Strain Measurement in an Aluminium Foam by Means of Digital Image Correlation - Visual Computing: Scientific Visualization and Imaging Systems - page 134

interpretation of the general behaviour, although some dissimilar results have been
found (e.g. [ 20 ], in which only evidence for the effect of the density is found).
It is obvious that, to assess the mechanical properties of a foam, in several cases
an effective measurement of the strain is needed. Only a mean value of the axial
strain can be obtained from the displacement of the crosshead (or equivalent piece
of equipment), whilst no information on the transverse strain is available. More-
over, for an inhomogeneous material as a foam, the local deformation values can
be very far from the mean. For the reasons already cited at the end of Sect. 7.1 ,
DIC appears as the ideal technique to measure the strain field; thus, for instance, it
has been used in [ 3 - 5 ]. Using a suitable high speed camera, measurement under
high rate of deformation can also be performed, as done in [ 21 ].
7.3 Mechanical Tests
The discussions reported in this chapter are based on experiments [ 22 ] carried out on
Alporas , an aluminium foam produced with the technique of generating bubbles in
the molten metal by means of TiH 2 [ 23 ]. The material has been cut in cubical samples
having side of 50 mm, which is one order of magnitude larger than the typical size of
the cavities, as usually recommended. The nominal density is 250 kg/m 3 ; however,
due to the inhomogeneity related to the production process, the value is lower in the
middle and higher at the edges of the plates in which the material is manufactured.
Since, in practice, each sample had a different density, in the range 232-306 kg/m 3 ,
to study the influence of the density the specimens were grouped in three classes of
density, termed ''low'' (235.4-238.5 kg/m 3 ), ''medium'' (248.8-252.8 kg/m 3 ) and
''high'' (270.1-272.5 kg/m 3 ). The classes defined in this way contained a sufficient
number of samples (about ten), enough similar each other and different from those of
the other classes. The reason for defining these classes was to have specimens
of practically equal density to be tested under different velocities.
All compression tests have been carried out with a servo-hydraulic machine of
100 kN maximum load and 100 mm/s maximum crosshead speed (Dartec
HA100), equipped with platens instead of the usual jaws, as shown in Fig. 7.2 . The
testing velocities have been chosen one order of magnitude different each other,
0.25 (low), 2.5 (intermediate), 25 (high) mm/s; the corresponding strain rates
referred to the initial specimen height are respectively 5 10 -3 ,5 10 -2 ,5 10 -1 s -1 .
Each test has been replicated three times under the same nominal conditions
(density and velocity). The total length of the compressing stroke is 40 mm,
corresponding to 80 % strain.
The load-displacement curves recorded during the tests are shown in Fig. 7.3 ,
each diagram corresponds to one velocity as labelled on top. Although, for each
velocity, the curves are not much different, from the plateau onwards they appear
stacked according to the density, and this appears more evident increasing the
velocity. These observations are consistent, although in this case the density varies
in a narrower range, with the findings reported in [ 15 ].