Stress and deformation during drying of wood - end-of-drying stage
At this time, the moisture content of the wood has been distributed fairly evenly along the layers of the wood section, and the moisture content gradient from the inside to the outside is small. Stop shrinking, and the inner layer of wood should shrink with the removal of absorbed water, so that the shrinkage of the inner layer of wood is limited by the outer layer of wood, and the stress of the inner layer being stretched and the outer layer being compressed, the internal stress The situation is just opposite to that at the initial stage of drying. At this time, if the test piece is sawed into a comb shape, some teeth in the middle of the test piece are free to shrink after breaking away from the shackles of the outer layer, and their size is shorter than that of the outer layer.
If the test piece is cut into two pieces, the two pieces bend inward when they are just cut, indicating that the inner side is shorter than the outer side, the inner side is under tension, and the outer side is under compression. When their moisture content is reduced and evenly distributed, the two pieces are bent inward. The degree is larger, indicating that there is a considerable residual stress.
Although the moisture content gradient at this stage is not large, as the drying continues, the internal stress will increase. If it is not eliminated in time, when the internal stress exceeds the strength limit of the wood, internal cracks will appear, that is, the inner layer of the wood will be stretched. If the stress exceeds the tensile strength limit of the inner layer, the inner wood will be damaged, and the cracked wood will lose its use value and cause serious waste. Therefore, the stress at this stage is very dangerous and should be eliminated in time.
The commonly used method is still spray steaming treatment, so that the surface wood will be re-moistened and softened under the action of high temperature and high humidity air in the kiln, and will be supplemented by dry shrinkage, so that the surface wood can shrink together with the inner wood, reducing the damage of the inner layer. Tensile and compressive stress on the outer layer. After the entire wood drying process, there may still be residual stress inside the wood. In order to eliminate these residual stresses, the wood will not be deformed, cracked and other defects in the future processing and use. A spray steaming treatment must be carried out to ensure the final drying quality.
In short, in the process of wood drying, internal stress exists at each stage, which is unavoidable. It is the cause of various drying defects. measures to bring it down to a safe level.
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