Mechano-sorptive Creep in Wood - Experimental Studies of the Influence of Material Properties
Abstract: Mechano-sorptive creep deformations appear when wood is loaded under varying moisture conditions. These creep deformations are known to be larger than the creep deformations under constant humidity conditions. The present thesis deals with mechano-sorptive creep in Norway spruce wood. Creep in bending, tension and compression was studied experimentally. The test material was well defined in terms of growth conditions of the trees (one slow-grown stand and one fast-grown stand) and the position of the specimens in the trees. Influence of material properties as annual ring width, shrinkage/swelling, density, grain angle, modulus of elasticity, knots and compression wood on the mechano-sorptive creep behaviour was studied. Additionally, the variability of the dynamic modulus of elasticity (along the grain) and the shrinkage/swelling properties was studied for spruce wood.Annual ring width and modulus of elasticity are the two parameters that correlate best with the relative creep in bending. Specimens cut near the pith of the tree displayed significantly larger relative creep in bending than specimens cut near the bark. For bending creep no significant difference was found between specimens from the different stands. Mechano-sorptive compression creep deformations were larger than corresponding tension creep deformations. In tension and compression no significant difference between specimens cut near the pith and specimens cut near the bark was found. For all three loading modes, creep deformation after a few moisture cycles was a good predictor of the creep deformation also after more moisture cycles. In bending, tension and compression the picture of the creep behaviour was substantially changed when the free shrinkage and swelling along the grain were subtracted. The dynamic modulus of elasticity and the shrinkage/swelling in the longitudinal direction displayed a strong individual variation with distance from the pith for specimens from individual trees. The eigenfrequency for vibrations in the longitudinal direction was the single best predictor of shrinkage/swelling in all three directions. Occurrence of compression wood reduced the dynamic modulus of elasticity on the average by 40%. The shrinkage/swelling in the longitudinal direction was doubled and in the transverse directions the shrinkage/swelling decreased by approximately 30% for specimens containing compression wood. Knots decreased the modulus of elasticity on the average by 40%.
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