Strain of wood
Internal stress, also known as internal force, is caused by uneven drying shrinkage. For example, when the surface of wood has started to dry and shrink, and the interior of the wood is still moist, the surface may dry and shrink while the interior does not, resulting in internal stress. When a straight and dry wooden board is sawed into a thin plate, it should immediately experience a phenomenon of warping, which is a manifestation of internal stress in wood. If a sawn wooden board has a convex surface facing inward, it indicates external tension and internal pressure.
Another situation is that if wood is rapidly dried at high temperature and low humidity, its surface will dry first and shrink, but the interior will not shrink due to moisture, and the tension stress caused by the extremely tight and long surface cannot be restrained. The surface will become rigid due to excessive tension, and the wood will continue to undergo rapid drying. The internal moisture will evaporate and begin to shrink, but due to surface rigidity, it cannot freely shrink. Therefore, tension stress will be generated inside the wood, and when the wood is sawn, the convex surface will deform outward. This deformation reflects the opposite change in the internal stress of the wood compared to the previous situation. When the surface of the wood becomes rigid and internal tension occurs, such as when the wood cells do not resist this internal stress, internal internal internal cracks occur. The aforementioned internal cracks are formed due to improper drying, which is caused by internal stress.
The presence and manifestation of internal stress are commonly used to check the drying condition of wood during drying.; For example, if the wood is sawn open after drying and has a convex surface that tilts inward or outward, it indicates that the wood has internal stress. On the contrary, unchanged indicates no internal stress and uniform drying. This type of wood can be processed immediately.
Drying internal stress can be released through steam treatment, but this treatment should be carried out appropriately. It is also advisable to use higher relative humidity and time than unnecessary, as this can easily cause the wood to undergo reverse surface hardening. Eliminating reverse surface hardening can immerse the wood in water to fully absorb moisture before drying.
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