What are Technical Properties of Wood?
TECHNICAL PROPERTIES OF WOOD
Anatomical structure, climate, soil condition, forest frequency, sunbathing, wood imperfections (prunes, factors such as developmental defects, crevices, self-shift, double oz) wood diseases (bacteria, fungi, insects, worms) affect the technical properties of wood.
PHYSICAL PROPERTIES OF WOOD
- Unit Volume Weight
- Temperature Expansion
- Thermal conductivity
- Electrical conductivity
1- Humidity: Water, which is abundant among the tree cells, exists in three different ways.
- a) Structural (Constituent) water: It is water in its chemical structure. It does not change with drying processes.
- b) Absorption water: Cellulose is a substance with high level of hydrophilicity, and it absorbs water very well and causes the wood to swell. The rate of absorption water is 28-30%.
- c) Free water (Capillary water): It is the water found between and inside the cells. The feeling of wetness on wood and boards is because of the excess of capillary water. As a result, when the moisture of wood is mentioned, Suction water and Free water are first two to come to mind.
The physical properties of wood are severely affected by humidity. As the wood dries, it shrinks and loses volüme, its hardness and strength increase, but its energy holding capacity decreases. The ideal and true properties of wood should be determined at 12-15% humidity.
For example, a wood in the contact with water, a newly cut coniferous wood, a commercially dried wood is, a wood dried by artificial drying consist 200%, 130-60%, 25-15%, and 12% moisture respectively. Due to the fact that the wood absorbs the humidity of the environment it is in, it is not possible to find it in an absolute dry state. The amount of water remains at a constant value of 30% at most. For this reason, the cellulose texture and bonds of the wood undergo swelling with the water entering the wood cells, and shrinkage with reduction. Therefore some cracks occur. These deformations are generally occur in the most tangential direction of the wood, and they exist more in broad-leaved than coniferous species. In addition, the increase in humidity has a vital role in reducing the mechanical strength of the wood.
2- Unit Volume Weight: The UVW of the wood and the humidity are interdependent. The unit volume weight corresponding to 15% moisture varies between 0.1t/m3 and 1.5t/m3 depending on the tree species.
The mechanical properties of woods with high UVW are also higher than others. However, they are difficult to process and work. They are resistant to animals such as fungi and insects tough.
Wood with low UVW has low mechanical strength. On the contrary, their workmanship is easy.
3-Temperature Expansion: Volume of wood expands with temperature and decreases with cooling.
4-Thermal conductivity: Wood is a bad conductor of heat due to its wood-celled structure and the fact that the material that forms the basis of the structure is cellulose. It is basically used for division and covering material.
5- Electrical Conductivity: The conductivity increases rapidly depending on the increase in humidity. The more humidity level of it has, the more conductivity level it holds. Dry wood is also used as insulation material at low voltage.
6- Durability: The durability of wood comes from its resistance to external factors, regardless of protective processes. Chestnut, oak, pine and hornbeam are durable due to the natural antiseptic substances in their structure. Ash, beech, sycamore, poplar willow, linden are more likely known as less resistant materials comparing to others.
Hardness: As the density increases, the hardness increases. The hardness is higher in the vertical direction of the fibers. The spring wood is softer than the summer wood, and the outer wood is softer than the inner wood. As the humidity decreases, the hardness increases. However, the humidity increases the hardness in soft woods and decreases it in hard woods.
Color-Brightness: The colors of the trees change in the inner wood and outer wood. Also, some trees change color after drying. The color may vary from tree to tree, even within the tree. Luster depends on the reflection of light by the wooden surface. The inner wood is the outer wood; the core section is brighter than the other sections.
Odor: The odor depends on the amount and type of secretion substances and may decrease over time. Mushrooms also cause odor.
CHEMICAL PROPERTIES OF WOOD
In the chemical composition of the cell wall consist of the followings with the given ratios.
- Cellulose 40-50%
- Hemicellulose 20-35%
- Lignin 20%
- Foreign matter 0 – 5%
Cellulose: It is the main additive of the cell wall. It is this substance that gives strength to the physical properties of wood against bending and pulling.
Hemicellulose: Hemicellulose is short polymers of pentose and hexos sugars. It strengthens the cell wall, acts as a storage material, adjusts the passage membranes. It is water absorbent.
Lignin: It is contained within the cellulose fibrils. It provides the wood’s resistance to pressure. It is an amorphous substance with the main structure of a phenol ring. It is a low water absorbent. Its color is brownish white.
- Wood texture is the most durable compared to other plant tissues.
- Wood stored in a dry place will last for many years. It is also highly durable in environments where there are no animal pests (in water).
- The decrease in resistance in wood is highly dependent on humidity (26-30%), whereas cotton deteriorates at 10% humidity.
- External wood secretions (such as starch) attract organisms.
- Internal wood secretions are usually poisonous and kill organisms.
- Substances such as tannin (chestnut, oak), resin (pine, fir, spruce) and creosote (cedar) do not keep microorganisms alive.
- Anti-rot secretions are formed during the transition period from outer wood to inner wood.
- The secretions that prevent decay are genetic; the wood remaining within the species lasts for many years. Moreover, in environments where there are no animal pests (in water), amorphous b, even a single tree varies.
- Durable wood does not contain carbohydrates.
- Lignination creates a physical barrier to enzymes.
- Correctly treated wood is superior to natural wood.
- Inner wood is more durable than outer wood; summer wood is more durable than spring wood.
- Trees with dark interior wood are more durable.
- Durability depends on the type of tree.
MECHANICAL PROPERTIES OF WOOD
Since wood is a heterogeneous and anisotropic material, it is difficult to examine its mechanical properties. All properties such as compressive and tensile strengths in the direction of the fibers are higher than their strength in the transverse direction.
Since wood is a material that swells and contracts as a function of its water content, its mechanical properties also change relatively.
The water in the cell spaces, which is called free water, evaporates in the days following slaughter. Suction water adhered to the cell wall remains in the wood for a long time. A log left to itself dries in 2 years for conifers and 4 years for leafy ones.
The resistance of the wood against the compressive forces in the direction perpendicular to the fibers is low. In the direction of the fibers, the resistance against shear force is low.
The properties of artificial wood materials made from wood are similar to those wood. However, such homogeneous and isotropic materials developed for production purposes cannot show varying values depending on the fiber directions as seen in natural wood.
Modulus of elasticity:
10000 N/mm2 parallel to the fibers in pines, 300 N/mm2 perpendicular to the fibers
Oak, beech fibers parallel to 12500 N/mm2, fibers perpendicular 600 N/mm2
The density of naturally dried 10-15% moist oak is 800 gr/dm3, while pine is 550-600 800 gr/dm3. In parallel with the fibers, the tensile strength of 1st class pine is 100-105 kg/cm3, pressure resistance is 85-100 100-105 kg/cm3. It wears out quickly in different weather effects. It is not resistant to fire.
DEFECTS OF WOOD
Wood is divided into classes such as I, II, III class woods in terms of the defects they contain. Knots from these defects cause a decrease in strength in the elements working under tension.
Accordingly, we can classify the defects as follows:
- Differences in age rings widths
- Decentralized growth of age rings
- Resin pockets
- Knots formed by branch places
- Circular cracks between age rings
- Longitudinal cracks
- Radial cracks in heartwood
- Twisted fibers
- Torsion in the stem, fibers not parallel to the shear axis