Metal Foams: A Design Guide

E/E bulk
σ peak /σ bulk
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
a
b
ERG Duocel
Alporas
ERG Duocel
Alporas
0.0
0 2 4 6 8 10 12 14 16 18
Normalized Size (L/d)
Characterization methods 27
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
3.3 Uniaxial compression testing
Uniaxial compressive tests are best performed on prismatic or cylindrical specimens
of foam with a height-to-thickness ratio exceeding 1.5. The minimum
dimension of the specimen should be at least seven times the cell size to avoid
size effects. Displacement can be measured from crosshead displacement,
by external LVDTs placed between the loading platens, or by an extensometer
mounted directly on the specimen. The last gives the most accurate
measurement, since it avoids end effects. In practice, measurements of Young’s
modulus made with an extensometer are about 5–10% higher than those made
using the cross-head displacement.
A typical uniaxial compression stress–strain curve for an aluminum foam
is shown in Figure 3.3. The slope of the initial loading portion of the curve
is lower than that of the unloading curve. Surface strain measurements
(Section 3.10) indicate that there is localized plasticity in the specimen at
stresses well below the compressive strength of the foam, reducing the slope
of the loading curve. As a result, measurements of Young’s modulus should be