Metal Foams: A Design Guide

28 Metal Foams: A Design Guide
Stress (MPa)
1.5
1.0
0.5
Unloading modulus
0.0
0.00 0.01 0.02 0.03 0.04 0.05 0.06
Stress (MPa)
5
4
3
2
1
Strain
(a)
0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
Strain
(b)
Figure 3.3 Stress–strain curve from a uniaxial compression test on a cubic
specimen of a closed-cell aluminum foam (8% dense Alporas): (a) to 5%
strain, (b) to 70% strain (from Andrews et al., 1999a)
made from the slope of the unloading curve, as shown in Figure 3.3, unloading
from about 75% of the compressive strength. The compressive strength of the
foam is taken to be the initial peak stress if there is one; otherwise, it is taken
to be the stress at the intersection of two slopes: that for the initial loading
and that for the stress plateau. Greasing the faces of the specimen in contact
with the loading platens reduces frictional effects and can give an apparent
compressive strength that is up to 25% higher than that of a dry specimen.
Variations in the microstructure and cell wall properties of some presentday
foams gives rise to variability in the measured mechanical properties. The
standard deviation in the Young’s modulus of aluminum foams is typically