32 Surface preparation and sample size

Metallic foam specimens can be machined using a variety of standard techniques. Cell damage is minimized by cutting with a diamond saw, with an electric discharge machine or by chemical milling. Cutting with a bandsaw gives a more ragged surface, with some damage. The measured values of Young's modulus and compressive strength of a closed-cell aluminum foam cut by diamond-sawing and by electric discharge machining are identical; but the values measured after cutting with a bandsaw are generally slightly lower (Young's modulus was reduced by 15% while compressive strength was reduced by 7%). Thus surface preparation prior to testing or microscopy is important.

The ratio of the specimen size to the cell size can affect the measured mechanical properties of foams (Figure 3.2). In a typical uniaxial compression test, the two ends of the sample are in contact with the loading platens, and the sides of a specimen are free. Cell walls at the sides are obviously less constrained than those in the bulk of the specimen and contribute less to the stiffness and strength. As a result, the measured value of Young's modulus and the compressive strength increases with increasing ratio of specimen size to cell size. As a rule of thumb, boundary effects become negligible if the ratio of the specimen size to the cell size is greater than about 7.

Shear tests on cellular materials are sometimes performed by bonding a long, slender specimen of the test material to two stiff plates and loading the along the diagonal of the specimen (ASTM C-273 - see Figure 3.5, below). Bonding a foam specimen to stiff plates increases the constraint of the cell walls at the boundary, producing a stiffening effect. Experimental measurements on closed-cell aluminum foams, and analysis of geometrically regular, two-dimensional honeycomb-like cellular materials, both indicate that the boundary effects become negligible if the ratio of the specimen size to the cell size is greater than about 3.

a ERG Duocel • Alporas

ERG Duocel Alporas

Figure 3.2 The effect of the ratio of specimen size to cell size on Young's modulus (above) and on compressive plateau stress (below) for two aluminum foams (Andrews et al., 1999b). The modulus and strength become independent of size when the sample dimensions exceed about seven cell diameters a

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