Wood Internal Stress at Different Drying Stages - Initial Stage of Drying
Wood Internal Stress at Different Drying Stages
The internal stress changes during wood drying can be divided into four stages.
At the beginning stage of drying, the wood has not yet produced stress, and the moisture content of each part of the wood is above the fiber saturation point. If the test piece is cut from the middle of the material, the test piece is sawn into a comb shape, and the height of each tooth is the same as the sawn tooth. The original size before opening is the same. If the test piece is cut into two halves, each piece remains straight, which shows that there is no moisture content stress and no residual stress in the wood.
In the initial stage of drying, at this stage, the free water on the surface of the wood has evaporated, and the absorbed water begins to be discharged and begins to shrink. However, the movement of water inside the wood is far behind the evaporation of surface water, and the moisture content of the inner layer is still higher than that of the fiber. Saturation point, so the outside has shrunk, but the inside does not shrink, the inner layer is compressed by the outer layer, and the surface layer is stretched. Just like a rubber ring is stretched and tightly wrapped on a wooden stick, the rubber ring will shrink, but the wooden stick will not shrink, which will inevitably cause the rubber ring to elongate, and the wooden stick will be compressed by the rubber ring.
Therefore, the internal stress in the initial stage of wood drying is the tensile stress on the outer layer and the compressive stress on the inner layer. If a test piece is cut from the middle of the wood and carved into a comb shape, it can be seen that the teeth on the surface are reduced in size due to drying shrinkage. , the teeth of the inner layers still maintain the size before drying without drying shrinkage. If the test piece sawn from the wood is cut into two pieces, and each of them bends outward immediately after cutting, it means that the outer dimension is shorter than the inner dimension. If the two pieces are put into a constant temperature box or placed in a ventilated place to reduce and make the moisture content uniform, due to the plasticity of the wood, the wood will undergo plastic deformation as soon as the internal stress is generated, and the original wood on the surface has been plasticized to a certain extent. When the moisture content of the inner layer is reduced, the original wood can shrink freely. Therefore, after the moisture content of the two halves is reduced and evenly distributed, the shape of the two pieces will be transformed into the opposite of the original one. become inwardly bent.
This kind of stress is allowed to exist in drying, because the movement of moisture inside the wood depends on the gradient of moisture content, and the gradient of moisture content will inevitably cause internal stress, but this stress should not be too large and the time should not be too long, otherwise it will cause Causes cracking of the wood surface.
At this stage, it is necessary to make use of the moisture content gradient without causing excessive stress on the wood. This requires regular spraying and steaming treatment and maintaining high humidity for a period of time to increase the moisture content of the wood surface and make the fixed plastic deformation Part resoftens and can shrink, thereby eliminating or reducing the tensile stress of the surface layer and the compressive stress of the inner layer.
Generally speaking, the initial stage of drying is the stage in which attention should be paid to prevent surface cracking during the drying process, and it is the stage in which the moisture content drops rapidly.
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