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Fig. 7.1 Schematic
representation of the
behaviour of a foam under
compression
densification
peak
stress
plateau
elasticity
strain
7.2 Aluminium Foams
Although metal foams [ 2 ] are getting popular, some related essential information
is given here for the benefit of a reader inexperienced with this class of materials.
In short terms, in a metal foam the metallic matrix is interrupted by cavities,
purposely generated by the manufacturing processes. This structure is obtained in
several ways, the most used are: blowing gas in the molten metal, introducing a
chemical agent which produces bubbles in the molten metal, consolidating metal
powder containing a foaming agent, casting the metal in a mould containing a wax
or polymer precursor of the cavities (protected by a coating of mould material).
According to the technology used, the cavities can be communicating or separate,
the resulting structures are called ''open-cell'' or ''closed-cell'' respectively.
Although several metals have been used to produce foams, the most important
applications are given by aluminium foams, in which the intrinsic lightness of the
metal is emphasised by the cellular structure; the result is an apparent density of
2-300 kg/m 3 . Aluminium foams are used to produce lightweight components in
aerospace applications, cores of sandwich panels, etc. The cellular structure
exhibits high thermal conductivity and is effective in managing vibrations or noise,
since it offers better stiffness (which increases the natural frequencies) and
damping than a solid panel of same mass per unit surface. A peculiar application of
the foam is to dissipate a considerable amount of energy when is crushed; therefore
it is used to produce impact absorbers, in form of pads or by filling the cavities of
thin-walled tubular elements. The stress-strain response of a foam under com-
pression is depicted schematically in Fig. 7.1 . Under increasing load, the graph
exhibits first an elastic rise, until a peak value is attained; then the stress stabilizes
at an approximately constant value (plateau); finally the stress increases notice-
ably, due to the closure of the voids deforming in compression (densification). The
actual behaviour is somewhat more complicated, as some inelastic phenomena
occur already in the initial rise.
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