in the foam, as well as the effect of the random distribution of the gas bubbles. Experimental results are used as a reference for developing numerical models using finite element approach. Firstly, a sensitivity analysis is performed in order to obtain a limit-but-bounded dynamic response, modelling the foam core as a homogeneous one. The experimental-numerical correlation is evaluated in terms of natural frequencies and mode shapes. Afterwards, an update of the previous numerical model is presented, in which the core is not longer modelled as homogeneous. Mass and stiffness are randomly distributed in the core volume, exploring the space of the eigenvectors.

"The insulated structural sandwich panel is a composite laminated structure, made of two stiff skin faces and lightweight thick core. The low-density low-shear rigidity laminated think core (foam, honeycomb) is structurally connected to the laminated skin faces by structural adhesive. The rigid skin (concrete, steel, wood) faces provide high bending stiffens for such lightweight structure, while the core resist the local buckling of the skin faces. The time-dependent deflection behaviour of the concrete insulated panel (CIP) varies with the change of the span-to-depth ratio for the instantaneous deflection, and the cyclic change of temperature and relative humidity over time. This paper discusses the nonlinear theoretical analysis for long-term creep deflection under sustained load that accounts for loads, time, temperature and relative humidity.

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.

Intelligent Engineering has designed an SPS Overlay solution to upgrade vessels to meet higher Ice Class requirements enabling them to operate in ice conditions.

Ship owners wishing to upgrade their vessels to satisfy new Ice Class operational requirements have previously faced undertaking major modifications to existing hull structures such as increased shell plate thickness and additional frames and stringers. 

By using SPS Overlay on the external surface of the shell plating in the ice belt region, higher Ice Class strengthening requirements can be met without major disruption to the hull structure. The use of SPS Overlay eliminates conventional crop-and-replace of the existing shell.  The inherent local stiffness of SPS Overlay ensures effective distribution of any localised peaks in the ice pressure loads. In addition to providing increased plate strength, SPS Overlay increases the section modulus of the framing plate/stiffener combination thus minimizing changes to the existing frames. SPS Overlay’s ability to absorb high impact loads makes it ideal for this application.  The system uses the existing hull as one side of a steel composite panel formed by a new top plate and an elastomer core, greatly reducing the complexity of the conversion, time out of service and total repair costs.

This paper describes the technical work carried out to design and achieve DNV-GL class approval, and install the SPS upgrades.

Fibre metal laminates (FMLs) have been widely used to manufacture airframe components. This work describes novel sisal fibre reinforced aluminium laminates (SiRALs) that have been prepared by cold pressing techniques and tested under tensile, flexural and impact loading. The pristine sisal fabric and the sisal fibre reinforced composites (SFRCs) were also tested to understand the difference in mechanical performance of the sisal fibre metal laminates. The SiRALs achieved not only the highest modulus and strength, but also the highest specific properties. The mean specific tensile strength and modulus of the SIRALs reached increases of 132% and 267%, respectively, when compared to the sisal fibre reinforced composites (SFRCs). Moreover, the mean specific flexural strength and modulus of the SiRALs were significantly higher than SFRCs, revealing increases of 430% and 973%, respectively. A delamination fracture mode was noted for SiRALs under bending testing. The SiRALs can be considered promising and sustainable composite materials for structural and multifunctional applications.

Three higher order refined displacement models are proposed for the free vibration analysis of sandwich and composite beam fabrications. These theories model the warping of the cross-section by taking the cubic variation of axial strain and they eliminate the need for a shear correction coefficient by assuming a quadratic shear strain variation across the depth of the cross-section. Numerical experiments for various lamination schemes, boundary conditions and aspect ratios are carried out to compare these models with the first order shear deformation theory, earlier investigations and also among themselves to ascertain the most efficient one. Numerical results for deep sandwich and composite beams are also presented for future references.

The fundamental frequencies and nonlinear dynamic responses of functionally graded sandwich shells with double curvature under the influence of thermomechanical loadings and porosities are investigated in this study. Two material models are considered. The continuity requirement of material properties throughout layers are fulfilled by newly introducing refined effects of two porosity types regarding the average of constituent properties weighted by the porosity volume fraction. The first-order shear deformation theory taking the out-of-plane shear deformation into account is employed to obtain the Lagrange equation of motions. The number of primary variables reduces from five to three after introducing the Airy stress function. The system of dynamic governing equations is obtained by utilizing the Bubnov-Galerkin procedure. The natural frequencies are analytically computed by solving eigenvalue problems, and the fundamental frequencies are acquired by further assumptions about the inertial force caused by the shell rotation variables. The nonlinear dynamic responses of the functionally graded spherical, cylindrical, and hyperbolic paraboloid shells under the influence of different geometry configurations, loading conditions, and porosity types and degrees are obtained by applying the fourth-order Runge-Kutta method. The numerical results are presented and verified with available studies in the literature. Although porosities are usually considered material defects weakening the structure performance, this study has proved clearly that porosities stiffen the shell structures to some extent.

An analytical framework has been proposed to analyze the effect of random structural irregularity in honeycomb core for natural frequencies of sandwich panels. Closed-form formulas have been developed for the out-of-plane shear moduli of spatially irregular honeycombs following minimum potential energy theorem and minimum complementary energy theorem. Subsequently an analytical approach has been presented for free-vibration analysis of honeycomb core sandwich panels to quantify the effect of such irregularity following a probabilistic paradigm. Representative results have been furnished for natural frequencies corresponding to low vibration modes of a sandwich panel with high length-to-width ratio. The results suggest that spatially random irregularities in honeycomb core have considerable effect on the natural frequencies of sandwich panels.

This paper investigates theoretically the differences in load-carrying behaviour between web-core sandwich plate, stiffened plate and isotropic plate. Buckling and post-buckling is studied. The study is carried out using two approaches, both solved with the finite element method. The first is a three-dimensional model of the plates. The second approach is the equivalent single-layer theory approach. First-order shear deformation theory is used. The second approach allows plates to the viewed through ABD- and DQ stiffness coefficients. Plates are axially loaded in the web plate/stiffener direction. Simply supported boundary condition is considered with loaded edges kept straight and unloaded edges free to move in-plane. The results show that the buckling load of sandwich plate is 42% to 65% higher than the stiffened plates. The reason is that sand-wich plate is a symmetrical structure where coupling between in-plane and out-of-plane displacements does not exist (B-matrix is equal to zero). Furthermore, breadth-to-thickness ratio (representing local plate slender-ness) is about two times lower in sandwich plate than in stiffened plate which prevents local buckling. On the other hand, buckling load of sandwich plate can be improved by increasing the transverse shear stiffness, e.g. by filling the voids in the core. For the same structural weight, post-buckling stiffness of stiffened plate is somewhat lower than in sandwich, also owing to the B-matrix. Isotropic plate of the same bending stiffness as sandwich plate has higher post-buckling stiffness due to larger in-plane stiffness (A-matrix) or structural weight.

Theoretical closed-form solutions and numerical results for nonlinear stability of the moderately thick functionally graded sandwich shells subjected to thermomechanical loadings are presented in this study. Two proposed material distribution models supported by elastic foundations are examined. The nonlinear strain field is deduced from the first-order shear deformation theory taking the stretching, bending and shear effects into consideration. The Bubnov-Galerkin procedure and harmonic balance principle are utilized to bring about the explicit algebraic expression for the shell static behaviors from governing equations derived from Hamilton's principle. Mechanical buckling loads and critical thermal rises for the shells in spherical, cylindrical, and hyperbolic paraboloid forms are obtained. The effect of geometry, elastic foundations, volume fraction index, material distribution models, buckling modes, and imperfections on the shell stability behaviors are considered in parametric studies. The yielding plateau in the thermal analysis of the spherical shells in case of temperature dependent characteristics is recognized for the first time. Verification studies are also conducted.

Analytical closed-form solutions for thermos-mechanical stability and explicit expressions for free-and forced-vibration of thin functionally graded sandwich shells with double curvature resting on elastic bases are investigated for the first time in this study. A core layer of ceramic and two cover layers of functionally graded materials constitute the shell structure. Governing equations are derived from the classical shell theory using Hamilton's principle admitting Volmir assumption and von Karman nonlinear displacement fields. Theoretical solutions are achieved by using the Bubnov-Galerkin procedure in solving differential equations. Parametric studies showing the effects of temperature-dependent features, material constituents, initial geometry imperfections , external thermos-mechanical loadings, elastic bases, and geometry configuration on static and dynamic behaviors of the shells are performed. Thin functionally graded sandwich spherical, cylindrical, and hyperbolic paraboloid shells are studied. Snap-through phenomena under load-control conditions are recognized in thin functionally graded sandwich cylindrical shells. The fourth-order Runge-Kutta method is employed to numerically solve dynamic problems and four analogies are drawn to validate theoretical formulations.

Effective use of natural fabric as reinforcement for production of ferrocement. Tensile behavior of ferrocement with natural fabric versus galvanized steel wire reinforcement. Sandwich composites comprising locally available materials for structural applications. a b s t r a c t A sandwich composite comprising ferrocement skins was developed as the primary structural module for building construction with indigenous materials. The indigenous reinforcement systems selected for use in ferrocement skins were jute burlap and chicken mesh (flexible galvanized steel wire). These reinforcement systems were characterized through performance of tension tests. The tensile strength, stiffness and ductility of jute burlaps were found to compare favorably with those of chicken mesh which is a viable reinforcement for use in ferrocement. Tension tests on ferrocement sheets indicated that indigenous reinforcement ratios above a threshold level could induce multiple cracking and strain-hardening behavior, producing a desired balance of tensile strength and ductility. The tensile strength of indigenous ferrocements with jute burlap reinforcement exceeded the theoretically predicted values, which could be attributed to the favorable interactions of the burlap reinforcement with the inorganic matrix, and the strengthening effects of hydrates precipitating within the yarn voids in burlap. Experiments were conducted to determine the bond strength and the required development length of the indigenous reinforcement in cementitious matrix. Indigenous sandwich composites comprising ferrocement skins with jute burlap reinforcement and an aerated concrete core made of lime-gypsum matrix and saponin foaming agent were fabricated and subjected to flexure testing. The sandwich composite provided relatively high flexural strengths; the flexural failure modes indicated that the relatively dense aerated concrete core makes important contributions to the flexural performance of the sandwich composite.

The NACA 4digit series aerofoil shapes are universally accepted standard designs generally used for wind turbine blades, helicopter rotor blades and car spoilers. The manufacturing of these complex shapes is a challenging task for the manufacturing engineers. These components need to be made of materials having high specific strength and fatigue properties. The composites with sandwich construction fulfill the above requirements. The main aim of the present Investigation is to select the best fiber orientation for the fabrication of automotive rear spoiler. The Design FOIL software provides different shapes of aerofoil from which NACA 2412 has been selected. The spoiler is modeled using CATIA software and is analyzed for the static deflection using ANSYS for various orientations of the fiber. The spoiler is then fabricated using woven E-glass fiber (with the best orientation) and epoxy resin. Static test is performed on the fabricated model to validate the results obtained from the software. This confirms the design feasibility and software adoptability for the design of sandwich aerofoil shape

Synthetic sandwich composite materials have been fabricated using carbon fiber–epoxy face sheet and polymeric foam core by mimicking the structure of a natural composite material, Palmetto wood. The foam core of the sandwich composites has been reinforced by pultruded carbon rods to replicate macrofiber reinforcement of Palmetto wood within the porous microstructure for enhanced flexural behavior and energy absorbance. Sandwich structures have been characterized to elucidate the multiscale deformation behavior under quasi-static three point bend test using multiscale Digital Image Correlation (DIC). The damage evolution in the sandwich materials has been evaluated using a model developed to decouple the effect of pore collapse and plastic strain. It has been observed that the longitudinal reinforcement of pultruded carbon rod in foam core by the bioinspiration from the hierarchical structure of Palmetto wood increases the flexural strength, elastic energy absorbance of the sandwich with bioinspired core compared to that with the conventional un-reinforced core, however, at the cost of damage initiation strain. The sandwich composites with bioinspired core exhibits an increase of approximately 100% in flexural stiffness compared to that of sandwich with conventional foam core. The strength and volumetric energy absorbed are found to increase by approximately 10× and 14× from the sandwich with conventional core. These results validated using Palmetto wood as a source of bioinspiration for increasing the energy absorbing capability of sandwich composites. The analysis of the experimental results also indicated that the macroscopic flexural response of the sandwich composites is similar to that of the Palmetto wood. This similarity is attributed to the evolution of damage mechanisms associated with pore collapse and plastic strain being nearly identical between the bioinspired core and Palmetto wood. Furthermore, it is found that the reinforcement does not significantly affect the damage evolution characteristics in the bioinspired core, only the damage initiation is affected similar to what was observed in Palmetto wood.

An experimental methodology is presented to determine tool-part frictional interaction of composite parts and the structural integrity of sandwich structures when subjected to temperatures and pressures similar to those of autoclave processing. This methodology includes the development of a testing rig that mimics the deformation mismatch between tools and parts, and quantifies shear stress-that is, tool-part friction or shear stress of sandwich structures.

The use of sandwich structures combines low weight with high energy absorbing capacity, so they are suitable for applications in the transport industry (automotive, aerospace, shipbuilding industry), where the "lightweight design" philosophy and the safety of vehicles are very important aspects. The goal of this paper was the analysis of the bending and the low -velocity impact response of aluminium foam sandwiches reinforced by the outer skins made of glass fibre reinforced epoxy matrix. The results were compared with those obtained for aluminium foam sandwiches without glass fibre skins. An analytical model for the peak load prediction under low velocity impact was developed and the predicted values are in good agreement with the experimental measurements. The impact response of the sandwiches was investigated using a theoretical approach, based on the energy balance model and the model parameters were obtained by the tomographic analyses of the impacted panels.

In this study we investigate the response of closed cell polymeric foam core sandwich panels with woven carbon/epoxy laminate facesheets under low velocity impacts. Two levels of energy, 10 J and 50 J simulating barely visible and visible impact damage categories are examined under cold, room and high temperature conditions. A finite element modeling framework is proposed for the sandwich composite comprising continuum damage, cohesive layers and crushable foam model with isotropic hardening for the core. The model shows how progressive damage in the facesheets, interlaminar damage between the layers and the crushable foam can be used to model low velocity impact and temperature effects. Ultrasonic testing and high speed cameras are used to determine the damage and delamination characteristics. The higher exposure temperatures result in larger damage zones for both the low and high impact energies. The elevated temperature enhanced the penetration rate and indentation depth showing degradations in the out of plane properties. The visible damage test results in significant damage in the core and facesheets dependent on the temperature ranges tested. In the barely visible damage category, the composite facesheet layers attached to the foam experience a tension failure under low energy impact despite surface indications showing no fiber fracture.

This paper investigates theoretically the compressive load-carrying behaviour of geometrically imperfect web-core sandwich plates. Slender plates, which first buckle globally, are considered. The study is carried out using two approaches, both solved with the finite element method. The first is the equivalent single-layer theory approach. First-order shear deformation theory is used. The second approach is a three-dimensional shell model of a sandwich plate. Plates are loaded in the web plate direction. Simply supported and clamped boundary conditions are considered with a different level of in-plane restraint on the unloaded edge. The results show that the behaviour of the sandwich plate is qualitatively equal to the isotropic plate of the same bending stiffness for deflections lower than the plate thickness. As the deflections increase, the lower in-plane stiffness of the sandwich plate results in lower post-buckling strength. Local buckling of face plates in the post-buckling range of the sandwich plate further reduces the structural stiffness.

The present study focuses on the effectiveness of steel-sand composite stiffened plates under impulsive loading. Dynamic response of steel-sand composite stiffened plates, with various stiffener layouts under impulsive loading is analysed using commercially available finite element software Abaqus/Explicit. The steel plates and stiffeners are modelled using shell elements and the effect of strain rates is incorporated through Johnson-Cook (J-C) material model. Sand is modelled as a continuum between the two steel plates. Sand response is simulated using the built-in Drucker-Prager plasticity model in Abaqus considering the strain rate effect. The contact planes between sand and the steel plates are modelled using the general contact formulation in Abaqus. Impulsive load is applied in the form of a rectangular uniform pressure pulse. The effect of different thicknesses of the sand layer in response mitigation has been investigated. The steel-sand composite stiffened plates exhibit lesser central point displacement under impulsive loading as compared to when no stiffeners are provided.

The ultra-microduplex structure was fabricated in a fully pearlitic Fe-0.8 wt% C steel after equal channel angular pressing (ECAP) at 923 K via the Bc route. The microstructures and mechanical properties, before and after deformation, were investigated using scanning electron microscopy and mini-tensile tests. The cementite lamellae are gradually spheroidized by increasing the number of ECAP passes. After four passes, the cementite lamellae are fully spheroidized. Microhardness and the ultimate tensile strength of pearlite increase with the strain, up to a peak value (after two passes) and then decrease significantly. The yield strength, elongation and percentage of reduction in area increase with the number of ECAP passes. The tensile fracture morphology changes gradually from brittle cleavage to typical ductile fracture after four passes.

Sandwich panels are extensively analyzed and used in the transportation field due to their specific strength and stiffness values, higher than the metal homogeneous. Often, it is highlighted that one of the main drawbacks is related to their increased acoustic emissions, which especially in the past has limited their use in the application where a high level of comfort is required. In the last decade, new types of core for sandwich panels have been designed to obtain good structural and acoustical performances. In order to create a reference for future analysis on new configurations of truss-core, in the present study the experimental modal analysis on sandwich panels with honeycomb and foam cores is carried out in the low-mid frequency range. The results obtained from modal analysis are used to estimate the acoustic power radiated by both panels. The panels’ faces are in carbon/epoxy so that all the attention is concentrated on the cores which represent the variables to be investigated. This activity has been developed inside the framework offered by the
SUPERPANELS project (www.superpanels.unina.it).

The desire to produce more geometrically complex components from pin-reinforced X-Cor Ò sandwich composite structures requires that the effects of curvature on the flexural behavior be investigated experimentally and numerically. This will enable development of validated design tools accounting for the effects of the load distributions associated with curved geometries on the efficacy of the pin reinforcement. Singly curved X-Cor Ò sandwich composite materials have been fabricated from pin reinforced foam core and composite facesheets. Flexural tests have been performed with curved X-Cor Ò sandwich composites of two different radii of curvature: 6 00 and 12 00 . Modeling of the flexural response has been performed using Finite Element Analysis (FEA) to predict the stiffness, strength and failure initiation. The reinforced core has been homogenized to simplify the core model and to account for the effects of pin reinforcements in the foam using a micromechanical modeling approach. The numerical results obtained by FEA have been compared with experimental measurements. A new stress interaction criterion has been used to predict the strength of the curved sandwich composite under bending. It has been found that the predictions from the numerical simulations correlated with experimental measurements.

In the present work we present a systematic experimental investigation of the generation and subsequent evolution of dynamic failure modes in sandwich structures subjected to low-speed impact. Model sandwich specimens involving a compliant polymer core sandwiched between two metal layers were designed and subjected to impact loading to simulate failure evolution mechanisms in real sandwich structures. High-speed photography and dynamic photoelasticity were utilized to study the nature and sequence of such failure modes. A series of complex failure modes was documented. In all cases, inter-layer (interfacial) cracks appeared first. These cracks were shear-dominated and were often intersonic even under moderate impact speeds. The transition from inter-layer crack growth to intra-layer crack formation was also observed. The shear inter-layer cracks kinked into the core layer, propagated as opening-dominated intra-layer cracks and eventually branched as they attained high enough growth speeds causing core fragmentation.

A higher-order refined model with seven degrees of freedom per node is presented in this paper for the free vibration analysis of composite and sandwich arches. The strain field is modeled through cubic axial, cubic transverse shear and linear transverse normal strain components. As the cross-sectional warping is accurately modeled by this theory, it does not require any shear correction factor. The stress-strain relationship is derived from an orthotropic lamina in a threedimensional state of stress. The proposed higher-order formulation is validated, in this first part, through arches with various curvatures, aspect ratios, boundary conditions and materials.

There is a great deal of interest in understanding the mechanics in pin-reinforced composite sandwich structures. In particular, characterizing the relationship of the mechanics in smaller to larger specimens due to changes in the pin response can expedite the development of more advanced models, as well as the assessment of new materials and processing conditions. In this investigation, specimens with a conventional symmetric pin configuration anchoring into each facesheet have been experimentally characterized under compressive loading. The effect of specimen size on the compressive response has been investigated through the number of active pins under loading. The stress–strain response of these structures exhibit initially high compressive stiffness followed by a significant load shedding and flow stress. The transition in the mechanical behavior is attributed to the failure of reinforced pins under a combination of axial compression and transverse bending due to their orientation relative to the facesheets. As the size of a specimen is decreased, the effective stiffness and strength decrease due to a decrease in effective pin stiffness. It was found that these effects on the compressive stress can be scaled through a stress scaling factor derived from the change in the structure of the specimen due to the number of active versus inactive pins per unit of area, which is scaled by the pin density related to the pin basis and spacing for the corresponding pin configuration. Using Digital Image Correlation (DIC), it was then possible to account for these size effects on compressive strain directly through the pin deformations. A stiffness scaling factor could then be determined from a model of a beam in bending under axial and transverse loading that was correlated to the DIC displacements to determine the effective pin stiffness and corresponding stiffness scaling factor for strains. A stretched exponential relationship was also found to describe the scaling of the effective pin stiffness with the number of active pins. The compressive stress–strain response was then corrected to show that similar scaled properties could be obtained for the compressive modulus. The Poisson’s ratio was also obtained from the DIC strains averaged over the full specimen, and a scaling relation similar to that for the effective pin stiffness was developed.

The strength of a web-core steel sandwich plate is potentially reduced in a corrosive environment. This study is dedicated to the influence of a reduction in the thickness of the plates as a result of general corrosion on sandwich plate buckling strength and first-fibre failure. Two scenarios are investigated in which corrosion reduces the thickness of (a) the outer sides of the face plates and (b) all surfaces, including the core. The laser weld between the face sheets and the core is assumed to be intact. The assumptions are made on the basis of earlier experimental findings. Critical buckling and geometric non-linear analysis are carried out with the finite element method, with the kinematics being represented using two approaches: (1) equivalent single-layer with first-order shear deformation theory, and (2) a three-dimensional model of the actual geometry of the structure, modelled using shell and connector elements. The former is used to identify the effect of corrosion on the stiffness coefficients and, consequently, the buckling strength. The latter is used for verification and for stress prediction during post-buckling. A rapid decrease in the buckling strength was found for corrosion affecting the outer sides of the sandwich plate. The decrease in the buckling strength doubled in the case of the diffusion of moisture (water) into the core. The shear-induced secondary bending of the faces was found to affect the first-fibre yield.

A higher-order arch model with seven degrees of freedom per node is proposed to study the deep, shallow, thick, and thin composite and sandwich arches under static loads. The strain field is modeled through cubic axial, cubic transverse shear, and linear transverse normal strain components. As the cross-sectional warping is accurately modeled by this theory, it does not require any shear correction factor. The stress-strain relationship is derived from an orthotropic lamina in a three-dimensional state of stress. The proposed formulation is validated through models with various curvatures, aspect ratios, boundary conditions, materials, and loading conditions.

Vibrations of angle ply laminated beams are studied using the higher order theory and isoparametric 1d finite element formulations through proper constitution of elasticity matrix. Subsequent to the validation of the formulation, deep sandwich and composite beams are critically analyzed for various boundary conditions. Frequencies classified based on their spectrum are presented along with those of first order theories for comparison.

A nonlinear analytical model for investigating localized interactive buckling in simply supported thin-face plate sandwich struts with weak cores is extended to account for local deformations in both face plates, which have been observed in experiments and finite element simulations. The original model is based on potential energy principles with large displacement assumptions. It assumes Timoshenko shear deformable theory for the core and approximates the overall mode as a half-sine wave along the length of the strut while the local face plate displacements are initially unknown and are found as solutions of the governing equations. The extended model is able to capture measurable local face plate displacements in the less compressed face plate, beyond the secondary bifurcation which leads to localized interactive buckling, for the case where overall buckling is critical. Moreover, the allowance of local displacements in both face plates allows the extended model to predict the post-buckling behaviour better in cases where local buckling is critical. The results from this model compare very well with nonlinear finite element simulations with respect to both the equilibrium paths and panel deformations.

The dynamic characteristics of sandwich composite plates with holes at various locations are investigated both experimentally and through numerical simulations. The critical parameters which affect the natural frequencies of such plates with holes or cutouts are the core and face sheet thicknesses, diameter and location of the holes and aspect ratio of the plate. A rigorous parametric study has been conducted to determine their interrelationship and presented in a concise manner. An experimental investigation has been conducted in the beginning to get an idea about the variations of the above mentioned parameters. The results can be utilized to determine the changes in dynamic characteristics of sandwich composite plates with holes with specified diameters and locations. The methodology can also be useful to control the dynamic responses of such plates by incorporating holes of chosen diameters at appropriate locations. The results may be used to create design curves for satisfactory dynamic performances of sandwich composite plate with holes.

Design optimization in sandwich composites is a less understood topic in research. The current study investigates the strength and stiffness of rigid foam core glass fabric/ epoxy sandwich composites with two different foam core materials, viz., polyurethane (PUF) and poly isocyanurate (PIR) of three different densities 64kg/m 3 , 125 kg/m 3 and 250 kg/m 3 respectively. Different skin to core weight ratios were maintained in the range of 1:1 to 4:1 for flexure tests. Simulation and analyses of various mechanical tests of sandwich panels were carried out using finite element software ANSYS TM v16. The results from finite element analyses were compared with results from the experimental work that were obtained for tensile, compressive, shear and flexural properties in sandwich specimens. The shear components were investigated through single lap, double lap and mode 1 simulations. Parameters like deflection, normal stress, bending and shear stress, shear strain etc., were determined for sandwich composites. Optimal load design was obtained by ANSYS Design e from which the optimal parameters were derived numerically and compared with the test data.

Pin-reinforced sandwich composites have recently attracted the attention for its advanced out-of-plane properties. Curved K-Cor polymer sandwich composite have been fabricated using a fixture and heat treatment. The controlled heat treatment process has been used to shape the sandwich composites without any damage during shaping. Experiments have been performed on the curved sandwich specimens under different boundary conditions at the edges. The effect of boundary supports have been observed in terms of increased load bearing capacity with supported at the edge compared to that supported at the bottom. Digital Image Correlation (DIC) has been used to determine the deformation fields from the images captured during deformation. Preliminary 3D DIC analysis has been performed to measure the curvature in the specimen. The effect of boundary conditions and foam in the core in the failure initiation has been discussed. Finite Element Analysis (FEA) has been performed to predict the response of the K-Cor sandwich composites and compare with that obtained from experimental measurements.

The corrosive marine environment is a threat to the ultimate strength of steel sandwich structures. Therefore, ultimate strength experiments were carried out in three-point bending for beams with different corrosion exposure times, i.e. one and two years. Standard laser-welded web-core sandwich beams are studied and different corrosion protection systems considered. The beams experienced general corrosion. The thickness reduction in unprotected plates and laser welds is around the typical 0.1 mm/year. This led to an ultimate strength reduction of 10% and 17% for beams with exposure times of one and two years, respectively. The experimental ultimate strength is in agreement with finite element simulations. The ultimate strength was maintained for the beams protected with coating or closed-cell polyurethane (PU) foam.

Sandwich panels are extensively analyzed and used in the transportation field due to their specific strength and stiffness values, higher than the metal homogeneous. Often, it is highlighted that one of the main drawbacks is related to their increased acoustic emissions, which especially in the past has limited their use in the application where a high level of comfort is required. In the last decade, new types of core for sandwich panels have been designed to obtain good structural and acoustical performances. In order to create a reference for future analysis on new configurations of truss-core, in the present study the experimental modal analysis on sandwich panels with honeycomb and foam cores is carried out in the low-mid frequency range. The results obtained from modal analysis are used to estimate the acoustic power radiated by both panels. The panels' faces are in carbon/epoxy so that all the attention is concentrated on the cores which represent the variables to be investigated. This activity has been developed inside the framework offered by the SUPERPANELS project (www.superpanels.unina.it).

We have designed an automatic complex equipped by a Cascade Microtech MPS150 micro-probing station and a PXIe-4143 source measure unit from National Instruments Corporation to investigate the electrical properties of metal/insulator/metal sandwich structures. We have also succeeded in implementing conditions for the current-voltage characteristics measurements and in simulating procedures for the multiple reading/writing of digital information by means of resistive switching inside the nanostructured layers. We have studied anodized-titanium-dioxide-based Ti/TiO2-NT/Au structures with diameter of 100 micrometers, synthesized by the mask method. For the micromemristors produced, we have estimated the resistance in the low-(<10 kOhm) and high-ohmic (> 50 kOhm) states.

Sandwich panels are extensively analyzed and used in the transportation field due to their specific strength and stiffness values, higher than the metal homogeneous. Often, it is highlighted that one of the main drawbacks is related to their increased acoustic emissions, which especially in the past has limited their use in the application where a high level of comfort is required. In the last decade, new types of core for sandwich panels have been designed to obtain good structural and acoustical performances. In order to create a reference for future analysis on new configurations of truss-core, in the present study the experimental modal analysis on sandwich panels with honeycomb and foam cores is carried out in the low-mid frequency range. The results obtained from modal analysis are used to estimate the acoustic power radiated by both panels. The panels' faces are in carbon/epoxy so that all the attention is concentrated on the cores which represent the variables to be investigated. This activity has been developed inside the framework offered by the SUPERPANELS project (www.superpanels.unina.it).

A higher order refined model with isoparametric elements is proposed to study the transient dynamic response of laminated arches/curved beams. The strain field is modeled through cubic axial, cubic transverse shear and linear transverse normal strain components. As the cross-sectional warping is accurately modeled by this theory, the shear correction factor is rendered redundant. The stress-strain relationship is derived from an orthotropic lamina in a three-dimensional state of stress, so that angle-ply laminates can be studied through one-dimensional elements. Consistent mass matrix is constituted for the equation of motion, which is solved by Newmark integration scheme. The higher order formulation is validated with available results and subsequently applied to arches with various curvatures, aspect ratios, boundary conditions, loadings and lamination schemes to evaluate its transient dynamic performance and suitable conclusions are drawn.

This paper presents analytical and case studies of sandwich structures. In this study, the Euler−Bernoulli beam equation is solved analytically for a four-point bending problem. Appropriate initial and boundary conditions are specified to enclose the problem. In addition, the balance coefficient is calculated and the Rule of Mixtures is applied. The focus of this study is to determine the effective material properties and geometric features such as the moment of inertia. The effective parameters help in the development of a generic analytical correlation for complex sandwich structures from the perspective of four-point bending calculations. The case study is built for a sandwich structure made of two materials; Aluminium and Steel. This case is solved using MATLAB R. The main outcomes of the Al-Steel sandwich structure are the maximum lateral displacements and longitudinal stresses varying with number of sandwich layers.

Previously published work on sandwich face wrinkling almost always considers isotropic or almost isotropic sandwich configurations. In that case the critical wrinkling load only needs to be evaluated in the principal compressive stress direction. For a sandwich panel with a higher degree of anisotropy this is not enough. This paper presents a method for estimating the wrinkling behaviour of highly anisotropic sandwich panels under multi-axial loading. The method is based on the assumption that wrinkling occurs at the angle where the ratio of applied load to sustainable wrinkling loads reaches a global maximum. In addition to the description of the analytical theories the paper also contains comparisons with finite element calculations and testing of real sandwich configurations. The results indicate that the derived model works excellent both for uni-and bi-axial loading, though a small factor of safety is required as with all other standard wrinkling theories.

This paper develops a new analytical solution to conduct the free vibration analysis of porous functionally graded (FG) sandwich plates based on classical plate theory (CPT). The sandwich plate made of the FGM core consists of one porous metal that had not previously been taken into account in vibration analysis and two homogenous skins. Design/methodology/approach: The analytical formulations were generated based on the classical plate theory (CPT). According to the power law, the material properties of FG plates are expected to vary along the thickness direction of the constituents. Findings: The results show that the porosity parameter and the power gradient parameter significantly influence vibration characteristics. It is found that there is an acceptable error between the analytical and numerical solutions with a maximum discrepancy of 0.576 % at a slenderness ratio (a/h =100), while the maximum error percentage between the analytical and experimental results was found not exceeding 15%. Research limitations/implications: The accuracy of analytical solutions is verified by the adaptive finite elements method (FEM) with commercial ANSYS 2020 R2 software. Practical implications: Free vibration experiments on 3D-printed FGM plates bonded with two thin solid face sheets at the top and bottom surfaces were conducted. Originality/value: The novel sandwich plate consists of one porous polymer core and two homogenous skins which can be widely applied in various fields of aircraft structures, biomedical engineering, and defense technology. This paper presents an analytical and experimental study to investigate the free vibration problem of a functionally graded simply supported rectangular sandwich plate with porosities. The objective of the current work is to examine the effects of some key parameters, such as porous ratio, power-law index, and slenderness ratio, on the natural frequencies and damping characteristics.

Pin reinforced foam cores with composite facesheets offer a viable means of structural construction owing to their low weight and high stiffness and strength. In this paper, various models are introduced to estimate the stiffness and strengths of such a sandwich structure, called K-Cor™, under out-of-plane compressive loading conditions. ince the reinforcing pins have a high modulus compared to the modulus of the facesheet in the transverse direction, the pins indent into the facesheets affecting the overall compressive stiffness of the sandwich structures. Models incorporating this interaction are constructed and their compressive properties are compared with experimental results for various sandwich specimens. The effect of pin locations and size of the sandwich on its compressive properties is also investigated.