Strain Measurement in an Aluminium Foam by Means of Digital Image Correlation - Visual Computing: Scientific Visualization and Imaging Systems - page 135

Fig. 7.2 Compression
testing of a foam block

7.4 Optical Strain Measurement

7.4.1 Displacements

The strain distribution in the specimen during the test was obtained by means of
DIC, as discussed in the introduction. Regarding the application of DIC to the
measurement on foams undergoing deformation, it must be emphasized that the
surface of the foam presents naturally different tones of gray, due to the pores,
that can act as a pattern of speckles for the measurement. If, on the one hand, due
to the typical cell size (some mm) the resolution of the measurement cannot be
very fine, on the other hand the displacements involved in the phenomenon are
large, and this facilitates the measurement. On the surface of the specimen a grid
of points, termed markers, is first identified. Then, a rectangular zone around each
marker, the subset, is considered with its tones of gray. The position of the subsets
is tracked during the mechanical test and this gives the displacements of the
markers, as shown schematically in Fig. 7.4 . In this scheme the subsets are chosen
aligned on their outlines, it can be remarked that they could also be chosen
overlapping or separated.
To evaluate the displacements, the procedure examines the gray scale over the
image, searching the similarity of the subsets. The position of the marker is that
which maximizes the normalized cross-correlation (NCC)

P

M a

P

N a

ð Þ Bm þ i ; n þ ð Þ
P

Am ; n

m¼1 n¼1

ri ; ðÞ ¼ s ð 7 : 1 Þ

M a

P

N a

P

M a

P

N a

Þ 2 Þ 2 Am ; n ð Bm þ i ; n þ j ð

m¼1 n¼1 m¼1 n¼1