Proceedings e report - Firenze University Press

ELASTIC PROPERTIES OF MINUTE SAMPLES OF WOOD ALONG THE THREE MATERIAL DIRECTIONS
At each imposed displacement increment, the indication of the load cell is noted down and an image of
the sample is grabbed. These images were subsequently treated by an image analysis software to
determine the strain tensor in the focal plane of the microscope [5]. The Young's modulus of the
sample was finally calculated from the linear part of the experimental force/ deformation curve.
Measuring the strain tensor by image correlation using the software MeshPore
The software MeshPore, developed in LERMAB [8] was used to determine the cell deformation of
wood sample at each displacement increment. An initial approximation of the strain tensor was
obtained by contour integral of a chain of points plotted on the initial image and on the deformed
image (Fig. 3). Depending on the image quality, three types of anatomical features have been used to
define the points of this chain [5]:
- the triple point which corresponds to the intersection of three cells;
- the mid-segment: the point situated halfway between two triple points. This was the solution adopted
when the image was of poor quality.
- the lumen centre.
Fig. 3 Correlation image by MeshPore
The approximate value of the strain field was subsequently refined using an inverse method. This
second step is the real image correlation procedure, whereas then first step allows us to avoid local
minima.
3. Material
The study was performed on a clone of poplar (Populus) that was ten years old and that came from an
in vitro culture carried out by the Forestery Physiological and Genetical Improvement Unit (INRA,
Orléans). Planted in 1996, it was artificially bent from 1998 in order to let it produce a highly located
area of tension wood in the cross section. Considering the small tree diameter and its age, samples are
all in juvenile wood.
Three kinds of wood were collected in this tree: reaction (tension); opposite (180° from the tension
wood zone) and normal wood (90° from the tension wood zone). From an anatomical point of view,
vessels of tension wood are fewer and smaller in diameter than the opposite and normal zones.
Tension wood exhibits a more important fibre ratio and thicker cell walls with very rounded contours
that result to smaller cell diameter than opposite and normal ones. Microscopic observations of tension
wood fibres revealed the presence of a gelatinous layer, the so-called G-layer.
3.1. Sampling for tests in longitudinal direction
Sampling was prepared in two successive stages. First, wood was sawed in the radial and the
longitudinal directions by a circular micro-saw (Fig. 4). The rotation speed and the sample advance
rate were 900 rounds/min and 250 μm /sec respectively. After, the previously machined block was cut
by a razor blade in the tangential direction.
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