Abstract:

In this paper, the rheological behavior of perpendicular to the grain of 3 mm-thick surface layer of fast-growing poplar lumber was discussed. Four kinds of drying temperatures(85℃, 105℃, 115℃and 125℃) were applied to dry the lumber. The results show that the total strain that occurs in lumber drying is the sum of four components:elastic strain, shrinkage strain, visco-elastic creep strain and mechano-sorptive creep strain. When the lumber is under the joint action of the high temperature and high humidity for a long time, it shows not only an elasticity and a plasticity but also a viscoelasticity. The authors improved the traditional slicing method for determination of the dryint strain of the lumber in accordance with the theory of the wood drying stress model, and put forward for the first time a new testing method to determine simultaneously different strain parameters of the established model on the same piece of squared lumber.(1)The surface layer of the lumber can be greatly plasticized in the high temperature drying and the casehardening is formed. The major factors which affect the drying stresses of the lumber are the maximum values of mechano-sorptive strains both in and after the drying procedure. (2) In the initial drying period, the linear expansion coefficient across the grain of the lumber increases due to the high temperature. This means that the lumber does not shrink but expands instead. As for the conventional drying method, the lumber does not expand due to a rather low temperature in the interior of the lumber, i.e. it shrinks as soon as the drying procedure begins. (3)The tensile strain of the surface layer of the poplar lumber is not so big but the compressive strain is much bigger than the tensile strain. This is why there is no cracking on the surface layer of the poplar lumber at the initial drying stage but internal cracking and shrinkage appear at the later drying stage, especially with a high temperature. (4) It can be found that, through comparison made among the different drying conditions, the viscoelasticity strain reduces as the drying temperature goes up. Judging from the overall variation characteristics of the strains, it is not difficult to find that the value of the creep strain of the viscoelasticity is rather small. (5)The pure strain in the lumber due to the drying is an algebraic sum of all strains excluding the self-shrinking strain of the lumber.

Key words: Elastic strain, Mechano-sorptive creep strain, Shrinkage strain, Visco-elastic creep strain