Wood shrinkage
When bound water is removed, the wood volume and linear dimensions decrease. This property is called shrinkage. Shrinkage is not caused by the removal of free water. A larger number of cell walls per unit volume of wood contributes to more severe shrinkage.
- Wood shrinkage must be taken into account when sawing logs into boards, the so-called shrinkage allowances. For example, when drying lumber, etc.
- Shrinkage is not the same in different directions. Thus, in the radial direction the shrinkage is 1.5-2 times less than in the tangential direction.
- Maximum shrinkage occurs when all bound water is removed.
Without the participation of external loads, internal stress arises in the wood, which is formed due to unequal changes in the volume of wood.
In the surface areas of the board, the humidity is lower than in the center. Therefore, due to the fact that free drying is constrained, “tensile” stresses arise. In this case, compressive stresses arise inside the board.
If the tensile strength across the grain, the tensile stress, is reached, cracks will appear in the wood. Internal and superficial.
