گسترش تنش هاي ارتوتروپ و گراديان كرنش باقي مانده در تخته هاي كششي و معمولي صنوبر در اثر خشك شدن

Orthotropic Drying Stresses and Residual! Strain Gradient in Poplar Tension and Normal Wood

Orthotropic drying stresses and residual strain gradient were investigated in tension and normal wood of P. nigra. Flat- and quarter sawn boards of each wood were conventionally dried under a constant dry-bulb temperature of 60°C and relative humidity of 50% inside a small laboratory kiln. The boards were dried to a final moisture content of about 8%, and the drying process was terminated without any conditioning treatment. Four surfaces of the boards were coated using polyurethane (PU)-coated aluminum foil to confine moisture flow through the board thickness. After drying, the degree of transverse stresses was evaluated by means of cutting U-shaped prongs. The degree of longitudinal stresses was measured by two methods: finger-test sample cut and strain-gauge method. The residual strain profile in the dried boards was also determined using slice-cutting method. Compared to normal wood, the more intense drying stresses developed in tension wood, whether in longitudinal or radial direction. Also, more unbalanced strain profile developed in tension wood compared to normal wood, and this was accompanied by a higher strain level in tension wood.