Proceedings e report - Firenze University Press

DIMENSIONAL CHANGES DUE TO FLOWS OF HEAT AND MOISTURE IN WOOD AND WOOD-BASED MATERIALS
Any physical stimulation leading to a surface deformation (for example vibrations or temperature
changes) changes the speckle pattern and consequently the superimposed image. By subtracting the
reference image from the active image, so-called correlation fringes are obtained which are lines of
equal displacement (Fig 2 left). A displacement of half the laser wavelength is sufficient to generate a
clear signal. Thus, the resolution of this technique is in the sub-micrometre range.
Fig. 2: Correlation fringes (left) and test frescoes mounted in a double climate chamber (right)
The setup shown in Fig.1 (right) allows the measurement of displacements perpendicular to the
surface only (z direction). Two similar set-ups are necessary to measure displacements parallel to the
surface (x and y direction). Fig. 2 (right) shows an application to the study of frescoes: A number of
test frescoes is mounted between the two chambers of a double climate chamber in the WKI.
Fig. 3: Displacement plots for a good (left) and a defective (right) test frescoes
Increasing the surface temperature by 5 K on one side resulted in a deformation of the frescoes. The
resulting displacements are plotted in Fig.3 for two different pieces measuring 30 cm x 30 cm. The
displacements in z direction is colour coded and given in µm. The in-plane displacement is depicted
by arrows. In the left part of the figure, the maximum z displacement is around 12 µm, whereas the
maximum in-plane deformation is lower than 2 µm. Both the displacements perpendicular to the plane
and in the plane vary little over the surface. This is typical for intact frescoes. In contrast, the fresco
shown in the right part of Fig. 3 shows deformations varying strongly over the surface, and the inplane
displacement even changes its direction over a short distance. Although the maximum
displacements are in the normal range (x: 5 µm, y: 6.5 µm, z: 3.5 µm) this is a clear evidence for the
beginning destruction of the fresco.
1.2. DICT
The digital image correlation technique is a photogrammetric method. In contrast to conventional
photogrammetry no externally applied markers are need. These are replaced by the surface features of
the object. Consequently, DICT will not work for extremely smooth, featureless objects. In cultural
heritage, however, this situation is encountered very rarely.
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