Thermal Expansion Of Moist Wood

H. Kubler, L. Liang, L. S. Chang

Abstract


Linear thermal expansion of wood, as measured with sealed cylindrical specimens at constant moisture content, generally increased with the amount of bound water. This could be expected on the basis of the low stiffness of moist wood. Near the fiber saturation point and above, shrinkage from internal drying was superimposed on true thermal expansion. At temperatures above 0 C during heating, water migrated out of fiber-saturated cell walls; the resulting shrinkage reduced the expansion significantly and between 20 and 70 C even led to contraction. At freezing temperatures during cooling, moisture diffused out of the saturated cell walls and condensed as ice crystals in the cell cavities; the resulting shrinkage supplemented true thermal contraction and caused large coefficients of thermal expansion in a broad sense.

Thermal expansion was largest tangentially and smallest in the longitudinal direction. Tangential coefficients of thermal expansion at 12% moisture content and room temperature averaged 53 X 10-1 mm/(mm C) for redwood, 70 X 10-6 mm/(mm C) for northern red oak, 38 X 10-6 mm/(mm C) for Douglas-fir and yellow birch. Coefficients in the longitudinal direction and for plywood were below 10 X 10-6 mm/(mm C) on the average.


Keywords


Betula alleghaniensis;Quercus rubra;Pscudotsuga menziesii;Sequoia sempervireus;plywood;thermal expansion coefficients;internal drying;shrinkage

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