Closed-cell Aluminum Alloy Foams Research Articles

Experimental investigation of the fatigue crack propagation in a closed-cell aluminum alloy foam

Fatigue crack propagation in a closed-cell aluminum alloy foam is experimentally investigated in this work. A series of fatigue tests are conducted to obtain the a-N curves (i.e. crack length vs loading cycle number) and da/dN-∆K curves (fatigue crack propagation rate vs stress intensity). Statistical analysis is carried out to assess the stochastic and scattering characteristics in the fatigue crack propagation of the foam, based on which a statistical fatigue crack propagation model is developed. Furthermore, the fracture surfaces of the foam specimens are characterized, as well as the fatigue fracture morphologies. It is demonstrated that the structural heterogeneity inherent in the material and the difference in the cell structures between different specimens account for the great scatter of the fatigue crack propagation in the close-cell aluminum alloy foam.

The temperature effect on the axial quasi-static compressive behavior of ex-situ aluminum foam-filled tubes

This study focuses on the effect of temperature on the mechanical behavior of closed-cell aluminum-alloy foam filled tubes (FFTs) under quasi-static compressive loads. The results of the compressive testing indicated that at each tested temperature the closed-cell aluminum foam improves the mechanical properties of the empty steel tubes. This behavior is related to the interaction effect between the aluminum foam as filler material and the empty tubes. Also, it was observed that the deformation mechanism of FFTs at all tested temperatures is axisymmetric concertina mode with formation of two folds. Due to the softening phenomenon of the steel tube matrix with increasing of temperature the distribution and size of propagated micro-cracks on both loading surfaces and peripheral folds decreased significantly from the order of millimeters up to micrometers. Finally, it was observed that the increasing of the working temperature reduces the ability FFTs to absorb energy during compression test.

Quasi-static compressive behavior of the ex-situ aluminum-alloy foam-filled tubes under elevated temperature conditions

This manuscript focuses on the uniaxial compressive performance of thin walled steel tubes filled with closed-cell aluminum-alloy foam (ex-situ FFTs) at high temperature. For this purpose, the axial compressive behavior of empty tubes, closed-cell aluminum foam, and ex situ FFTs was evaluated under quasi-static loading conditions at 300°C. The FFTs were compressed according to the concertina mode with the formation of two folds at the tested temperature. Also it was concluded that inserting closed-cell aluminum foam as a filler material inside the empty steel tube improved its energy absorption by 23% at 300°C, as well as reducing crack initiation and propagation in the steel tube.

Effect of heat treatment on the low velocity impact response of A356/SiCp composite foam

In this work, the effect of heat treatment on the low velocity impact response of closed-cell A356 aluminum alloy foam was investigated. A number of aluminum foam specimens were processed by T6 aging treatment. The drop weight impact test with a hemispherical striker tip was carried out on both as-received and T6-aged specimens. The load-time history data obtained showed a significant increase in the plateau load level for the heat-treated specimens. The length of plateau region is decreased for these specimens. It was also found that the heat treatment can significantly raise the energy absorption capacity of this cellular metal. Furthermore, the impact behavior of tested foams was simulated through a 3D finite element code. The numerical results were compared with the experimental ones and validated. The results showed that the numerical model can predict accurately the plateau load level, the impact time, the energy absorbed by the foam and the failure mode of the material.

The Microstructure and Compressive Properties of Aluminum Alloy (A356) Foams with Different Al-Ti-B Additions

Closed-cell aluminum alloy (A356) foams with different percentages of Al-Ti-B are prepared by melt foaming method, using Ca and TiH 2 as thickening agent and foaming agent, respectively. SEM and Quasi-static compression tests are performed to investigate the effect of Al-Ti-B on the microstructure and compressive properties of aluminum alloy (A356) foams. The results show that foams with Al-Ti-B percentage of 0.3 wt.% possess good combinations of micro hardness, yield strength, plateau strength, densification strain and energy absorption capacity under the present conditions. The reasons are mainly due to the foams with Al-Ti-B percentage of 0.3 wt.% possess optimal eutectic Si morphology (with eutectic Si existing in the forms of particles or short fiber). DOI: http://dx.doi.org/10.5755/j01.ms.22.3.8559

Modelling of aluminium foam core sandwich panels under impact perforation

Aluminium foam core sandwich panels are good energy absorbers for impact protection applications, such as light-weight structural panels, packing materials and energy absorbing devices. In this study, the high-velocity impact perforation of aluminium foam core sandwich structures was analysed. Sandwich panels with 1100 aluminium face-sheets and closed-cell A356 aluminium alloy foam core were modelled by three-dimensional finite element models. The models were validated with experimental tests by comparing numerical and experimental damage modes, output velocity, ballistic limit and absorbed energy. By this model the influence of foam core and face-sheet thicknesses on the behaviour of the sandwich panel under impact perforation was evaluated.

Compressive behaviour of unconstrained and constrained integral-skin closed-cell aluminium foam

The aim of this paper is to evaluate the quasi-static and dynamic compressive crush performance of integral-skin closed-cell aluminium alloy foam with and without radial constraints. The foam specimens were prepared by the powder compact foaming method. The behaviour under different loading conditions (loading velocity and radial constraints) has been determined by an extensive experimental program. The results show a significant increase in the collapse stress of the integral-skin closed-cell aluminium foam under quasi-static loading when radial constraints are applied. The radial constraint induces a significant strain hardening of the foam, where the densification occurs at lower strains, consequently enhancing the energy absorption per unit volume of the deformed foam. The strain hardening is also sensitive to the foam density, increasing with the density.

Investigation of microstructural and mechanical properties of cell walls of closed-cell aluminium alloy foams

This study investigates the influence of microstructure on the strength properties of individual cell walls of closed-cell stabilized aluminium foams (SAFs). Optical microscopy (OM), micro-computed X-ray tomography (µ-CT), electron backscattering diffraction (EBSD), and energy dispersive X-ray spectroscopy (EDS) analyses were conducted to examine the microstructural properties of SAF cell walls. Novel micro-tensile tests were performed to investigate the strength properties of individual cell walls. Microstructural analysis of the SAF cell walls revealed that the material consists of eutectic Al-Si and dendritic a-Al with an inhomogeneous distribution of intermetallic particles and micro-pores (void defects). These microstructural features affected the micro-mechanism fracture behaviour and tensile strength of the specimens. Laser-based extensometer and digital image correlation (DIC) analyses were employed to observe the strain fields of individual tensile specimens. The tensile failure mode of these materials has been evaluated using microstructural analysis of post-mortem specimens, revealing a brittle cleavage fracture of the cell wall materials. The micro-porosities and intermetallic particles reduced the strength under tensile loading, limiting the elongation to fracture on average to ~3.2% and an average ultimate tensile strength to ~192MPa. Finally, interactions between crack propagation and obstructing intermetallic compounds during the tensile deformation have been elucidated.

Impact Response of Aluminium Alloy Foams Under Complex Stress States

A series of dynamic tests were conducted on a closed-cell aluminum alloy foams in order to determine experimental failure surface under impact loading conditions. Quasi-static tests have also been performed to investigate failure mechanism under different stress paths. Three typical types of deformation modes can be observed, which corresponds to the different failure mechanism. The failure loci of the foam in principal stress plane are explored from quasi-static to dynamic loading conditions. A significant strength enhancement is identified experimentally. The expansion of the failure locus from the quasi-static to the dynamic test is almost isotropic. A modified failure criterion for the metallic foam is proposed to predict failure locus as a function of strain rate. This rate-dependence failure criterion is capable of giving a good description of the biaxial failure stresses over a wide range of the strain rates.

Fatigue damage of closed-cell aluminum alloy foam: Modeling and mechanisms

The objective of this work is to experimentally investigate the damage evolution and damage mechanism in closed-cell aluminum alloy foam under tension–tension fatigue loading. Constant amplitude fatigue tests are performed for the aluminum alloy foam, and experimental results indicate the large scatter of the fatigue damage in the aluminum alloy foam. To describe the fatigue damage with large scatter, a statistical fatigue damage model is developed on the basis of continuum damage mechanics. It is seen that the statistical damage model can describe the fatigue damage of the foam. Scanning electron microscopy (SEM) observation on the fracture surface of the tested specimen is carried out to understand the damage mechanisms of the foam. Four major categories of fatigue damage mechanism are concluded, i.e. damage initiates from the material surface, damage initiates on the cell wall, damage initiates at the intersection of several cell walls and damage initiates from the edge of cell. The high-resolution SEM images reveal that the fatigue mechanisms of the foam are mainly governed by the cell structure.

A new class of closed-cell aluminium foams reinforced with carbon nanotubes

This manuscript reports on the fabrication of closed-cell aluminium alloy foams reinforced with carbon nanotubes (CNTs) through a novel approach that combines the powder metallurgy method with colloidal processing step that grants uniform dispersion of CNTs into the aqueous suspension of all powder components. Spraying the as prepared suspension into liquid nitrogen followed by lyophilisation enables obtaining homogeneous spherical granules to be used in the powder metallurgy method. Besides ensuring good dispersion of all powder components in the system, the non-agglomerated form of CNTs and the expansion upon foaming foster their structural integrity under stretched conditions in the final foams for an efficient load transfer.

Experimental investigation of the fatigue of closed-cell aluminum alloy foam

Tension–tension fatigue tests are carried out to investigate the fatigue of closed-cell aluminum alloy foam. The stress–life (S–N) curve of the foam is obtained, in which large scatter is observed and discussed in detail. To understand the scatter of fatigue life of the foam, fracture surface is examined. The fatigue life decreases as the number and the size of large cells increase. The irregularity of inner cell structure of the foam attributes to the scatter of fatigue life. Then, a statistical stress–life model is developed, and a series of statistical probability–stress–life (P–S–N) curves are obtained. The P–S–N curve is efficient in describing the fatigue of foam materials.

Variation of quasi-static and dynamic compressive properties in a single aluminium foam block

The variation of the structural and compressive properties through a single rectangular closed-cell aluminium alloy foam block prepared by the powder-compact foaming technique was evaluated using quasi-static and dynamic loading conditions. For this study, this foam block was cut into identical representative foam cubes in three horizontal layers. Both quasi-static and dynamic compression tests were carried out on foam specimens in two orientations (parallel and perpendicular) related to foaming directions. The properties of these cubic representative specimens were also compared to integral-skin foam cubes of the same dimensions prepared using the same foaming temperature. The results showed that a large size variation of cellular pores with irregular cell shape is observed within the large foam block in height and width, due to the non-uniform foam growth. The variation of the properties is associated with the processing of the foams. Additionally, the relation between pore size, density and compressive properties has been studied. The results also show that the effect of the structural imperfections and defects on the compressive response is more pronounced for foams without skin. This is manifested in a higher scatter of the stress plateau. The results also demonstrate that the compressive properties and the energy absorption capabilities increase with the density and the strain rate. Such higher energy absorption capability during dynamic compression is beneficial for impact energy absorbing structures. Furthermore, the advantages of the application of integral-skin foam are demonstrated.

Experimental Determination of Mechanical Properties of Aluminium Foams Using Digital Image Correlation

This paper presents an experimental characterization of three different types of closed-cell aluminium alloy foams (AlMg1Si0.6, AlSi12Mg0.6 and AlMg0.6Si0.3) under static compressive loading. This study was carried out on half-cylindrical specimens with skin. The influence of foam density on compressive behaviour was investigated for densities ranging from 430 kg/m3 to 935 kg/m3. The compression tests were performed at room temperature (23°C) with a constant crosshead speed of 0.5 mm/min. Strain distribution, yield stress and compressive modulus values were recorded using Digital Image Correlation. Experimental results show that the mechanical properties (Youngs Modulus, yield stress and plateau stress) increase with density.

Variation of Quasi-static and Dynamic Compressive Properties in Single Aluminium-alloy Foam Block

The variation of the density and compressive properties through a single closed-cell aluminium alloy foam block were evaluated. For this purpose a rectangular block of aluminium foam was fabricated and cut into identical small cubic representative volume specimens in three horizontal layers (bottom, middle and top). The mechanical characteristics of these cubic representative volume specimens were determined using quasi-static and dynamic compression tests, parallel and perpendicular to the foaming direction. The visual observation of the cubic representative volume specimens revealed that the pore size and the relative density vary across the original foam block, particularly on the different horizontal layers. Accordingly, the variation of the compressive properties and the energy absorption characteristics also proved to be significant.

Uniaxial and biaxial failure behaviors of aluminum alloy foams

Combined shear–tensile test have been performed on a closed-cell aluminum alloy foams with three relative densities over a wide range of loading rates in order to probe their failure behaviors under biaxial loading conditions. Quasi-static uniaxial compressive and tensile tests have also been conducted to investigate uniaxial failure behaviors of the aluminum alloy foams. The materials exhibit uniaxial failure stress asymmetry due to different failure mechanism in the uniaxial tensile and compression. Comparison is made between three phenomenological failure criteria and the measured failure stresses under different loading conditions to verify these criteria. The experimental failure surfaces of the aluminum alloy foams provide support for the three phenomenological failure criteria when suitable Poisson’s ratio is employed. The shape of the experimental failure surface in principal stress plane was not significantly influenced by variation in the relative density. The slight expansion of the failure surfaces with loading rate happened to be isotropic for this investigated closed-cell aluminum alloy foams in combined shear–tensile testes.

Loading rate effect on yield surface of aluminum alloy foams

This paper presents a combined compression-shear test for closed-cell aluminum alloy foams with three different relative densities in order to determine their initial yield surfaces and investigate the influence of the loading rate on the initial yield surface under multiaxial loading conditions. Uniaxial compression and tensile tests have also been performed to determine yield stress in simple load condition. The experimentally measured yield surfaces are compared with three phenomenological yield criteria for metallic foams. The comparison result indicates that three phenomenological yield criteria give a good description of the multiaxial yield of closed-cell aluminum alloy foams when proper plastic Poisson's ratio is employed. The experimentally measured yield surface of the tested foams in principal stress plane is independent of relative density in the range investigated. The experimental yield surfaces of three types of aluminum alloy foams all exhibited a slight expansion over the range of loading rates studied.

Compressive behavior of closed-cell aluminum alloy foams at medium strain rates

Compressive behavior of closed-cell aluminum alloy foams at strain rates of 10−3–450s−1 has been studied experimentally. The fully stress–strain curves of specimens at medium strain rates were obtained using the High Rate Instron Test System, which can maintain a constant loading rate. The experimental results show that plateau stress and energy absorption capacity are remarkable dependent on strain rate, while the densification strain has a negligible dependence.

The dependence of damping property of fly ash reinforced closed-cell aluminum alloy foams on strain amplitude

Closed-cell aluminum alloy foams with 1.5, 3.0 wt.% fly ash particles have been manufactured by molten body transitional foaming process. The room temperature damping properties of fly ash reinforced aluminum alloy foams were measured at different strain amplitude in two directions using the forced vibration mode. The results show that the damping properties of fly ash reinforced aluminum alloy foams increase with FA content. A critical strain amplitude ε crit was observed and ε crit decreases with increasing FA content. Moreover, the damping property in the transverse direction is higher than that in the longitudinal direction. The related mechanism has been discussed.

Experimental analysis of compressive notch strengthening in closed-cell aluminum alloy foam

The notch strengthening effect is studied experimentally in closed cell aluminum foams. The limit loads, net section strength were found for a set of double-edge-notched (DEN) and single-edge-notched (SEN) specimens loaded in compression. In addition, the evolution of the deformation is monitored through a digital image correlation procedure. The influence of independently varying the net section width, b, and the crack length to width ratio, a/ W is examined. The DEN specimens showed notch-strengthening behavior, while the SEN specimens were found to be notch-insensitive. From the plastic deformation measurements the dominant deformation mode for the SEN specimens was found to be a crushing/twinning mode. The constraint imposed on this twinning mode by the DEN geometry leads to the observed notch-strengthening behavior. A phenomenological model is developed to rationalize the observed strength enhancement for the DEN-specimens, featuring a dependence on the ligament width to cell size ratio, b/ d.