Proceedings e report - Firenze University Press

DIMENSIONAL CHANGE OF UNRESTRICTED WOOD AT DIFFERENT LEVELS OF EQUILIBRIUM MOISTURE CONTENT
S = published value for shrinkage (%)
Linear Dimensional Change
Considering that all dimensions in every orientation would undergo radial contraction/expansion, the
extent of tangential contraction/expansion needs to be found. Formula (3) giving the linear
dimensional change [4]was used to arrive at the final increase/decrease for every dimension recorded.
D
f
D ⎡
⎤
i ⎛⎛
Vt
−Vr
0 ⎞ ⎞
× ⎢100
− ⎜⎜
× O ⎟ + V ⎟⎥
100 ⎣ ⎝⎝
90 ⎠ ⎠⎦
= r
where: Df = final dimension
Di = initial dimension
Vt = calculated tangential shrinkage (%)
Vr = calculated radial shrinkage (%)
O O = orientation (in degrees)
2.3. Reconstruction of the Panel End Grain presenting Contraction/Expansion and
Warping on AutoCAD
The results obtained as outlined above, by using a Microsoft Excel spreadsheet, were then transferred
back to AutoCAD. The resulting outline (based on the new set of data) led to the indications of
dimensional change as well as any cupping the panel would undergo as a result of changes in the T
and the RH. Longitudinal contraction/shrinkage is deemed as insignificant, especially over short
lengths, but it can still be predicted by using the same computer routine.
3. Limitations of the computer analysis
The computer analysis only leads to approximate results. These results are only reliable under given
conditions while certain other conditions affect the behaviour of wood. Such instances outlined
hereafter.
The wood panels should be free from natural defects such as: cross grains, knots, compression/tension
wood, juvenile wood, etc.
The wood should not be restricted by crossbars, frames and/or mounting hardware.
Preparation and painting layers (as in panel paintings) lower the accuracy of the results obtained from
this computer analysis, especially when cupping is evaluated.
Published literature data for the coefficient of shrinkage (tangential, radial and longitudinal) as well as
the FSP are all average values, which might result to be quite different in reality. Dimensional
response of aged wood may result to be slightly lower due to decrease in hygroscopicity and/or the
mechanical effects of repeated shrinkage/swelling cycles. [5] In the case of panel paintings,
unpredictable cupping may take place even on radially cut boards due to rapid moisture adsorption by
the untreated back. [6]
4 . Possibility of errors
As the grain orientation may change, adjacent sides of sections may have slightly different
dimensions. Such discrepancies are negligible and are not visible to the naked eye. They may be
eliminated by narrowing the sections and, consequently, any changes in grain orientation result to be
more gradual.
5. Testing the computer procedure
5.1. Aim
This test was carried out in order to simulate situations of contraction/expansion as well as warping.
Literature data for Shellbark Hickory (Carya laciniosa) were chosen for this test, due to the high
tangential and radial shrinkage values, i.e. 12.6% and 7.6% respectively. [7] The test was also carried
out on actual pine samples.
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