Strain Measurement in an Aluminium Foam by Means of Digital Image Correlation - Visual Computing: Scientific Visualization and Imaging Systems - page 141

corresponding to the four crushing stages of Fig. 7.6 . Observation of the contour
plots gives an interesting insight on how the deformation in the material evolves
during the test. The process is, obviously, governed by the axial displacement
imposed by the lower platen, moving upwards. The longitudinal component
(Fig. 7.7 ) is, in absolute value (the negative values indicates compression), more
than one order of magnitude larger than the transverse component (Fig. 7.8 ). At the
beginning of the crushing the y strain (Fig. 7.7 a) distribution grows in absolute value
from top to bottom of the specimen; the x component (Fig. 7.8 a) is in general very
small except for the bottom left corner, where a localised collapse occurs. As the
crushing reaches (Figs. 7.7 b, 7.8 b) and exceeds half of the total stroke (Figs. 7.7 c,
7.8 c), the distribution of both strains is very irregular, this reflects the inhomogeneity
of the foam. The values of the two strain components remain quantitatively very
different. At the end of the crushing, as the material has been almost compacted, the
y strain (Fig. 7.7 d) tends to uniformity, although bands are noticeable; the x strain
(Fig. 7.8 d) is again very inhomogeneous and its values remain small.
7.5 Conclusions
The chapter has presented an example of application of the DIC to the study of an
aluminium foam. The main goal was to assess if strain rate and density changes,
considering for the former the range of low values 10 -3 -10 -1 s -1 , and for the
latter the intrinsic change of values from centre to periphery (232-306 kg/m 3 ),
affect the properties of the foam. Compressive tests have been carried out on cubic
specimens of material; it is observed that the effect on the response (expressed as
force-displacement diagram) is noticeable, although small.
Digital Image Correlation has been applied to study the strain field in the
sample during the compression tests. The displacements of the marker points are
detected, then the strains are calculated by adopting an interpolation of the same
type as that used in finite element modelling. The results given by the application
of the DIC to this kind of material are valuable to assess the strain distribution in
the sample surface, which would be impossible to obtain with other experimental
contact techniques (e.g. strain gauges).
References
1. Pan, B., Qian, K., Xie, H.M., Asundi, A.: Two-dimensional digital image correlation for in-
plane displacement and strain measurement: a review. Meas. Sci. Technol. 20, 1-17 (2009)
2. Ashby, M.F., Evans, A.G., Fleck, N.A., Gibson, L.J., Hutchinson, J.W., Wadley, H.N.G.:
Metal Foams: A Design Guide. Butterworth-Heinemann, Oxford (2000)
3. Bastawros, A.F., Bart-Smith, H., Evans, A.G.: Experimental analysis of deformation
mechanisms in a closed-cell aluminum alloy foam. J. Mech. Phys. Solids 48, 301-322 (2000)