Metal Foams: A Design Guide

46 Metal Foams: A Design Guide
purposes, it is helpful to know that the tensile modulus Et of metal foams is
not the same as that in compression Ec; the tensile modulus is greater, typically
by 10%. Anisotropy of cell shape can lead to significant (30%) differences
between moduli in different directions.
Open-cells foams have a long, well-defined plateau stress, pl , visible on
Figures 4.2 and 4.3. Here the cell edges are yielding in bending. Closed-cell
foams show somewhat more complicated behavior which can cause the stress
to rise with increasing strain because the cell faces carry membrane (tensile)
stresses. The plateau continues up to the densification strain, εD, beyond which
the structure compacts and the stress rises steeply. The plateau stress, pl, and
the densification strain, εD, scale with density as:
� �m � �
pl ³ ⊲0.25 to 0.35⊳ y,s
εD ³ 1 ˛1
⊲4.2⊳
s
For currently available metfoams m lies between 1.5 and 2.0 and ˛1 between
1.4 and 2. As a rule of thumb, m ³ 1.6 and˛1 ³ 1.5. These properties are
important in energy-absorbing applications, to which metal foams lend themselves
well (see Chapter 11).
The tensile stress–strain behavior of metal foams differs from that in
compression. Figure 4.4 shows examples. The slope of the stress–strain curve
before general yield is less than E, implying considerable micro-plasticity even
at very small strains. Beyond yield (yield strength: y) metal foams harden
up to the ultimate tensile strength ts beyond which they fail at a tensile
ductility εt.
Stress (MPa)
12
10
8
6
4
2
ρ/ρ s = 0.4
Longitudinal
ρ/ρ s = 0.2
0
0 1 2 3.5 4
Strain (%)
Transverse
Figure 4.4 Tensile stress–strain curves for Alulight foams
The damping capacity of a metal foam is typically five to ten times greater
than that of the metal from which it is made. This increase may be useful,
although the loss factor is still much less than that associated with polymer
s