Issue 36

J. Kováčik et alii, Frattura ed Integrità Strutturale, 36 (2016) 55-62; DOI: 10.3221/IGF-ESIS.36.06 56 of the cracks, which leads to the macroscopic failure in the disordered system is in general a non-equilibrium and non- linear phenomenon. The scaling model [9] considers that the compression strength of the disordered material is a function of the compression strength of solid material and the degree of disorder (porosity or volume fraction of solid material). Benguigui, Ron and Bergman [10] in 2D and Sieradzki and Li [11] in 3D found experimentally that the compression strength  cs of disordered solids scales according to the power law:   f T c CS p p    (1) where p is the degree of disorder, p c is a percolation threshold, i.e., the value of disorder below which the compression strength vanishes, and T f is a critical exponent for the compression strength. In 3D Sieradzki and Li [11] obtained T f = 1.7 ± 0.1 for the system composed of a 2 mm thick aluminium plate with holes punched at positions corresponding to the triangular networks. On the other hand, Bergman [12] proposed the theoretical bounds for T f , which in 3D give 2.58 < T f (d=3) < 2.76. Using computer simulation, Sahimi and Arbabi [13] found T f ~ 2.64 ± 0.3 in 3D, which agrees with the bounds proposed by Bergman. They assumed that the lower value of T f = 1.7 ± 0.1 by Sieradzki and Li [11] was due to the fact that the measurements were done far from p c and also due to the significant size effect. Recently, author [14] used the scaling model for the study of the tensile strength of copper and nickel foams. The obtained results for T f agree reasonably with the Sahimi and Arbabi estimated value T f ~ 2.64. Unfortunately, the data are far from the percolation threshold and the obtained values can be over or under estimated. The scope of this work is to study the scaling of the compression strength of the aluminium foams prepared by powder metallurgical route [15]. The metallic foams are the disordered solids consisting of a metal matrix filled with gas pores. The metallic foams around 0.1 and less volume fraction of metal can be prepared thus enabling to study the scaling of the compression strength near the percolation threshold. E XPERIMENTAL INVESTIGATIONS o investigate the scaling of the compression strength, the metallic foams of various densities were prepared by powder metallurgical route [15]. The samples were made from aluminium alloy powders Al 99.96, AlMg1Si0.6 and AlSi11Mg0.6, which were mixed together with a foaming agent (0.4 wt.% of titanium hydride), CIP-ed and then continuously hot extruded at 450 °C into a foamable precursor. The precursor was expanded into the porous cellular solid by hydrogen, which is released from the foaming agent, during the heating of the precursor above the melting temperature of the metal matrix. Then the rapid cooling process takes place to freeze the obtained cellular structure. The samples of various geometry were prepared: cylinders with the diameter of 20 mm and the length of 10, 20, 25, 30 and 40 mm without surface skin, with the diameter of 28 mm and the length of 32 mm with surface skin and finally with the diameter of 40 mm and the length of 51 mm with surface skin. The prepared foam samples were usually from the range of 0.07 - 0.45 volume fraction of metal (see Tab. 1). The metallic surface skin always covers the foam prepared by the powder metallurgical route. The surface skin was removed via the electric discharge machining thus excluding the possible effect of the skin on the compression strength scaling. It enabled also to evaluate the effect of the surface skin using a couple of samples with the skin. The compression test was carried on INSTRON testing machine with constant ram speed of 10 mm/min. From the load-deflection curve, the stress-strain curve was obtained and the compression strength of the foam sample was determined (see Fig.1). R ESULTS AND DISCUSSIONS To model the compression strength of aluminium foams, the following assumption was made: because the foams with 0.1 and less volume fraction of metal can be successfully prepared, the percolation threshold was set to zero and Eq. (1) was rewritten in the following way: f T 0 0CS CS            (2) T