Strain Measurement in an Aluminium Foam by Means of Digital Image Correlation - Visual Computing: Scientific Visualization and Imaging Systems - page 133

Due to the practical interest, and to the relative novelty of this class of mate-
rials, much research work has been spent in the last two decades to investigate
their mechanical properties and response under several types of loading. A short
survey (limited to years from 2,000 onwards) is reported in the following.
Numerous researches have contributed to establish a wide experimental
knowledge of the mechanical properties. In [ 3 ] the deformation mechanisms of
closed-cell foams is analysed, accounting for cases of distortion alone or accom-
panied by rotation and shear; it is also remarked that the deformation initially
appears in spots, then spreads out over bands. The evolution from localised
deformation to compaction bands is studied in [ 4 ]. In [ 5 ] the failure mechanism of
closed-cell foams is studied and it is noticed that the cracks can nucleate and
propagate through the brittle precipitates embedded in the walls. Regarding the
effect of the cell size, in [ 6 ] it is found that, comparing foams of equal density, the
strength is higher in case of larger diameter of the cell, due to the higher wall
thickness.
More complex testing conditions have been adopted to assess the response of the
foams under multiaxial loading; the obtained results support the definition of
empirical models to predict the yield and post-yield behaviour, and validate the
models proposed on a theoretical basis. Results of this kind are found in [ 7 ] and [ 8 ]
by means of a hydraulic apparatus, while in [ 9 ] a biaxial testing machine is used.
Maybe the largest experimental database on the mechanical behaviour of the foams,
established to support the finite element code LS-DYNA, is described in [ 10 ]. Other
contributions have assessed the capability of known formulae to predict the moduli
of the foams [ 11 ], or have proposed regression models [ 12 ]. A model to predict
elastic modulus and plateau stress is proposed and validated in [ 13 ].
As stated previously, a prominent use of the aluminium foams is to increase the
absorption of energy in impact conditions. Therefore, many works have been
dedicated to the effect of the strain rate on the adhesive performance. Several of
the studies are based on experiments performed by means of the Split Hopkinson
Pressure Bar (SHPB), which is required to obtain strain rates above 10 s -1 .In[ 14 ]
the dependence is assessed over a wide range of strain rate, of relatively low
values, and it is noticed that at low strain rate the deformation is localised in some
zones, while at higher strain rate deformation bands appear and the effect of the
inertia becomes predominant. In [ 15 ] it is found that the dependence on the strain
rate is higher for the foams with higher density, and the reason is ascribed to the
gas flowing through the orifices in the wall during the progressive failure. Con-
versely, in [ 16 ], for a closed-cell foam, the plateau stress is found to be almost
insensitive to the strain rate but is affected by the density. In [ 17 ] three different
foams are compared; the dependence on the strain rate is significant for one foam
and partial for another, conversely the third foam is insensitive. Foams obtained by
sintering metal powder have been studied in [ 18 ], also in this case it has been
found that the mechanical properties depend on strain rate and density. The
response of a foam to static and dynamic loading, considering different densities, is
investigated in [ 19 ] and the evidence is that the dependence on the strain rate is
higher in case of high density. This is also the remark that can be made as an