composition
Wood is a complex organism with a main component of about 44% carbon, about 6% hydrogen, about 42.5% oxygen, and less than 0.5% nitrogen and less than 1% mineral ash.
Experimental formula for wood: C3H4O2
The main components of wood are lignin, cellulose and hemicellulose.
Utilization of wood physics and mechanical properties
Material
Artificial board
Plywood, pellet board, fiberboard
Firewood
Utilization of wood chemistry
Pulp and paper
Wood chemical improvement
Wood can be divided into two categories: coniferous trees and broad-leaved trees. Chinese fir and various pines, spruce and fir are conifers; eucalyptus, ash, citron, eucalyptus and various birch, nanmu and poplar are broad-leaved trees. There are many species of trees in China, so the wood species commonly used in engineering in various regions are also different. In the northeastern region, there are mainly Korean pine, larch (Huanghuasong), fish scale spruce, red spruce, and ash. The Yangtze River Basin mainly includes Chinese fir and Masson pine; in the southwest and northwest, there are mainly fir, spruce and hemlock.
The trunk consists of bark, forming layers, xylem (ie wood) and medulla. The annual ring surrounding the medulla can be seen from the xylem of the trunk cross section. Each round is generally composed of two parts: the light part is called early wood, which grows in the early season, the cell is larger and the material is sparse; the dark part is called the late wood, which is grown in the late season, the cell is more Small, dense material. Some wood, in the middle of the trunk, is darker in color and is called a heartwood; at the side, it is lighter in color and is called sapwood. The coniferous tree material is mainly composed of tracheid, wood ray and axial thin-walled tissue, and the arrangement is regular and the material is relatively uniform. The broad-leaved tree material is mainly composed of ducts, wood fibers, axial thin-walled tissues, wood rays, etc., and the structure is complicated. Since the cells constituting the wood are oriented, the difference between the grain and the grain is formed. The horizontal stripes can be distinguished by a radial direction consistent with the wood rays; a chord direction perpendicular to the wood rays. Coniferous trees generally have tall trunks, straight texture, easy processing, easy drying, less cracking and deformation, and are suitable for structural materials. Some broad-leaved trees have a hard texture, beautiful texture and are suitable for decoration.
Also known as rickets, can be divided into three categories:
1 Natural defects. Such as knots, oblique textures, and defects due to growth stress or natural damage. The knot is the part of the branch that is wrapped in the xylem when the tree grows. The oblique texture of the log is often referred to as a twist, and the sawn timber is called a twill.
2 Biological defects. There are mainly decay, discoloration and insects.
3 Defects caused by drying and machining. Such as dry cracking, warping, slash injury and so on. Defects reduce the value of wood use.
In order to use wood reasonably, the type, size and quantity of defects allowed in the wood are usually restricted according to the requirements of different uses, and the wood is classified. Decayed and wormed wood is not allowed to be used in construction, so the defects affecting structural strength are mainly knots, twill and cracks.
The main physical properties of wood are:
1 Density. Refers to the weight of wood per unit volume. The weight and volume of wood are affected by the moisture content. The ratio of the dry weight of the wood sample to its volume when saturated with water, the volume after drying and the volume at the time of drying is referred to as the basic density, the dry density and the dry density. The ratio of the weight of wood after air drying to the volume after air drying is called the air dry density of wood. Wood density varies with tree species. Most woods have an air dry density of about 0.3 to 0.9 g/cm3. Wood with high density generally has a high mechanical strength.
2 Wood moisture content. Refers to the percentage of water in the wood that accounts for the weight of the dried wood. The moisture in the wood can be divided into two parts, one part is in the cell wall of the wood cell, called adsorption water; the other part exists between the cell cavity and the cell gap, 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 from tree to species, ranging from 23 to 33%. When the water content is greater than the fiber saturation point, the effect of moisture on the properties of the wood is small. As the water content decreases from the fiber saturation point, the physical and mechanical properties of the wood change. 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 water content, called equilibrium moisture content. The equilibrium moisture content of wood varies with regions, seasons and climate, and is between 10 and 18%.
3 expansion and contraction. The wood expands in volume after absorbing moisture, and shrinks when it loses moisture. The dry shrinkage rate of the wood from the fiber saturation point to the oven dryness is about 0.1% in the direction of the grain, about 3 to 6% in the radial direction, and about 6 to 12% in the chord direction. The difference in radial chord-to-dry shrinkage is the main cause of cracks and warpage in wood.
Wood has good mechanical properties, but wood is an organic anisotropic material, and the mechanical properties of the direction of the grain and the direction of the grain are very different. The tensile strength and compressive strength of the wood are higher, but the horizontal tensile strength and compressive strength are lower. Wood strength also varies with tree species and is affected by factors such as wood defects, load time, water content and temperature, with wood defects and loading time being the most affected. Due to the different size and position of the knot and the different stress properties (pull or pressure), the strength of the knotted wood can be reduced by 30 to 60% compared with the unstructured wood. The long-term strength of wood under long-term loading is almost half of the instantaneous strength.
In addition to the direct use of logs, wood is processed into sheet metal or other products. In order to reduce the deformation and cracking of the wood during use, the square of the board is usually dried naturally or manually. Natural drying is the stacking of wood for air drying. Manual drying is mainly carried out by a dry kiln method, and a simple drying and baking method can also be used. The drying kiln is a drying chamber equipped with circulating air equipment that regulates and controls the temperature and humidity of the air. The dried wood in the drying kiln has good quality and the water content can reach below 10%. Wood that is prone to decay in use should be preservative treated in advance. Gluing can be used to synthesize sheet rubber into large components for wood structures, stakes, and the like. Wood can also be processed into plywood, shredded wood, fiberboard, and the like.
In ancient buildings, wood is widely used in temples, palaces, temple towers and residential buildings. Among the existing ancient buildings in China, the most famous one is the East Hall of Foguang Temple in Wutai Mountain, Shanxi Province, which was built in 857 AD; the wooden tower of Yingxian County, Shanxi Province, was built in 1056, with a height of 67.31 meters. In modern civil engineering, wood is mainly used for building wooden structures, wooden bridges, formwork, poles, sleepers, doors and windows, furniture, building decoration, etc.
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