① density. Refers to the weight of wood per unit volume. The weight and volume of wood are affected by water content. The ratio of the dry weight of wood samples to the volume when saturated with water, the volume after drying and the volume during drying is called basic density, absolute dry density and drying density respectively. The ratio of the weight of wood after air-drying to the volume after air-drying is called wood air-drying density. Wood density varies with tree species. The air-dried density of most wood is about 0.3 ~ 0.9 g/cm3. The mechanical strength of high-density wood is generally higher.

② Moisture content of wood. Refers to the percentage of water weight in wood to the weight of dry wood. The moisture in wood can be divided into two parts, one of which exists in the cell wall of wood cells and is called adsorbed water; The other part exists between the cell cavity and the cell gap, which is called free water. When the adsorbed water reaches saturation without free water, it is called fiber saturation point. The fiber saturation point of wood varies with tree species, which is about 23 ~ 33%. When the moisture content is greater than the fiber saturation point, the moisture content has little effect on the wood properties. When the moisture content begins to decrease from the fiber saturation point, the physical and mechanical properties of wood also change. Wood can absorb or evaporate moisture in the atmosphere, and it is consistent with the relative humidity and temperature of the surrounding air, reaching a constant moisture content, which is called equilibrium moisture content. The equilibrium moisture content of wood varies with region, season and climate, which is about 10 ~ 18%.

③ Extensibility. Wood expands in volume after absorbing water and contracts when losing water. The drying shrinkage of wood from fiber saturation point to drying is about 0. 1% along grain direction, 3-6% in radial direction and 6- 12% in chord direction. The difference of radial and chord shrinkage is the main reason for wood cracking and warping.

Mechanical properties of wood:

Wood has good mechanical properties, but it is an organic anisotropic material, and the mechanical properties along the grain direction are quite different from those along the grain direction. Wood has high tensile strength and compressive strength along grain, but low tensile strength and compressive strength along grain. Wood strength also varies with tree species, and is influenced by wood defects, loading time, moisture content and temperature, among which wood defects and loading time have the greatest influence. Due to the different size and position of joints and different mechanical properties (tension or pressure), the strength of jointed wood can be reduced by 30 ~ 60% compared with that of non-jointed wood. Under long-term load, the long-term strength of wood is almost half of the instantaneous strength.