Physical and mechanical characteristics of wood which are controlled by specific anatomy, moisture content, and to a lesser extent, mineral and extractive content. The properties are also influenced by wood's directional nature, which results in markedly different properties in the longitudinal, tangential, and radial directions or axes. Wood properties within a species vary greatly from tree to tree and within a single axis.
Physical properties
The physical properties (other than appearance) are moisture content, shrinkage, density, permeability, and thermal and electrical properties.
Moisture content is a major factor in the processing of wood because it influences all physical and mechanical properties, and durability and performance during use. Normal in-use moisture content of processed wood that has been dried ranges 8–13%. Moisture content for wood is expressed on either a fractional or percentage basis. Moisture content is defined as the ratio of the mass of water contained in the wood to the mass of the same sample of dry wood.
Shrinkage occurs when wood loses moisture below the fiber saturation point. Above that point, wood is dimensionally stable. The amount of the shrinkage depends on its direction relative to grain orientation and the amount of moisture lost below the fiber saturation point. Wood shrinks significantly more in the radial and tangential directions than in the longitudinal direction.
The density of wood is determined by the amount of cell wall substance and the volume of voids caused by the cell cavities (lumens) of the fibers. Density can vary widely across a growth or annual ring. The percentage of earlywood and latewood in each growth ring determines the overall density of a wood sample.
Permeability is a measure of the flow characteristics of a liquid or gas through wood as a result of the total pressure gradient. Permeability is influenced by the anatomy of the wood cells, the direction of flow (radial, tangential, and longitudinal), and the properties of the fluid being measured. Permeability is also affected by the species, by whether the wood is sapwood or heartwood, and by the chemical and physical properties of the fluid.
The primary thermal properties of wood are conductivity, specific heat, and coefficient of thermal expansion. The conductivity of wood is determined by density, moisture content, and direction of conduction. Thermal conductivity in the transverse directions (radial and tangential) is approximately equal. Conductivity in the longitudinal direction is greater than in the transverse directions. For most processing operations, the dominant heating direction is transverse. Thermal conductivity is important to wood processing because heating—whether for drying, curing, pressing, or conditioning—is an integral step. Specific heat of wood is dependent on moisture content and, to less extent, on temperature. See also Specific heat.
Dry wood is an excellent insulator. By measuring wood's electrical resistance, electrical moisture meters accurately determine the moisture content of wood in the 5–25% range. Two other electrical properties of interest are the dielectric constant and the dielectric power factor for alternating current. These dielectric properties are dependent on density, moisture content, frequency of current, grain orientation, and temperature. The power factor is a measure of the stored energy that is converted to heat.
Mechanical properties
The mechanical properties of wood include elastic, strength, and vibration characteristics. These properties are dependent upon species, grain orientation, moisture content, loading rate, and size and location of natural characteristics such as knots.
Wood is both an elastic and plastic material. Elasticity manifests itself during loading and at moisture contents and temperatures that occur in most service uses of wood. The elastic stiffness or modulus of elasticity of wood is dependent on grain orientation, moisture content, species, temperature, and rate of loading. The stiffness of wood in the longitudinal (fiber) direction is utilized in the manufacture of composite products such as oriented strand board, in which the grain or fiber direction is controlled. See also Elasticity.
The strength of wood, like its elastic properties, is dependent upon rate of loading, species, moisture content, orientation, temperature, size and location of natural characteristics such as knots, and specimen size. The strength of individual wood fibers in the longitudinal direction can be significantly greater than that of larger samples with their complex anatomy and many defects. As with stiffness, the excellent strength characteristics of wood in the direction of the fiber can be maximized during the manufacture of wood composites by controlling fiber alignment.
Damping and sound velocity are two primary vibration phenomena of interest in structural applications. Damping occurs when internal friction dissipates mechanical energy as heat. The velocity of a sound wave through wood can be used to estimate mechanical stiffness and strength: the higher the velocity, the higher the stiffness and strength. Like other properties of wood, the velocity of sound along the three principal axes differs. Sound velocity in the longitudinal direction is two to four times greater than in the transverse directions. See also Wood anatomy.
