Strain Measurement in an Aluminium Foam by Means of Digital Image Correlation - Visual Computing: Scientific Visualization and Imaging Systems - page 131

7.1 Introduction
Full-field, non-contact, optical techniques can add a substantial contribution to
materials science for what concerns the measurement of deformation. The main
advantage is that the measurement is achieved on a wide region of the sample
surface and can be carried out in parallel, while a mechanical testing (e.g. a tension
or compression test) is underway. In general terms, these techniques can be
classified as interferometric and non-interferometric. In the case of the interfero-
metric technique, the measurement is based on the phase difference (a coherent
light source is used) between the light reflected by the surface of the specimen
before and after deformation. The technique can detect very small displacements
and strains; its main shortcoming is that an elevated level of insulation from
vibrations is required. The non-interferometric technique is based on the com-
parison of the pattern of ''speckles'' before and after deformation; it is accom-
plished by processing electronically the related images and, usually, referred to as
Digital Image Correlation (DIC). The speckles can be applied (e.g. by spraying a
suitable paint) or formed by the natural texture of the specimen surface. This
technique cannot be as sensitive as the previous, but is less demanding in terms of
experimental setup for what concerns insulation from vibrations and the equipment
is less sophisticated. Moreover, the measurement can be performed also with
natural light. Thanks to the progress of the electronic equipment, in the last decade
both CCD cameras and computers have become more and more powerful and
inexpensive so that acquisition and processing of the images is easily achieved.
The core of the technique lies mainly in the algorithm adopted to track the speckles
in the deformed state and evaluate the displacements, a wide survey of these
aspects—and of the features of DIC as well—can be found in [ 1 ]. In its two-
dimensional formulation DIC requires a single camera, having the plane of its
CCD parallel to that of the specimen surface. This setup is suitable as long as the
specimen is flat and the out-of-plane motion is negligible; otherwise the more
sophisticated 3D DIC, based on a binocular observation, must be applied.
This chapter reports an experience obtained by applying DIC to the measure-
ment of the strain during compression testing of aluminium foam. For this case,
contact techniques would be unsuitable, due to the porous nature of the specimen
surface and the inhomogeneity of the material. On the contrary, the optical tech-
nique is fit to the purpose, taking also advantage from the naturally speckled
surface of the specimens. Moreover, since displacements and strains are large, the
lower resolution of DIC with respect to interferometry does not constitute a
drawback. The text is organised in the following way: first, a brief description of
the aluminium foams is presented, especially for what concerns the assessment of
their mechanical properties; second, the results of the compression tests carried out
in the work are reported; then, the application of DIC is described.