1. Stress and deformation caused by inconsistent wood shrinkage in radial and chord directions
According to the characteristic that the shrinkage coefficient in the chord direction of wood is approximately twice that of the radial direction, the drying shrinkage, stress and deformation of the three types of wood are analyzed.
A diameter cutting board
The surface of the board is all radial section, the thickness of the board is chord direction, the shrinkage is uniform, and no additional stress and deformation are caused.
B string cutting board
The outer surface (the surface close to the bark) is close to the chord direction, and its dry shrinkage is greater than the inner surface (the surface close to the pith) that is close to the radial direction. Therefore, the force of the outer surface is warped during drying, but it is actually dry During production, the plates are stacked into piles, and press blocks are placed on the top of them to prevent warping and deformation of the wood. Due to the weight of the piles and briquettes, additional pressure is exerted on the plates to restrain their warpage, so the outer surface of the plates Additional tensile stress is generated, and additional compressive stress is generated on the inner panel surface. This stress has nothing to do with the water content gradient. The superimposition of the additional tensile stress on the outer plate surface with the surface tensile stress caused by uneven moisture content can easily cause surface cracks on the outer plate surface.
C Square material with pith
Its four surfaces are close to the chord section, and the shrinkage is larger than that in the diameter direction. The dry shrinkage of the four surfaces is inhibited by the wood in the inner diameter direction during drying. As a result, additional tensile stress is generated in the surface area, and additional compression is generated in the center area. stress. This stress also has nothing to do with the water content gradient. This additional surface layer tensile stress is superimposed on the tensile stress caused by the moisture content gradient in the initial stage of drying, and it is easy to cause surface cracks and radial cracks. Therefore, square timber with pith core is prone to defects when it dries. Stress and deformation caused by uneven moisture content in wood thickness.
2. Stress and deformation caused by uneven moisture content in wood thickness
During the drying process, the anisotropy of wood shrinkage is not considered, and it is assumed that only the moisture movement occurs in the thickness of the wood, then the moisture content distribution, stress and deformation changes in the thickness can be analyzed in four stages:
A. The stage where stress has not yet been generated at the beginning of drying. In this stage, although the moisture content of the surface layer is low and the moisture content of the thickness is unevenly distributed, they are all above the fiber saturation point, and no shrinkage occurs, so no stress occurs.
B. In the initial stage of drying, the stress is externally pulled and internally compressed. After the drying process begins, the free water on the surface of the wood evaporates first. After a short period of time (depending on the temperature and relative humidity of the drying medium), the surface moisture content drops below the fiber saturation point, and the moisture content gradient on the section increases and appears "Wet line", the area outside the "wet line" drops below the fiber saturation point, and the area inside is still higher than the fiber saturation point. As the drying progresses, the "wet thread" continues to move inward.
Because the moisture content of the wood surface layer is below the fiber saturation point, it will shrink, but the moisture content of the internal layers is higher than the fiber saturation point and the size is unchanged, thus restricting the drying shrinkage of the surface layer. Therefore, the surface layer is subject to tensile stress due to this restraint. Then it is under compressive stress at the same time. And because the area where the moisture content drops below the fiber saturation point is thinner at the initial stage of drying, the area corresponding to the tensile stress is smaller, and the area receiving the compressive stress is larger, and the total tension is balanced with the total pressure , The internal compressive stress per unit area is small, while the tensile stress per unit area of the surface layer is quite large, and it develops quickly and reaches a larger tensile stress. When the stress is greater than the surface tensile strength limit, cracks will occur. This is also the main reason for the cracking of the surface at the early stage of drying.
Because wood is an elastic-plastic body, when the tensile stress of the surface layer exceeds its proportional limit, plastic deformation will occur, or although the tensile stress does not exceed the proportional limit, creep will occur after a long time of stress, which will cause plasticization and fixation.
As the drying process progresses, the "wet line" keeps moving inward, that is, some areas within the surface layer gradually drop below the fiber saturation point, the tensile stress area gradually expands, and the internal compressive stress above the fiber saturation point The area of effect is gradually reduced. Therefore, the tensile stress per unit area of the surface layer gradually decreases, while the compressive stress per unit area of the inner layer gradually increases and reaches a larger value, but the internal lamination stress develops slowly.
C. During the dry period, the internal and external stresses are temporarily balanced. At this stage, the surface layer is severely stretched and plasticized and fixed, resulting in limited dry shrinkage, and the comb tooth length of the surface layer is longer than the size that should be achieved by free dry shrinkage. Due to its free shrinkage or compression and plastic deformation, its size gradually approaches and is temporarily equal to the outer layer size. At this time, the stresses of the inner and outer layers in the wood are temporarily balanced.
D. In the late drying stage, the stress is externally pressed and internally pulled. At this stage, the “wet line” continues to move inward, and the moisture content gradient on the cross section of the wood slows down. Because the surface layer is plasticized and fixed, the dry shrinkage has stopped, so the hardened surface layer and the core layer above the fiber saturation point contain the shrinkage of the middle layer. The middle layer produces tensile stress, the surface layer and the core layer produce compressive stress, and the surface layer and the middle layer undergo a stress transition. Continue to dry, the "wet line" disappears, that is, the moisture content of each part of the wood drops below the fiber saturation point. At this time, the internal layers are restricted by the surface layer to produce limited shrinkage, so all are under tensile stress, and the surface layer quickly reaches a relatively high level. Large compressive stress and internal tensile stress have also reached larger values one after another. When the tensile stress is greater than the internal tensile strength limit, cracks occur. This is also the reason why internal cracking is easy to occur in the later stage of drying. It can be seen that the internal fissure is mainly caused by severe plasticization and fixation in the early stage of drying.