The effect of wood moisture content on strength and shrinkage

Another characteristic of wood is that the water content directly affects the strength and volume of the wood. The water content of the wood is the ratio of the weight of the moisture of the wood to the dry weight of the wood. It is also called the water content. 4.16Kg, it is dried to absolute dry state, and the weight is 3.4Kg, then the dry weight of the wood is 3.4Kg, and the weight of the moisture is 4.16-3.4=0.76Kg. The moisture content of this piece of wood is:
Moisture content (w%) = (weight of water-containing wood - weight of dry wood) / (weight of dry wood) x 100% = 0.76 / 3.4 x 100% = 22.3%

Newly cut wood, the cell gap is filled with water, and the moisture content of the wood is above 100%. When the site is stacked, the water in the cell cavity evaporates first. At this time, the total weight of the wood is reduced, but the volume and strength are not changed. At some point, the water in the cell cavity is evaporated, and the cell wall is still filled with water. This condition is called "fiber saturation." This is about 30% moisture content. For convenience, the water content of 30% is defined as the "saturation point of the fiber". The water contained in the cell wall continues to evaporate, causing changes in the cell wall. At this time, the wood not only loses weight, but also begins to shrink in volume and the intensity begins to increase.

Wood strength varies with water content because the colloids between the cell walls are "hydrophilic". After the water evaporates, the colloidal plasticity decreases, the cementing force increases, and the fiber can resist the external force. The change of water content has little effect on the tensile strength of the grain, and has a great influence on the compressive strength and bending strength of the grain. For example, the compressive strength of pine at the fiber saturation point is about 3KN/CM2.

The phenomenon of volume shrinkage caused by the decrease of water content in wood is called dry shrinkage. Dry shrinkage is also called “anisotropic”. For example, when the fiber saturation point is reduced to 0%, the dry shrinkage is very small, 0.1~0.3%. The radial shrinkage of the horizontal grain is 3.66%, the maximum chord shrinkage is 9.63%, and the volume shrinkage is 13.8%. Therefore, when the wood texture is not straight and uneven, the evaporation speed of the surface and internal water is inconsistent, and the degree of shrinkage of each part At different times, irregular deformation such as bending and twisting may occur, and cracks may occur when the shrinkage is uneven.

The change of wood strength and shrinkage have brought many inconveniences to the use of wood. It is impossible for us to eliminate this objective and unfavorable change, but we can understand the laws of change and control this change. The moisture of the wood can be evaporated into the air, and the moisture in the air will be sucked in. The latter phenomenon is “hygroscopic”, and the moisture absorption is the characteristic of the wood, mainly because the moisture content of the wood reaches a relative saturation point, and the moisture content is too high, or Low can cause disadvantages to the basic physical properties of wood. The picture shows the relationship between air humidity and relative humidity.

Corresponding to a certain air humidity and relative humidity, there is a wood moisture content value, which is called "balanced water content". Example: The local indoor average humidity is 32. The relative humidity is 55%. From the figure, we found that the equilibrium moisture content is 10%, and the furniture high-grade materials generally have a moisture content of 15%. Generally, wood products (including wooden packaging), the relevant departments set about 18%~25% for the standard products, because of the wood. Under the moisture content of 18% or less, wood rot fungi cannot survive and multiply.

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