4. Unnotched material: mechanical properties and
energy absorption
Typical stress–strain curves are shown in Fig. 2(a)
for the Alporas foam and in Fig. 2(b) for the Alcan
foam. We note that the overall shapes of the tensile
and compressive stress–strain curves are insensitive to
the choice of material. Alporas foam was found to be
almost isotropic, with a variation of 915% in modulus
and yield strength with regard to specimen orientation.
In contrast, the Alcan foam is orthotropic; the
tensile and compressive stiffness and strength are least
in the through-thickness (TT) direction.
In compression, both foams show a local peak stress
at yield, followed by a fairly constant plateau stress.
The plateau stress spl was taken as the average compressive
stress for compressive nominal strains within
the range 5–30%. In tension, both materials strain
harden within the plastic range, prior to tearing at a
strain of only a few percent. We found that the ultimate
tensile strength sUTS is within 25% of the compressive
plateau stress spl for both foams. The
unloading Young’s modulus E in both tension and
compression tests was measured from the slope of the
unloading curve after a small plastic strain, of the
order of 1%; it was found that the tensile and compressive