Metal Foam Core Research Articles

Convective Heat Transfer in Open Cell Metal Foams

Convective heat transfer in aluminum metal foam sandwich panels is investigated with potential applications to actively cooled thermal protection systems in hypersonic and re-entry vehicles. The size effects of the metal foam core are experimentally investigated and the effects of foam thickness on convective transfer are established. Four metal foam specimens are utilized with a relative density of 0.08 and pore density of 20 pores per inch (ppi) in a range of thickness from 6.4mmto25.4mm, in increments of approximately 6mm. An exact-shape-function finite element model is developed that envisions the foam as randomly oriented cylinders in cross flow with an axially varying coolant temperature field. A fully developed velocity profile is obtained through a semi-empirical, volume-averaged form of the momentum equation for flow through porous media, and used in the numerical analysis. The experimental results show that larger foam thicknesses produce increased heat transfer levels, but that this effect diminishes for thicker foams. The finite element simulations capture the thickness dependence of the heat transfer process and good agreement between experimental and numerical results is obtained for larger foam thicknesses.

Responses of damage and energy of sandwich and multilayer beams composed of metallic face sheets and aluminum foam core under bending loading

This present article deals with bending deformation and failure behavior of sandwich and multilayer beams composed of aluminum foam core and metallic face sheets analyzed byin-situ surface displacement analysis (SDA). The effect of beam structure on the failure mode of beam and the energy absorbed by beam failure were investigated and discussed. The SDA results revealed that collapse of the sandwich beams is by two basic modes, indentation (ID) and core shear (CS). The ID is localized deformation on the beam adjacent to the inner or outer roller in four-point bending, where displacement and compressive strains are at the maximum. As for CS mode, failure occurs in the core between inner and outer rollers, which corresponds to the maximum shear strain; discontinuous displacements in both the vertical and horizontal directions are the primary factors for shear crack initiation, growth, and broadening. The failure of the multilayer beams depends on whether the face sheets show ID mode or otherwise. If a single layer core sandwich fails in ID mode, the multilayer beams with similar face sheets show mixed ID + CS modes. If a single layer core sandwich fails fully in CS mode, no ID characteristic appears in the similar face sheet multilayer beams. The deformation energy of the beams relates strongly to the structure and geometry of beam. The predication of the bending fracture workWx of a beam is given by $$W_x /W_5 = 0.3875C + 0.9383$$ whereW5 is the deformation energy of the beam with a 5-mm core, andC is the thickness ratio of the corex to 5 mm.

Impact and post-impact vibration response of protective metal foam composite sandwich plates

Composite sandwich structures composed of a metal foam core possess the ability to provide significant energy absorption under impact from flying debris, routine operation, tool drop, flying projectiles, or crash conditions. Sandwich constructions of aluminum foam core and a variety of facesheets are being increasingly used in a number of applications ranging from turbine shrouds, heat exchangers, pneumatic tool mufflers, energy absorbers for blast shock waves, and silencers for jet engine intakes. In the present work, vacuum assisted resin transfer molding (VARTM) was used to produce composite sandwich plates with laminate facesheets and aluminum foam core. Aluminum foam of two different thicknesses and densities were studied in conjunction with facesheets composed of either S2-glass, E-glass, aramid, or carbon fiber reinforced vinyl ester (VE) resin. Low velocity impact (LVI), up to 5 m/s, and intermediate velocity impact (IVI), up to 100 m/s, tests were conducted to simulate impact conditions resulting from tool drop to blunt objects, and flying debris. It was concluded from the impact tests that the sandwich construction with S2-glass/VE facesheets in conjunction with aluminum foam was optimal for resisting low and intermediate velocity impact. The vibration response of composite sandwich plates composed of laminate facesheets and aluminum foam core was also studied under a free–free boundary condition. The vibration response (natural frequency and damping ratio) is reported as a function of impact to the sandwich plate.

Functional grading of metal foam cores for yield-limited lightweight sandwich beams

Keywords: Aluminium ; Bending test ; Foams ; Functionally graded materials (FGM) ; Sandwich structure Note: Ecole Polytechnique Federale de Lausanne (EPFL), Laboratory for Mechanical Metallurgy, Station 12, CH-1015 Lausanne, Switzerland13596462 (ISSN) Reference LMM-ARTICLE-2006-004doi:10.1016/j.scriptamat.2005.10.050View record in Web of ScienceView record in Scopus Record created on 2006-10-09, modified on 2017-05-10

The response of clamped sandwich plates with metallic foam cores to simulated blast loading

The dynamic responses of clamped circular monolithic and sandwich plates of equal areal mass have been measured by loading the plates at mid-span with metal foam projectiles. The sandwich plates comprise AISI 304 stainless steel face sheets and aluminium alloy metal foam cores. The resistance to shock loading is quantified by the permanent transverse deflection at mid-span of the plates as a function of projectile momentum. It is found that the sandwich plates have a higher shock resistance than monolithic plates of equal mass. Further, the shock resistance of the sandwich plates increases with increasing thickness of sandwich core. Finite element simulations of these experiments are in good agreement with the experimental measurements and demonstrate that the strain rate sensitivity of AISI 304 stainless steel plays a significant role in increasing the shock resistance of the monolithic and sandwich plates. Finally, the finite element simulations were employed to determine the pressure versus time history exerted by the foam projectiles on the plates. It was found that the pressure transient was reasonably independent of the dynamic impedance of the plate, suggesting that the metal foam projectile is a convenient experimental tool for ranking the shock resistance of competing structures.

A Comparison of the Structural Response of Clamped and Simply Supported Sandwich Beams With Aluminium Faces and a Metal Foam Core

Plastic collapse modes for clamped sandwich beams have been investigated experimentally and theoretically for the case of aluminium face sheets and a metal foam core. Three initial collapse mechanisms have been identified and explored with the aid of a collapse mechanism map. It is shown that the effect of clamped boundary conditions is to drive the deformation mechanism towards plastic stretching of the face sheets. Consequently, the ultimate strength and level of energy absorption of the sandwich beam are set by the face sheet ductility. Limit load analyses have been performed and simple analytical models have been developed in order to predict the postyield response of the sandwich beams; these predictions are validated by both experiments and finite elements simulations. It is shown experimentally that the ductility of aluminium face sheets is enhanced when the faces are bonded to a metal foam core. Finally, minimum weight configurations for clamped aluminium sandwich beams are obtained using the analytical formulas for sandwich strength, and the optimal designs are compared with those for sandwich beams with composite faces and a polymer foam core.

Size Effects in Metal Foam Cores for Sandwich Structures

The shear response of aluminum foam, including size effects, is measured and quantified for a closed-cell aluminum foam. The shear stiffness is shown to depend linearly on density, whereas the strength exhibits a power law dependence. The linear response is shown to be independent of strain rate up to rates of 0.17/s, whereas the strength and energy absorption increase with increasing strain rate. The density dependence of the stiffness is reproduced analytically based on the composite cylinders model. Optical techniques are used to measure the strain field of the experimental specimens throughout the loading program. By evaluation of concentric subregions of the sample, a sample size of 18 mean cell diameters is determined to be the dimension below which the uncertainty in the predicted shear modulus of an aluminum foam sample increases significantly. This length scale threshold is replicated in a periodic finite element structure with randomly distributed imperfections. I. Introduction M ETAL foams represent an attractive alternative for sandwich structure cores for multiple reasons. First, with metal foam cores, the adhesive substrate of a sandwich structure may be eliminated with in-production integral bonding to metallic face sheets, stiffening the sandwich and broadening its range of operating environments. Second, metal foams exhibit a compressive stress‐strain response that is ideal for energy absorption and impact alleviation with a long, constant stress, plastic strain plateau. 1 Third, an opencell metal foam offers an opportunity to eliminate the catastrophic nature of water or cryogenic gas permeation that has crippled the long-term use of sandwich constructions with honeycomb cores. 2 Fourth, an open-cell construction also allows for active cooling of the sandwich structure, elevating its range of acceptable operating temperatures. For integration into sandwich structures, the shear behavior of metal foam must be understood. Some disparate results regarding shear behavior currently exist in the literature. One study found a linear relationship between shear strength and density, 3 whereas a cubic lattice model subjected to shear loading predicted a nonlinear power law dependence. 4 Another investigation offers only a few data points for shear stiffness and strength of melt-foamed aluminum. 5 Furthermore, these experiments involved thin specimens, with no account for size effects. The present paper offers the full shear response curves for a broad range of density. The density dependence of stiffness and strength are found experimentally with the former being reproduced analytically. The strain rate dependence of the shear response is also considered. The effect of specimen size, relative to the mean cell size, is analyzed experimentally with a unique approach involving digital image correlation. The observed behavior is reproduced with a finite element model. These analyses identify a threshold in the ratio of specimen size to cell size, below which the shear response of a given sample is associated with a significant amount of uncertainty.

The Collapse Response of Sandwich Beams with Aluminium Face Sheets and a Metal Foam Core

AbstractPlastic collapse modes of simply supported and clamped sandwich beams have been investigated experimentally and theoretically, for aluminium face sheets and Alporas foam core. The effect of clamped boundary conditions is to induce axial stretching after the initial yield mechanism. Hence, face sheet ductility dictates the level of energy absorption of the beam. Numerical and analytical predictions are validated by the available experimental evidence.

Collapse mechanisms of sandwich beams with composite faces and a foam core, loaded in three-point bending. Part I: analytical models and minimum weight design

Analytical predictions are made for the three-point bending collapse strength of sandwich beams with composite faces and polymer foam cores. Failure is by the competing modes of face sheet microbuckling, plastic shear of the core, and face sheet indentation beneath the loading rollers. Particular attention is paid to the development of an indentation model for elastic faces and an elastic–plastic core. Failure mechanism maps have been constructed to reveal the operative collapse mode as a function of geometry of sandwich beam, and minimum weight designs have been obtained as a function of an appropriate structural load index. It is shown that the optimal designs for composite–polymer foam sandwich beams are of comparable weight to sandwich beams with metallic faces and a metallic foam core.

Creep Response of Sandwich Beams with a Metallic Foam Core

The creep response of metallic foam sandwich beams in 3-point bend is investigated numerically for the case of a metallic foam core and two steel faces. The face sheets are treated as elastic, while the foam core is modeled by a viscoplastic extension of the Deshpande-Fleck yield surface. This power-law creeping constitutive law has been implemented within the commercial finite element code ABAQUS. It is found that the beams creep by a variety of competing mechanisms, depending upon the choice of material properties and the geometric parameters. A failure map is constructed and effect of rate dependence on the load-deflection curves is quantified, and compared against the available experimental data.

Influence of imperfections on the performance of metal foam core sandwich panels

Sandwich panels and beams are used in bending and compression dominated components. The retention of their load capacity in the presence of imperfections is a central consideration. To address this issue, sandwich beams with metallic foam cores have been tested in four-point bending following the introduction of imperfections, created by impressing the face sheets. Limit load expressions for face yielding, core shear, and indentation failure have been developed and used to construct failure mechanism maps. From these maps, specimen designs were determined. Imperfections were introduced by indenting to varying penetrations. The indents were located on both the compressive and tensile side of bending configurations. Experimental measurements of the load/deflection response are obtained and compared with finite element results.

Creep of sandwich beams with metallic foam cores

The steady state creep deflection rates of sandwich beams with metallic foam cores were measured and compared with analytical and numerical predictions of the creep behavior. The deflection rate depends on the geometry of the sandwich beam, the creep behavior of the foam core and the loading conditions (stress state, temperature). Although there was a considerable scatter in the creep data (both of the foams and of the sandwich beams made using them), the data for the sandwich beams were fairly well described by the analysis.

Size effects in metallic foam core sandwich beams

Foams exhibit size effects: if the specimen dimensions are of the same order as the cell size, the moduli and strength depend on specimen size. Metallic foams have particularly large cells (typically 2–20 mm), potentially giving rise to size effects when they are used in sandwich beams. Previous studies have shown that the shear strength of metallic foams bonded to rigid plates increases by over 50% if the specimen thickness is equal to the cell size. In this study, sandwich beams with aluminum alloy faces and aluminum alloy foam cores were tested in three-point bending to characterize the effect of the beam depth on the limit load. Beams of constant ratio of core thickness to span length but different absolute values of core thickness were tested, and the measured limit loads were compared to analytical values. The analysis gives a good description of the measured limit load when the shear size effect is accounted for.

Collapse of truss core sandwich beams in 3-point bending

Sandwich beams, comprising a truss core and either solid or triangulated face-sheets, have been investment cast in an aluminium–silicon alloy and in silicon brass. The macroscopic effective stiffness and strength of the triangulated face-sheets and tetrahedral core are estimated by idealising them as pin-jointed assemblies; tests show that this approximation is adequate. Next, the collapse responses of these sandwich beams in 3-point bending are measured. Collapse is by four competing mechanisms: face-yield, face-wrinkling, indentation and core shear, with the active collapse mode dependent upon the beam geometry and yield strain of the material. Upper bound expressions for the collapse loads are given in terms of the effective properties of the faces and core of the sandwich beam; these upper bounds are in good agreement with the measured beam response, and are used to construct collapse mechanism maps with beam geometrical parameters as the axes. The maps are useful for selecting sandwich beams of minimum weight for a given structural load index. The optimisation reveals that truss core sandwich beams are significantly lighter than the competing concept of sandwich beams with a metallic foam core.

Failure of sandwich beams with metallic foam cores

Abstract Sandwich beams with metallic foam cores can fail by several modes: face yielding, face wrinkling, core yielding and indentation. We estimate the initial failure load, corresponding to the first deviation from linearity in the load–deflection curve as well as the peak load for each mode. Failure mode maps are constructed which illustrate the dominant failure mode for practical beam designs. The results of the analysis are compared with experiments on sandwich beams with aluminum foam cores in three-point bending. The peak loads and the failure modes are described well by the analysis.

The plastic collapse of sandwich beams with a metallic foam core

Plastic collapse modes of sandwich beams have been investigated experimentally and theoretically for the case of an aluminium alloy foam with cold-worked aluminium face sheets. Plastic collapse is by three competing mechanisms: face yield, indentation and core shear, with the active mechanism depending upon the choice of geometry and material properties. The collapse loads, as predicted by simple upper bound solutions for a rigid, ideally plastic beam, and by more refined finite element calculations are generally in good agreement with the measured strengths. However, a thickness effect of the foam core on the collapse strength is observed for collapse by core shear: the shear strength of the core increases with diminishing core thickness in relation to the cell size. Limit load solutions are used to construct collapse maps, with the beam geometrical parameters as axes. Upon displaying the collapse load for each collapse mechanism, the regimes of dominance of each mechanism and the associate mass of the beam are determined. The map is then used in optimal design by minimising the beam weight for a given structural load index.

Buckling of cylindrical sandwich shells with metal foam cores

Buckling of cylindrical sandwich shells subject to axial compression is addressed for shells having foamed metal cores. Optimal face sheet thickness, core thickness and core density are obtained which minimize the weight of a geometrically perfect shell with a specified load carrying capacity. Constraints imposed by wrinkling and yielding of the face sheets and yielding of the core are all considered. The range of the structural load index is identified for which the sandwich shells have a competitive weight advantage over stringer stiffened shells. In most of this range, the minimum weight design has elastic buckling simultaneous with face sheet yielding. Imperfection sensitivity of the shells is assessed with special emphasis on the role of plasticity in degrading strength, especially in light of the coincidence of elastic buckling and face sheet yielding in the optimally designed perfect shell. The purpose is to examine the interaction between imperfections and plastic yielding to see if buckling load knockdowns should be larger than those expected for elastic shells.