Metal Foams: A Design Guide

26 Metal Foams: A Design Guide
most informative for open-cell foams (Figure 3.1(b)). Closed-cell foams often
present a confusing picture from which reliable data for size and shape are
not easily extracted. For these, optical microscopy is often better.
X-ray Computed Tomography (CT) gives low magnification images of
planes within a foam which can be assembled into a three-dimensional
image (Figure 3.1(c)). Medical CT scanners are limited in resolution to about
0.7 mm; industrial CT equipment can achieve 200 µ. The method allows
examination of the interior of a closed-cell foam, and is sufficiently rapid
that cell distortion can be studied through successive imaging as the sample
is deformed.
3.2 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, twodimensional
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.