Density
Density is the mass of an object per unit volume, usually in g/cm3; or kg/m3; Denote. Wood is a porous substance, and its shape and volume are composed of cell wall materials and pores (cell cavity, intercellular space, pores, etc.), so the density is divided into wood density and wood cell material density. The former is the mass per unit volume of wood (including pores) and the latter is the mass per unit volume of cell wall material (excluding pores).
Wood density: It is an important indicator of wood properties, according to which the actual weight of wood is estimated, and the process properties of wood and the physical and mechanical properties of wood such as dry shrinkage, expansion, hardness, and strength of wood are inferred. Wood density, basic density and air-dry density are the most commonly used.
1. Basic density
The basic density is suitable for comparing the properties of wood because the weight of the dry timber and the volume of the raw timber (or impregnated timber) are relatively stable, and the measurement results are accurate. It is also practical in the wood drying and preservative industry.
2. Air-dry density
Air-dry density is the ratio of the weight of air-dried wood to the volume of air-dry wood, usually with the density of wood with a moisture content of 8%~20%. Wood air-dry density is the basic basis for the comparison of wood properties and the production and use of wood in China.
The density of wood is affected by a variety of factors, and the main influencing factors are: the size of wood moisture content, the thickness of the cell wall, the width of the growth ring, the level of fiber ratio, the amount of extract, the position and age of the trunk, the site conditions and forestry measures. According to the air-dry density of wood (when the moisture content is 15%), the wood is divided into five grades (unit: g/cm³):
Small: ≤0.350, Small: 0.351-0.550, Medium: 0.551-0.750, Large: 0.751-0.950, Large: >0.950.
Water Content
Refers to the percentage of water weight in wood to the weight of dried wood. The water in wood can be divided into two parts, one is found in the cell wall of the wood cell, called adsorbed water, and the other part is found between the cell cavity and the intercellular space, called free water (free water). When the adsorbed water reaches saturation and there is no free water, it is called the fiber saturation point. The fiber saturation point of wood varies depending on the tree species, and is about 23~33%. When the moisture content is greater than the fiber saturation point, the effect of moisture on the properties of the wood is very small. When the moisture content decreases from the fiber saturation point, the physical and mechanical properties of the wood change accordingly. Wood can absorb or evaporate water in the atmosphere, and adapt to the relative humidity and temperature of the surrounding air to achieve a constant moisture content, which is called equilibrium moisture content. The equilibrium moisture content of wood varies with region, season and climate and other factors, and is about 10~18%.
Expansion
When wood absorbs water, it expands in volume and shrinks when it loses moisture. The dry shrinkage of wood from the fiber saturation point to the furnace dry is about 0.1% in the grain direction, 3~6% in the radial direction, and 6~12% in the tangential direction. The difference in the dry shrinkage rate in the radial and chordal directions is the main cause of cracks and warpage in wood.
Mechanical properties
Wood has very good mechanical properties, but wood is an organic anisotropic material, and the mechanical properties of the grain direction and the horizontal grain direction are very different. The tensile and compressive strength of timber are high, but the tensile and compressive strength of the sgrain are low. Timber strength also varies from tree species to tree species and is affected by factors such as timber defects, load action time, moisture content and temperature, among which wood defects and load action time are the most affected. Due to the different size and position of the knots, and the different force properties (tension or compression), the strength of knotted wood can be reduced by 30~60% compared with knotless wood. The long-term strength of the timber is almost half of the instantaneous strength under long-term load.
