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Fig. 7.7 Distribution of the y strain obtained from the optical measurement at four stages of
crushing: a 10 mm; b 20 mm; c 30 mm; d 40 mm
It is worth noting that, with this procedure, the subset is searched as a rect-
angular area, offset from the original position, as shown in Fig. 7.4 . More
sophisticated alternatives [ 24 ] are the ''Newton-Raphson method'' and ''gradient-
based method'', which achieve a better accuracy but are computationally heavier
(especially the former); in these methods the subset is tracked in a deformed shape.
In the measurements described here, each image is not too different from the
previous one (approximately 10 images per 1 mm of stroke are considered), thus
keeping the subset undeformed does not introduce an important error and sim-
plifies the calculations.
7.4.2 Strains
After that the displacements of the markers have been tracked, the strain distri-
bution is determined by means of an interpolation scheme similar to that used in
finite element modelling. The markers are regarded as nodes of a finite element
mesh (Fig. 7.5 ), their displacements u are known from the tracking procedure. The
specimen surface is divided in quadrilateral elements, thus the strain in each of
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