Wood is a popular material and is found in many different home accessories. Wood is also an easy-to-work material that you can unleash your creativity on any accessories and furniture in the home. But for beginners, grasping the properties of wood is something you should do first before embarking on any project.
We have compiled some information on the properties of this material and hope it will help your project.
Basic properties of wood materials
Wood has an isotropic structure, so its mechanical properties are not uniform in different directions. The mechanical properties of the wood depend on many factors such as: moisture content, volume weight, percentage of early and late wood layers, defect status, and so on.
Humidity and hygroscopicity
Humidity has a great influence on the properties of wood. Water in wood has 3 forms: capillary water (free), adsorbed water and chemically bound water. Free water resides in a cell, the spaces between cells and inside ducts. The adsorbed water is located in the cell envelope and the space between the cells. Chemically bound water is in the chemical composition of the wood-forming substances.
In a growing tree contains both adsorbed water and free water, or only adsorbent water. The state of wood containing maximum adsorption water and no free water is called the grain saturation limit (Wbht). Depending on the type of wood, the grain saturation limit can range from 23 to 35%.
When drying, water slowly separates from the outside surface, water from the inner wood layer gradually changes to replace. When the wood dries, it absorbs water from the air.
The degree of water vapor absorption depends on the temperature and relative humidity of the air. Because the humidity of the air is not constant, the moisture content of the wood is always changing. The moisture that wood receives when it is kept for a long time in a constant humidity and temperature atmosphere is called equilibrium humidity.
The equilibrium humidity of dry wood in the room is 8 ÷ 12%, and that of dry wood in the air after long drying in the air is 15 ÷ 18%.
Since the properties of wood (volume weight, intensity) vary with humidity (within the limits of the amount of water absorbed), so for comparison, it is often converted to standard humidity (18%).
The density for all woods is usually the same and its mean is 1.54 g / cm3.
Volumetric volume of wood depends on porosity (coniferous wood porosity: 46 ÷ 81%, broadleaf: 32480%) and moisture. It is converted volumetric mass of wood at any moisture (W) to volumetric mass at standard humidity (18%) according to the formula:
W0180γ = γ [1 + 0.01 (1- K0) (18 – W)]
– Volumetric mass of wood with moisture content W and humidity 18%. 180γW0γ
– K0 – Volumetric shrinkage factor.
Based on volume volume, wood is divided into five categories: very light wood (γ0 <400kg / m3), light wood (γ0 = 40 ÷ 500 kg / m3), medium light wood (γ0 = 500 ÷ 700 kg / m3) ), heavy wood (γ0 = 700 ÷ 900 kg / m3) and very heavy wood (γ0> 900 kg / m3).
Very heavy types of wood such as grinding wood (γ0 = 1100 kg / m3), wood (γ0 = 1080kg / m3). Very light types of wood such as: fig wood, white rosewood.
Wood shrinkage is the reduction in length and volume upon drying. The capillary water evaporation does not cause the wood to shrink. Shrinkage occurs only when the wood loses its adsorbed water. Meanwhile the thickness of the cell shell reduces the micelles, causing the size of the wood to decrease.
Enlargement is the ability of the wood to increase in size and volume when absorbing water into the cell wall. The wood swells when it absorbs water to the limit of grain saturation. The elongation is like the shrinkage is not the same in different directions (figure 8-3): Along the grain 0.1 ÷ 0.8%, normal: 3 ÷ 5%, tangent line 6 ÷ 12%.
#Color and texture
Each wood has a different color. Based on color, it is possible to preliminarily assess the quality and type of wood. Examples: Mahogany wood, dark ebony and black wood; dark pink, cheesy wood; pine and linden are white. The color of the wood also changes according to the status of fungi and the impact of wind and rain.
Wood grain is also very rich and varied. The coniferous wood is simple, the broadleaf is complex and beautiful (the slice of flowers has a cloudy pattern, the slice is like a pearl). The beautiful veined wood is used in handicrafts.
Wood’s thermal conductivity is not great and depends on porosity, moisture and grain direction, wood type, as well as temperature. Wood conducts heat in the longitudinal direction of the grain, 1.8 times larger than horizontally.
Average heat conduction coefficient of wood is 0.14 ÷ 0.26 kCal / m0C.h. As the volumetric volume and moisture content of the wood increases, so does its thermal conductivity.
Wood is a good transmitting material. Wood transmits sound 2 -17 times faster than air. Sound travels fastest along the grain, in the slowest tangential direction.
Mechanical Properties of Wood
Wood has an isotropic structure, so its mechanical properties are not uniform in different directions. Mechanical properties of wood depend on many factors such as: humidity, volume, percentage of early and late wood layers, defects, etc.
Since the mechanical properties of the wood depend on moisture, the strength of the test at a certain humidity (σW) must be converted to the intensity at the standard humidity (σ18) according to the formula:
σ18 = σW [1 + α (W – 18)]
In which: α – Humidity adjustment coefficient, representing the percentage change in intensity of wood when the humidity changes by 1%. The α value varies according to the intensity and direction of the grain.
W- Wood moisture (%), W≤Wbht.
The compressive strength includes: vertical grain compression, horizontal compression of the normal grain (radial), horizontal compression of the tangent grain, and compression of the grain.
In practice, it is very common to see compression along the grain (house columns, bridge poles, scaffolding, etc).
The radial tensile strength is very low, and when tangential tension is only connected between the working fibers, its strength is also smaller than pulling and compressing along the grain.
The bending strength of the wood is quite high (less than the longitudinal tensile strength and greater than the vertical compressive strength). Common bending-resistant working structures are beams, beams, embankments, etc.
Flexural strength is calculated according to bending moment M (kG.cm) and bending moment W (cm3).