Showing papers in "Journal of Sandwich Structures and Materials in 2012"

A new four-variable refined plate theory for thermal buckling analysis of functionally graded sandwich plates:

Abstract: The novelty of this paper is the use of a new four-variable refined plate theory for thermal buckling analysis of functionally graded material (FGM) sandwich plates. Unlike any other theory, the present new theory is variationally consistent and gives four governing equations. The number of unknown functions involved is only four, as against five in case of other shear deformation theories. In addition, the theory, which has strong similarity with classical plate theory in many aspects, accounts for a quadratic variation of the transverse shear strains across the thickness and satisfies the zero traction boundary conditions on the top and bottom surfaces of the plate without using shear correction factors. Material properties and thermal expansion coefficient of the sandwich plate faces are assumed to be graded in the thickness direction according to a simple power-law distribution in terms of the volume fractions of the constituents. The core layer is still homogeneous and made of an isotropic material. ...

Elastic modulus and Poisson\'s ratio determination of micro-lattice cellular structures by analytical, numerical and homogenisation methods

Abstract: The mechanical properties of micro-lattice structures made of interconnected metallic struts are calculated analytically, in order to derive their elastic modulus and Poisson’s ratio in the three Cartesian directions. The geometry of the investigated unit cell is a body centered cuboid, which is a more generic case of the well-known body centered cubic geometry. The Bernoulli–Euler and Timoshenko beam theories are used for the analytical solution of the unit cell response under complex loading. The derived elastic constants are compared to the relative numerical ones. The influence of geometrical parameters on the stiffness coefficients of the cellular structure is parametrically determined. The derived elasticity matrices are introduced in a homogenised numerical model of the core and its results are compared to a numerical model with explicit representation of the core geometry indicating an excellent accuracy, while the solution time is considerably reduced.

Shear stresses in honeycomb sandwich plates: Analytical solution, finite element method and experimental verification

Abstract: Honeycomb composite structures are used in airplanes, railway cars and vehicles. The sandwich panels consist of two stiff face sheets of aluminium, which are bonded to a very lightweight honeycomb ...

A high-order finite element for dynamic analysis of soft-core sandwich plates:

Abstract: The dynamic behavior of soft-core sandwich plates is investigated. A high-order finite element concept that has been developed for the dynamic analysis of multi-layered plate structures with stiff and compliant layers is applied to the soft-core sandwich plate. The application to sandwich plates aims to validate the general model through comparison with experimental and analytical benchmarks and to throw light on the unique structural response of the sandwich plate. The model introduces the core’s three-dimensional stress and deformation fields using a high-order kinematic assumption that is based on the closed-form solution of the static governing equations of the core. The first-order shear deformation laminated plate theory is used for the face sheets. The combination of the high-order theory with the finite element concept aims to extend the application of the theory to more general layouts, to reduce the computation effort needed for a three-dimensional analysis, and to address some of the obstacles ...

Experimental impact damage study of a z-pinned foam core sandwich composite

Abstract: This paper assesses the impact damage and post-impact compression properties of a foam core sandwich composite reinforced with through-thickness z-pins. Low-speed flat-wise compression tests perfor...

Crashworthiness optimization design of honeycomb sandwich panel based on factor screening

Abstract: This paper presents a crashworthiness robust optimization design of aluminum honeycomb sandwich panel with regular hexagonal core cell based on explicit finite element analysis. First, the crashworthiness of honeycomb sandwich panel and the bare honeycomb core are compared with each other in the aspects of specific energy absorption and peak crushing force. The comparative results show that honeycomb sandwich panel can absorb more energy than the bare honeycomb core, though the peak crushing force of honeycomb sandwich panel is larger. Then the factor screening experiments are carried out to find out variables that have significant effects on the crashworthiness of honeycomb sandwich panel. The ultimate purpose of factor screening experiments is to decrease the computational expense by reducing the number of design variables. Finally, the dual response surface method and the crossed array design are employed to formulate the complex robust optimization design problem. The regression expressions of specific energy absorption and peak crushing force are defined as the objective and constraint function respectively in the robust design of crashworthiness optimization. Language: en

Localized indentation of sandwich beam with metallic foam core

Abstract: The localized behavior of sandwich structures with foam core subjected to indentation loading is investigated in this article. Based on the principle of virtual velocities, concisely explicit solutions for the indentation forces and shape functions of deformation zones of sandwich beams are derived. Both flat and cylindrical indenters are considered. The indentation force varies linearly with the square root of indenter displacement and thesurface deformation profiles are proportional to the square of the distance from the contact center along the beam direction. Finite element models have been established using the ABAQUS/Explicit code to verify the validity and applicability of the analytical solutions. The theoretical predictions of the profiles of deformation zones and the denting loads of indenters are in good agreement with those by numerical simulation.

Investigation of sandwich crack stop elements under fatigue loading

Abstract: This paper deals with the investigation of crack stop elements for carbon-fibre-reinforced polymer (CFRP)/foam core sandwich structures manufactured by using liquid composite moulding technology un...

A critical evaluation of mechanical models for sandwich beams

Abstract: We focus on the description of the stress state of sandwich beams under bending and shear, a non-trivial task if Saint-Venant's principle does not hold, as it is the case if the skins are somewhat stiffer than the core. Each of the analytical structural models available in literature turns out to be accurate for a limited range of relative stiffness between core and skins, or sandwich heterogeneity. For a simply supported sandwich beam subject to uniform transversal load, we evaluate the stress by means of (a) the classical theory relying on the linear cross-section kinematics, appropriate if Saint-Venant's principle holds, (b) the structural theory based on the zig-zag warping (e.g. Krajcinovic D. Sandwich beam analysis. J Appl Mech, Trans ASME 1972; 39(3): 773–778), and (c) the higher-order theory of Frostig et al. (Frostig Y, Baruch M, Vilnay O, et al. High-order theory for sandwich-beam behavior with transversely flexible core. J Eng Mech, Trans ASCE 1992; 118(5): 1026–1043), the latter usually approp...

Analysis of sandwich plates by Radial Basis Functions collocation, according to Murakami’s Zig-Zag theory

Abstract: In this article, the static analysis of sandwich plates is performed by radial basis functions collocation, according to the Murakami's Zig-Zag function theory. The Murakami's Zig-Zag function theory accounts for through-the-thickness deformation, by considering a Zig-Zag evolution of the transverse displacement with the thickness coordinate. The equations of motion and the boundary conditions are obtained by the Carrera's unified formulation, and further interpolated by collocation with radial basis functions.

Pressure Pulse Response of Composite Sandwich Panels with Plastic Core Damping

Abstract: An analytical technique is presented for obtaining the large amplitude, damped response of a foam-core composite sandwich panel subjected to uniform pressure pulse loading. The predicted panel resp...

Measuring thickness changes of edgewise compression loaded corrugated board panels using digital image correlation

Abstract: This study examines thickness changes in web-core sandwich panels under edgewise compressive loading Both undamaged and damaged panels were examined Three-dimensional full-field digital image correlation systems were used to determine deflections on both sides of loaded panels The change in thickness at any given point in the panel was obtained as the difference between the two displacement fields It was observed that the thickness was reduced in the post-buckling regime Damage introduced into the corrugated core by lateral compression proved to significantly reduce the load-carrying capability panels and elevate the thickness reduction of the panels

Sandwich composites made of syntactic foam core and paper skin: Manufacturing and mechanical behavior

Abstract: Novel sandwich composites made of syntactic foam core and paper skin were developed as potential building materials. Interface bonding between core and skin was controlled by varying starch content. Two different microsphere size groups were employed for syntactic foam core manufacturing based on the pre-mold processing method. Properties of skin paper with starch adhesive on were found to be affected by drying time of starch adhesive. Mechanical behavior of manufactured sandwich composites in relation with properties of constituent materials was studied. Skin paper contributed to increase up to 40% in estimated flexural strength over syntactic foams, depending on starch content in adhesive between syntactic foam core and paper skin. Small microsphere size group for syntactic foam core was found to be advantageous in strengthening of sandwich composites for a given starch content in adhesive. This finding was in agreement with calculated values of estimated shear stress at interface between skin and core. Failure process of the sandwich composites was discussed in relation with load–deflection curves. Cracking of syntactic foam core was detected to be the first event in sandwich composite failure sequence. Hygroscopic behavior of syntactic foam panels was investigated. Moisture content in the foam was measured to be high for high starch content in the foam panels. No significant moisture effect on flexural strength of syntactic foam panels after being subjected to moisture about 2 months was found for both microsphere size groups. However, substantial decrease (28%) in flexural modulus was found for the foam panels made of large microspheres although not much moisture effect was found on that of small microspheres.

Optimizing sandwich beams for strength and stiffness

Abstract: This article presents a method for minimizing structural mass by optimizing the geometry and core density of sandwich beams comprising composite face sheets and polymer foam cores, loaded in three-...

First-order shear deformation analysis of the sandwich plate twist specimen

Abstract: An analytical solution of the deflection of the sandwich twist specimen based on first-order shear deformation theory is presented. The solution utilizes a Fourier series approach for the loading and displacements. Parametric analysis of the influences of face and core stiffnesses, core thickness and panel in-plane size on the plate compliance is conducted. The results are compared to finite element analysis. The first-order shear deformation compliance results are in good agreement with finite element analysis for sandwich panels with soft, shear deformable cores, whereas for stiff cores the first-order shear deformation solution underpredicts the panel compliance. Reasons for the discrepancy and limits for the use of the analysis are discussed.

Evaluation of micromechanical methods to determine stiffness and strength properties of foams

Abstract: The stiffness and strength properties of foams with tetrakaidecahedral unit cells are evaluated using both finite element-based micromechanics and analytical methods. The finite element analysis models the varying cross section of the struts exactly. The analytical methods assume the struts have constant cross section along the length. Equivalent constant cross section of the strut can be obtained by either matching the densities or by using harmonic averaging of the stiffness properties. A method in which the moment of inertia of the cross section is averaged is also considered. The comparison of properties obtained for the different equivalent constant cross-section foams shows the inability of the various averaging schemes to match the shear modulus and the strength properties, while the Young’s modulus matches to some extent. The failure of the weakest cross section in the varying cross-section strut of the unit cell leads to a lower tensile strength in the actual foam compared to the uniform cross-se...

Fabrication and properties of stainless steel foams for sandwich panels

Abstract: Open-celled metallic foams with their specific structural properties are attractive candidates for a wide range of applications in the field of catalyst supports, process, and energy technologies. ...

Effects of time and stress state on fracture of closed-cell polyvinyl chloride foam:

Abstract: The effects of time and stress state on fracture behaviors and mechanisms of polyvinyl chloride foam have been evaluated by varying displacement rates (0.1 to 1000 mm/min) and specimen thicknesses ...

Cohesive zone modeling of coupled buckling – Debond growth in metallic honeycomb sandwich structure:

Abstract: Buckling-induced skin–core debond growth in honeycomb sandwich cantilever beam is demonstrated using a cohesive zone model. The input parameters for the analysis are interfacial bond strength, mode...

Development of damage in some polymeric foam-core sandwich beams under bending loading:

Abstract: The damage mechanisms in sandwich plates with polymer foam cores are investigated. There has been a long-standing controversy on what is the weakest element of sandwich plates, the skin-core bonding or the foam core. Under four-point bending of a sandwich plate, we found that in all cases cracks are nucleated inside the polymer foam core as monitored by a high-speed camera. Several separated small cracks develop in the core; then the small cracks are connected and form the main crack that propagates through the core. We complemented the optical recording by the acoustic emission study which provides an additional insight in the fracture process. In addition, the influence of sub-zero temperature of the ultimate load of each material is examined at discrete values of sub-zero temperature. The results revealed that both materials become increasingly brittle with decreasing temperature.

Failure of uniformly compression loaded debond damaged sandwich panels — An experimental and numerical study

Abstract: This paper deals with the failure of compression-loaded sandwich panels with an implanted circular face/core debond Uniform compression tests were conducted on intact sandwich panels with three di

Analysis of sandwich plates: A three-dimensional assumed stress hybrid finite element:

Abstract: An eight-node assumed stress solid element with rotational degrees of freedom is employed for analyses of sandwich plates consisting of stiff face and a comparatively flexible core material with the aim to accurately and efficiently capture stresses. The element formulation is based directly on an eight-node element. This direct formulation requires fewer computations than a similar element that is derived from an internal 20-node element in which the midside degrees of freedom are eliminated by expressing them in terms of displacements and rotations at corner nodes. The formulation is based on Hellinger-Reissner variational principle. Numerical examples are presented to show the validity and efficiency of the present element.

Methodology for exciting dynamic shear and moment failure in composite sandwich beams

Abstract: The increasing use of composites in high performance structural applications subjected to dynamic loading drives the need for understanding basic material failure properties and structural response...

Strength of aluminum single-lap bonded joints in various disbond size at circular and semi-circular notches:

Abstract: Applications of assembly bonded technique have been extended currently to various industrial fields. Adhesively bonded joints are widely used in the aeronautics and astronautics industry as a resul...

Analytical approach for modal characteristics of honeycomb sandwich beams with multiple debond

Abstract: An analytical formulation for the evaluation of vibration characteristics of sandwich beams with multiple debond at the interface between the carbon fiber reinforced plastic (CFRP) face sheets and honeycomb core is obtained and verified through test. The well-known split theory is modified considering the core stiffness of sandwich beam model and accordingly the equations of motion are set up for both the undebonded and the debond regions to solve the Eigenvalue problem. Honeycomb sandwich beams with aluminium as well as quasi-isotropic carbon epoxy laminate face sheets of same thickness and core height are fabricated and tested. In the case of cantilever beam with single debond, the region of validity of the analytical solution for natural frequencies and mode shapes are found to be within a debond size of 12% of the beam length for sandwich beams with either CFRP or aluminium skin. Studies on multiple debond reveal that a single long debond can result in significant reduction in the frequencies of the b...

Effects of pressure and temperature on the vibration behavior of sandwich cylindrical shells

Abstract: In this study, damping characteristics of viscoelastic sandwich cylindrical shell under internal/external pressure and internal temperature are investigated. By implementing finite difference method for irregular grids in the sandwich cylinder, the temperature distribution at each layer is determined. A non-linear model for displacement fields at the viscoelastic core layer is developed to describe the damping characteristics of sandwich cylinder with thin/thick core layer. The effect of dissipation in a pressurized cylinder under harmonic variation of the pressure is also investigated. As shown, deflection increases with time due to the dissipated energy and consequently loss factor is not unique even at a constant frequency.

Evaluation of Low-Velocity Impact Response of Honeycomb Sandwich Structures Using Factorial-based Design of Experiments

Abstract: A response surface analysis of data from factorial experiments is used to determine the effect of different design factors on the low-velocity impact response of the honeycomb sandwich structures. A low-cost, out-of-autoclave manufacturing process is utilized to manufacture aerospace quality honeycomb sandwich panels. CYCOM® 5320 out-of-autoclave prepreg and FM® 300-2M are used as facesheet and adhesive, respectively. The out-of-autoclave process uses the vacuum bag pressure, thus, avoiding costly tooling and making the process more economical. A completely randomized design is used while manufacturing and testing the samples. Three design factors: angle difference between successive prepreg layers, number of prepreg layers, and number of adhesive layers, are selected as experimental variables. Each variable is considered at three levels, yielding a 33 factorial design. To investigate the effect of the experimental variables on the three response variables: energy absorbed (X); peak contact force (Y) and ...

Design and analysis of multifunctional structures with embedded electronics for thermomechanical loads

Abstract: Multifunctional structures, of various forms, are being used in aerospace industry and there have been increasing efforts to enhance their performance. The design and analysis of a composite sandwich beam embedded avionics and integrated cooling systems is presented in this article. The integrated electronics inside a sandwich beam reduces the overall weight of a vehicle by eliminating most of the avionics housing, cables, interconnects, etc. The foam core of a sandwich beam is modified with a cavity to embed avionics. Since the presence of a cavity degrades the strength of the structure, various methods of reinforcement have been presented. The heat dissipation system has been designed to protect the structure from excessive thermal loads. The design of heat dissipating system consists of two parts, thermal interface materials and a highly efficient heat transfer device. Design guidelines for a thermal interface material consisting of particulate composites are presented here. Among various choices, heat...

Optimization for sandwich damping composite structure: Used in sprockets of crawler vehicles

Abstract: A parametric study of dynamic optimization is carried out for a vibration absorber which consists of sandwich composites with a tube configuration and be used in the sprocket of crawler vehicles. A decoupling method is suggested, in which the geometrical parameter, Y, and viscoelastic material loss factor, β, are given first, then the shear parameter, x, is determined by analyzing the coupling equations of structural parameters. Based on the temperature–frequency effect about the viscoelastic materials, the structure optimization of the vibration absorber is performed by taking the structural dimensions as variables, and the structural loss factors, η, and, Y, as the objective functions. A dynamic test is executed for the optimized vibration absorber, and the testing data show good agreement with the optimized theoretical results. A contrast experiment is conducted in a practical bulldozer, and the results present that the vibration level is decreased significantly by the optimized vibration absorber.

On the piercing force enhancement of aluminum foam sandwich plates under impact loading

Abstract: This article tends to explain the observed piercing force enhancement under impact loading of the sandwich panels, which is made of rate-insensitive aluminum skin sheet and Cymat foam core. Using an elastic–plastic damageable model for the skin and an isotropic foam constitutive model for the core, a numerical model is proposed and validated by the comparison with experimental results. Virtual tests allow revealing that the damage evolution in the skin sheet is strain rate dependent because of the wave propagation effects. Under impact loading, the damage area is larger and it leads to a larger deflection of the skin plate at its breaking. Therefore, the foam cores are more compressed under impact loading and induce the piercing force enchancement of the sandwich.