Scaling of compression strength in disordered solids metallic foams.PDF (original) (raw)
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Scaling of compression strength in disordered solids: metallic foams
The scaling of compression strength with porosity for aluminium foams was investigated. The Al 99.96, AlMg1Si0.6 and AlSi11Mg0.6 foams of various porosity, sample size with and without surface skin were tested in compression. It was observed that the compression strength of aluminium foams scales near the percolation threshold with Tf ≈ 1.9-2.0 almost independently on the matrix alloy, sample size and presence of surface skin. The difference of the obtained values of Tf to the theoretical estimate of Tf = 2.64 ± 0.3 by Arbabi and Sahimi and to Ashby estimate of 1.5 was explained using an analogy with the Daoud and Coniglio approach to the scaling of the free energy of sol-gel transition. It leads to the finding that, there are two different universality classes for the critical exponent T f : when the stretching forces dominate T f = f = 2.1, respectively when bending forces prevail T f = .d = 2.64 seems to be valid. Another possibility is the validity of relation T f ≤ f which varies only according to the universality class of modulus of elasticity in foam.
High-temperature compression of closed cell aluminium foams
The compression behaviour of closed cell aluminium foams (Al99.5, AlMg1Si0.6 and AlSi12 matrix alloys, TiH2 foaming agent) prepared by powder metallurgy was studied in the temperature range of 20–550 • C. It was observed that the temperature increase results in the decrease of the compression strength and energy absorption and increase of densification strain (plateau length) at constant density. The dependence of compression strength on foam density and temperature was successfully modelled using new proposed equation. The activation energy for compression of aluminium foams seems to be density dependent with a maximum at certain density range depending on foam composition. It was also found that the characteristic exponent T f for the compression strength of aluminium foams is temperature dependent variable. The strain at compression strength (deformation up to the macroscopic failure of foam) is nearly temperature independent or decreases at constant density depending on aluminium alloy matrix. The absorbed energy per unit volume of aluminium foams decreases with increasing temperature significantly due to the decrease in the value of plateau/compression strength at constant density. K e y w o r d s : aluminium foam,
Compression Test Evaluation Method for Aluminium Foam Parts of Different Alloys and Densities
2010
In general, the stress-strain curves of aluminium foam obtained from uniaxial compression tests are not smooth and the expected plateau is missing. Moreover, the curve often exhibits lot of peaks with locally dropping stress instead of slowly increasing stress before final densification. The paper is therefore aimed to describe and explain these effects and is based on the experimental compression testing of several hundred samples. It has been shown how the structural non-uniformities in foam structure affect the stress strain curve in compression, i.e. collapse stress, densification stress, corresponding strains, and finally the potential of foam in crash energy absorption. Moreover, a way for objective evaluation of the tests was suggested.
Compressive behaviour of aluminium foams at low and medium strain rates
Composite Structures, 2002
Compressive behaviour of CYMAT aluminium foams with relative densities ranged from 5% to 20% has been studied experimentally in this paper. An MTS machine is employed to apply a compressive load at strain rates of 10 À3 -10 þ1 s À1 to these closed-cell aluminium foams. It has been found that the plateau stress is insensitive to the strain rate and is related to the relative density by a power law. Deformation is not uniform over the whole sample: it first occurs in the weakest band, followed by the next weakest bands after the first one has been completely crushed.
High strain rate compressive behaviour of aluminium alloy foams
International Journal of Impact Engineering, 2000
The high strain rate compressive behaviour of two cellular aluminium alloys (Alulight and Duocel) has been investigated using the split Hopkinson pressure bar and direct impact tests. It is found that the dynamic behaviour of these foams is very similar to their quasi-static behaviour. The plateau stress is almost insensitive to strain rate, for strain rates up to 5000 s\. Deformation is localised in weak bands in the Alulight foam but is spatially uniform for the Duocel foam, over the full range of strain rates 10\ s\))5000 s\.
Materials Science and Engineering: A, 2010
The porous structure of aluminum foam manufactured through the foaming of precursors containing blowing agent is stochastic in nature, usually with a random distribution of pores of different size and shape, creating difficulties in the modeling and prediction of foam properties. In this study, the effect of the initial location of the precursor material in the mold on the foam structure and compression behavior was investigated. Structural characterization showed that the porosity distribution, surface skin thickness and pore orientation was affected by the location of the precursors in the mold and by the extrusion direction of the precursors. Moreover, compression tests demonstrated a significant effect of the structural anisotropy on the collapse stress and its dispersion. The collapse stress of the foam increased if the loading was performed parallel to the thicker surface skin or parallel to the preferential pore orientation, leading to a 20% difference in collapse stress. The dispersion of the collapse stress could be significantly decreased if the loading was performed with regard to the structural anisotropy.
2011
Original scientific paper In this paper, the possibility to create uniform porous structure for nonsymmetric precursor heating was investigated together with changes in collapse stress and their reproducibility. The statistic relative standard deviation (RSD) of pore size was used to determine the structural uniformity. It was shown that proper adjustment of the foaming parameters leads to formation of the uniform foam structure. The compression testing showed that standard deviation of collapse stress was higher for uniform structure than those obtained for nonuniform structure.
Uniaxial stress–strain behaviour of aluminium alloy foams
Acta Materialia, 1999
ÐThe tensile and compressive stress±strain behaviour of closed cell aluminium alloy foams (trade name``Alulight'') has been measured and interpreted in terms of its microstructure. It is found that the foams are anisotropic, markedly inhomogeneous and have properties close to those expected of an open cell foam. The unloading modulus and the tensile and compressive yield strengths increase non-linearly with relative density. The deformation mechanisms were analysed using image analysis software and a d.c. potential drop technique. The scatter in results is attributed to imperfections within the foam. These include non-uniform density, weak oxide interfaces, and cell faces containing voids and cracks.
Compressive Behaviour of Closed-Cell Aluminium Foam at Different Strain Rates
Materials
Closed-cell aluminium foams were fabricated and characterised at different strain rates. Quasi-static and high strain rate experimental compression testing was performed using a universal servo-hydraulic testing machine and powder gun. The experimental results show a large influence of strain rate hardening on mechanical properties, which contributes to significant quasi-linear enhancement of energy absorption capabilities at high strain rates. The results of experimental testing were further used for the determination of critical deformation velocities and validation of the proposed computational model. A simple computational model with homogenised crushable foam material model shows good correlation between the experimental and computational results at analysed strain rates. The computational model offers efficient (simple, fast and accurate) analysis of high strain rate deformation behaviour of a closed-cell aluminium foam at different loading velocities.
Effect of cell morphology on the compressive properties of open-cell aluminum foams
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2000
The mechanical properties of open-cell 6101 aluminum foams with different densities ( 5 -10%) and morphologies (4-16 cells cm − 1 ) were characterized in compression. It was found that density is the primary variable controlling the modulus and yield strength of foams. The effects of other variables such as cell size and shape were also studied. Whereas the cell size appears to have a negligible effect on the strength of foams, at a fixed density, the cell shape was shown to effect the strength of foams. In the present paper, theoretical models are offered to explain the differences in modulus and strength caused by the differences in cell shape and size.