Aluminum Foam Samples Research Articles

Micro-scale finite element modeling of ultrasound propagation in aluminum trabecular bone-mimicking phantoms: A comparison between numerical simulation and experimental results

The present study investigated the accuracy of micro-scale finite element modeling for simulating broadband ultrasound propagation in water-saturated trabecular bone-mimicking phantoms. To this end, five commercially manufactured aluminum foam samples as trabecular bone-mimicking phantoms were utilized for ultrasonic immersion through-transmission experiments. Based on micro-computed tomography images of the same physical samples, three-dimensional high-resolution computational samples were generated to be implemented in the micro-scale finite element models. The finite element models employed the standard Galerkin finite element method (FEM) in time domain to simulate the ultrasonic experiments. The numerical simulations did not include energy dissipative mechanisms of ultrasonic attenuation; however, they expectedly simulated reflection, refraction, scattering, and wave mode conversion. The accuracy of the finite element simulations were evaluated by comparing the simulated ultrasonic attenuation and velocity with the experimental data. The maximum and the average relative errors between the experimental and simulated attenuation coefficients in the frequency range of 0.6−1.4MHz were 17% and 6% respectively. Moreover, the simulations closely predicted the time-of-flight based velocities and the phase velocities of ultrasound with maximum relative errors of 20m/s and 11m/s respectively. The results of this study strongly suggest that micro-scale finite element modeling can effectively simulate broadband ultrasound propagation in water-saturated trabecular bone-mimicking structures.

Experimental investigation of convective heat transfer in an open-cell aluminum foams

Experimental results of heat transfer and pressure drop for a three open-cell aluminum foam blocks are presented for turbulent forced convection. The aluminum foam samples are inserted in a rectangular channel in a staggered manner on its bottom and top walls. A constant heat flux of 2W/cm2 is maintained in the test section on the bottom wall. The wall temperatures along the flow direction as well as the average inlet and outlet temperatures of the air were measured. Measurements of pressure drop across the aluminum foam sample with a grade of 40 PPI and a porosity of 93% in a blower were used to determine the intrinsic properties of this foam and to allow the determination of its permeability and inertial coefficient. The air velocity was varied from 1 to 5ms−1 while two heights of the porous blocks respectively 16mm and 20mm were used. Additionally a configuration of solid blocks made of aluminum was investigated.The results show that inserting metallic foams in a turbulent air flow improves the heat transfer by approximately 300% compared with an empty channel with adopting lower velocity, which reduces the supplied power. This improvement is even more important as the sample height is great. Increasing the sample size of the metallic foam, accentuate the turbulent kinetic energy levels inside the porous matrix and into the inter-baffle space, which increased the heat dissipated in the wall. In addition, the use of metallic foam increased the thermal conductivity of the coolant. Finally, compared with solid baffles, the aluminum foam created lower pressure losses because of their permeability and dissipate 2 times more heat.

Interaction of a shock wave with a closed cell aluminum metal foam

The present investigation examines the interaction of shock waves with closed cell aluminum foam samples in a conventional shock tube. The effect of the sample thickness on shock wave attenuation and/or enhancement and the use of the foam in the sandwich structure is studied. Results in terms of incident and reflected shock pressures are obtained, and the effectiveness of the samples with and without the foam is compared. It is demonstrated that the foam density and thickness, as well as the placement of cover plates of the same material in front of and behind the foam have the most significant effect on the reflected shock pressure. It is concluded that the closed cell aluminum metal foam can be effectively used as a sacrificial layer in blast protection of structures.

Application of metal hydrides as pore-forming agents for obtaining metal foams

One of the well-known ways of metal foam production consists of adding powder which releases gaseous decomposition product at high temperatures into a melted metal. When producing aluminum foams, titanium hydride powder is most frequently used, while its main disadvantage is its insufficient thermal stability. This results in a premature gas loss in the process of the powder introduction into a melted metal matrix. The paper discloses approaches to suppression of the undesirable premature titanium hydride decomposition by means of its preliminary thermal treatment, as well as the external pressure elevation over that of the system being processed during the foam formation. A method of using the thermally treated powder is proposed, in order to obtain aluminum foam samples having a required geometry and sufficiently uniform porous structure.

Effect of local mass distribution on modal behavior of Aluminum foam

Free vibration analysis of aluminum foam sample is performed in the present paper. Mechanical properties of metal- lic foams depend on their relative densities. The discrete mass distribution of metallic foam causes discontinuities in their local properties. X-ray computer tomography (XCT) revealed the 3-D inhomogeneous mass distribution in metallic foams. The densi- ty mapping method is used to approximate the aluminum foam trade name ALPORAS by a continuous, 3-D density distribu- tion at the mesolevel. The density recorded by XCT is homogenized over an averaging volume. A continuum model is imple- mented using the finite element (FE) method to simulate the effect of local mass distribution on the modal behavior of metallic foam sample. The effect of measured density distributed model (MDDM) Coarse sub-domain density model (CSDM), uniform average density model (UADM), and three random distributed density model (RDDM) on eigen frequencies of the aluminum foam sample is studied. Density distribution of aluminum foam measured by X-ray computer tomography was used as input to the corresponding FE model of investigated specimen. All numerical investigations were carried out by means of the FE pack- age ANSYS.

Experimental investigation on the strain-rate effect and inertia effect of closed-cell aluminum foam subjected to dynamic loading

Abstract In this paper, a series of experiments were conducted to clarify the effects of the strain rate and inertia on the deformation behavior of closed-cell aluminum foams under impact. The quartz-crystal technique was employed to analyze the stress uniformity of aluminum foam samples under split Hopkinson pressure bar (SHPB) loading. It was revealed that the condition of stress uniformity is hard to reach for a thicker foam sample, and the strength of aluminum foam is sensitive to strain rate. Two different direct-impact Hopkinson pressure bar (DHPB) methods were introduced to validate the three deformation modes, i.e. homogeneous mode, transitional mode and shock mode. Results displayed that the stress at the front surface increased dramatically than that at the back surface as the impact speeds increased from 16 m/s to 113 m/s. The axial-inertia effect became more important than the strain rate effect under high speed impact. The dynamic deformation processes were recorded by a Phantom-675 camera and were analyzed through the digital imaging correlation (DIC) method. The deformation of aluminum foam in homogeneous mode was presented by the evolution of global distributed failure, but it was dominated by the local failure in shock mode.

Enhanced sound absorption of aluminum foam by the diffuse addition of elastomeric rubbers

The sound absorption properties of open cell aluminum foams are understood to be significant (Ashby et al. Metal Foams: A Design Guide, 2000) with theoretical models presented in the literature [J. Acoust. Soc. Am. 108, 1697–1709 (2000)]. The pores that exist in metal foams, as artifacts of the manufacturing process, are left unfilled in the vast majority of cases. Work done by the US Navy (US patent 5895726 A) involved filling the voids with phthalonitrile prepolymer, resulting in a marked increase in sound absorption and vibration damping. The work presented here involves adding small amounts of elastomeric rubbers to the metal foam, thereby coating the ligaments of the foam with a thin layer of rubber. The goal is to achieve an increase in sound absorption without the addition of cost and weight. The work involves testing aluminum foam samples of various thicknesses and pore sizes in an impedance tube, with and without the added rubber. A design of experiment model was employed to gauge the effect of the various manufacturing parameters on the sound absorption and to set the stage for a physics-based predictive model.

Effect of Organic Space Holder in Fabrication of Cloced-Cell Aluminium Foam

In this work, the closed-cell aluminium foams were fabricated via powder metallurgy route with organic space holder which also known as carbamide (urea). Carbamide was added with aluminium powder at different compositions from 50wt% to 70wt%. Then the influences of sintering temperatures were also investigated at 500°C, 550°C and 600°C. Physical and mechanical tests were conducted on the fabricated closed cell aluminium foam samples. The result showed that the porosity inversely proportional with density, that means the porosity decreases as the density of the foam increased. Also, it was revealed that the highest compressive strength was obtained at sintering temperature of 550°C. However, the compressive strength of the closed-cell aluminium foam decreased at the sintering temperature of 600°C. This is due to the occurrence of oxidation problem. It can be concluded that carbamide can be proposed as one of the organic space holder material in the fabrication of closedcell aluminium foam. Then, the experimental parameters such as sintering temperature, type of space holder and composition gave great influence to the mechanical and physical properties of the fabricated closed-cell aluminium foam.

Metal foam regenerators; heat transfer and storage in porous metals

Abstract

Air forced convection through metal foams: Experimental results and modeling

Metal foams are a class of cellular structured materials with open cells randomly oriented and mostly homogeneous in size and shape. In the last decade, several authors have discussed the interesting heat transfer capabilities of these materials as enhanced surfaces for air conditioning, refrigeration, and electronic cooling applications. This paper experimentally characterizes twenty-one aluminum and copper foam samples with different number of pores per linear inch (PPI), which vary between 5 and 40 and with porosity ranging between 0.896 and 0.956; samples of different heights (20 and 40mm) have been studied. The most important parameters both geometrical (porosity, pore density, and foam core height) and operative (air mass flow rate, imposed heat flux) affecting the heat transfer and fluid flow behavior of these enhanced surfaces are analyzed and discussed. The results are presented with reference to different performance parameters: overall and interstitial heat transfer coefficients, foam finned surface efficiency, normalized mean wall temperature, pumping power per area density, and pressure gradient. From the experimental measurements two correlations for the heat transfer coefficient and pressure drop calculations have been developed, validated and here proposed. These models can be successfully used to optimize different foam heat exchangers for any given application.

An assessment on forced convection in metal foams

Metal foams are a class of cellular structured materials with open cells randomly oriented and mostly homogeneous in size and shape. In the last decade, several authors have discussed the interesting heat transfer capabilities of these materials as enhanced surfaces for air conditioning, refrigeration, and electronic cooling applications. This paper reports an assessment on the forced convection through metal foams presenting experimental and analytical results carried out during air heat transfer through twelve aluminum foam samples and nine copper foam samples. The metal foam samples present different numbers of pores per linear inch (PPI), which vary between 5 and 40 with a porosity ranging between 0.896–0.956; samples of different heights have been studied. From the experimental measurements two correlations for the heat transfer coefficient and pressure drop calculations have been developed. These models can be successfully used to optimize different foam heat exchangers for any given application.

Influences of Processing Factors on Pore Structure of Aluminum Foams

Orthogonal experiments were employed to confirm the interactive effect of the addition of magnesium powder, milling time of aluminum powder, oxidization time of TiH2 on the pore structure of Aluminum foam, which prepared by PCM method. The results show that the three factors arragned in important order as the follows: oxidization time, milling time, and the quantity of magnesium powder. The high quality aluminum foam samples, with average porosity of 84.2%, average equivalent circle diameter of 1.9 mm and average circularity of 0.754, was made on the condition of milling time as 85 minutes, oxidization time as 10 minutes and magnesium powder content as 0.5 wt%.

Numerical prediction of the radiative behavior of metallic foams from the microscopic to macroscopic scale

A parallel C++ numerical code was implemented to quickly and accurately calculate the radiative properties (reflectance and transmittance) of metallic foams that had scattering domains (pores, struts) with mean sizes larger than the wavelengths associated with thermal radiation. For application, the code required experimental knowledge of the material characteristics on several length scales ranging from the microscopic (approximately one micrometer) to macroscopic (approximately one centimeter). In particular, the 3D voxelized image of the investigated material was used, which was previously acquired using x-ray μ-tomography. The radiative properties were obtained by tracking a large number of rays within the numerical medium. On the local scale, rays interacted directly with the fluid/solid interface, which was obtained prior to the calculation by the application of a robust marching cubes algorithm to the entire set of voxels. The code was applied to aluminum foam samples, for which the experimental radiative properties were previously measured at room temperature with a customized instrument based on an infrared spectrometer. The code performance is discussed and the results enabled determination of the effects caused by the fluid/solid interface on the interfacial reflection.

Acoustic properties of air-saturated porous materials containing dead-end porosity

This study examines the acoustic properties of materials with complex micro-geometry containing partially open or dead-end (DE) porosity. One of these kinds of materials can be obtained from dissolving salt grains embedded in a solid metal matrix with the help of water. The solid matrix is obtained after the metal, in liquid form, has invaded the granular material formed by the salt particles at negative pressure and high temperature, and after cooling and solidification of the metal. Comparisons between theoretical and experimental results show that the classical Johnson-Champoux-Allard model does not quite accurately predict the acoustic behavior. These results suggest that the assumptions of the Biot theory may not all be fulfilled and that cavity resonators and dead ends can be present in the material. The first part of the study proposes a simple model to account for this geometry. Based upon this model, two acoustic transfer matrices are developed: one for non-symmetric and one for symmetric dead-end porous elements. It is thought that this model can be used to study the acoustic absorption and sound transmission properties of the type of material previously described. In the second part, a series of simplified samples are proposed and tested with a three-microphone impedance tube to validate the exposed model. Finally, the third part compares the predictions of the exposed model to the impedance tube results on a real aluminum foam sample containing dead-end pores. These first results are encouraging and show that this simple model also provides a good prediction for these materials with more complicated microstructure.

Effect of Confining Wall on Properties of Gas Flow Through Metal Foam: An Experimental Study

While the Darcy and Forchheimer relations are widely applied to determine the permeability and the form drag coefficient of open-cell metal foam, they both assume that the porous medium is infinite in all directions, i.e., large enough so that the effect of any confining walls is negligible. Many researchers, however, pay little or no attention to the size of metal foam samples in pressure-drop studies. The size of a foam sample perpendicular to the flow direction may be small enough such that wall effects are significant. This article experimentally investigates the wall effect on the permeability and form drag coefficient for two types of open-cell aluminum foam subjected to airflow entering the foam in the Forchheimer regime. The Forchheimer equation was recast in two different manners, which resulted in new non-dimensional numbers that correlated very well with the diameter of the foam samples measured in cells. The correlations are valid for a confining-tube-diameter-based Reynolds number ranging from approximately 13,000 to 105,000, and for diameters ranging from 12 to 36 cells and 24 to 60 cells for 10- and 20-pore per inch foam, respectively. The obtained correlations allow for determining pressure drop given only the velocity and the diameter of an aluminum foam sample.

High-velocity impact experiment of aluminum foam sample using powder gun

Porous materials such as aluminum foam have been investigated for possible use as impact shock absorbers in transportation aeronautic applications. However, the response of aluminum foam during impacts at high velocities of more than 100 m/s is not yet fully understood. A high-velocity impact experiment was therefore carried out to clarify impact shock absorption properties of aluminum foam. A one-stage powder gun was used to accelerate an aluminum foam sample to impact a rigid wall. Velocity and deformation of the aluminum foam sample during impact was studied using a digital high-speed video camera, while the pressure wave in the aluminum foam sample was measured using a PVDF gauge. The experimental observations revealed uneven collapse of the aluminum foam sample structure during high speed impact with a general stress plateau effect, typical for cellular material structures when subjected to quasi-static loading.

Thermal conductivity and contact resistance of metal foams

Accurate information on heat transfer and temperature distribution in metal foams is necessary for design and modelling of thermal-hydraulic systems incorporating metal foams. The analysis of heat transfer requires determination of the effective thermal conductivity as well as the thermal contact resistance (TCR) associated with the interface between the metal foam and the adjacent surfaces/layers. In this study, a test bed that allows the separation of effective thermal conductivity and TCR in metal foams is described. Measurements are performed in a vacuum under varying compressive loads using ERG Duocel aluminium foam samples with different porosities and pore densities. Also, a graphical method associated with a computer code is developed to demonstrate the distribution of contact spots and estimate the real contact area at the interface. Our results show that the porosity and the effective thermal conductivity remain unchanged with the variation of compression in the range 0–2 MPa; but TCR decreases significantly with pressure due to an increase in the real contact area at the interface. Moreover, the ratio of real to nominal contact area varies between 0 and 0.013, depending upon the compressive force, porosity, pore density and surface characteristics.

Static and Dynamic Crushing Behaviors of Aluminum Foam

Quasi-static and dynamic axial crushing tests were carried out to investigate the energy absorption capacity of aluminum foam with several densities. First, aluminum foam samples with three densities were crushed quasi-statically to study their compression characteristics, the stress strain relationship curves and energy absorption capacities were achieved from the test results; then dynamic crushing tests were carried out to check the crushing behaviors of the corresponding samples; finally, the energy absorption capacities of tested samples were compared. According to the tests results, all the aluminum foam samples showed very good isotropy and very high mass specific energy absorption capacities. For the crushing tests, the dynamic crushing behaviors were very similar to the static ones, which meant aluminum foam was insensitive to the crash velocity tested.

Resistance network-based thermal conductivity model for metal foams

A network model for the estimation of effective thermal conductivity of open-celled metal foams is presented. A nodal network representation of three aluminum foam samples from DUOCEL – 10 ppi, 20 ppi and 40 ppi – is constructed out of X-ray microtomography data obtained by computed tomography (CT) scanning of the samples using a commercial CT scanner. Image processing and 3D skeletonization are performed with commercially available image processing software. The effective thermal conductivity is estimated through a 1D conduction model, representing individual ligaments as an effective thermal resistance using the topological information from the scan data. The effective thermal conductivity data thus obtained are compared with the Lemlich theory and other pore-based models. Further, microstructural characterization of foam features – pore size, ligament thickness, ligament length and pore shapes – is performed. All the three foam samples are observed to have similar pore shapes and volumetric porosity, while the other features scale with the pore size. For a given porosity the computed permeability is found to scale as the square of the pore diameter, as also noted by previous researchers.

Effect of sample’s length on flow properties of open-cell metal foam and pressure-drop correlations

Many applications require fluid flow through the open pores of metal foam. The foam is usually treated as a porous medium for which the Darcy law and the Hazen-Dupuit-Darcy (or Forchheimer) equation are used to describe the pressure drop, and for obtaining the two important flow properties, i.e., the permeability and the form drag coefficient. Little or no attention is paid to the length (or thickness) of the porous medium in the flow direction. This paper establishes a minimum length necessary for the foam to have length-independent (or bulk) permeability and form drag coefficient. This minimum length is obtained experimentally for various types of open-cell aluminum foam subjected to airflow in the Forchheimer regime. Below this thickness values of the two key flow properties are not constant, and they include entrance/exit effects, which may explain some of the discrepancies in the reported values in the literature. The Forchheimer equation was recast in two different manners, which resulted in new non-dimensional numbers- one representing the form drag and the other the viscous drag. These numbers correlated very well with the thickness of the porous medium. The obtained correlations allow for determining the pressure drop given only the velocity and the thickness of an aluminum foam sample.