III. Read and translate the following sentences.

1. Mechanical characteristics of wood depend on its structure and moisture

content.

2. Density and permeability are physical properties of wood.

3. Wood shrinks most in the direction of the annual growth rings (tangentially)

4. The elasticity and strength properties depend on fibre orientation, loading rate,

size and location of natural characteristics such as knots.

5. Strength of wood is the ability of wood to resist changes in size or shape when

it is affected by mechanical forces.

6. Hardness and plasticity are called technological properties of wood.

7. Shrinkage of wood can result in its warping, checking and splitting.

8. The ability of wood to be deformed without failure is different in various wood

species.

IV. Read and translate the text. Physical, Mechanical and Technological Properties of Wood

Physical and mechanical characteristics of wood depend on its structure, moisture content and mineral content. Wood is anisotropic material, which means that wood has different properties in the longitudinal, tangential and radial directions or axes of a piece of wood. Wood properties within one species vary greatly from tree to tree and within the same axis.

Physical properties. Physical properties are moisture content, shrinkage, density, permeability, and thermal and electrical properties. Moisture content is an important factor in the processing of wood because it influences all physical and mechanical properties. Normal moisture content of dry wood ranges from 8 to 13 per cent. Shrinkage occurs when wood loses moisture below the so-called fibre saturation point. The moisture content at which only the cell walls are completely saturated with water and the cell cavities are free from water, is called fibre saturation point. The more moisture is lost below the fibre saturation point, the greater shrinkage is. As anisotropic material, wood shrinks more in the radial (perpendicular to the annual rings) and tangential directions than in the longitudinal direction. The shrinkage can result in warping, checking, splitting. The density of wood is determined by the mass of cell wall substance per unit volume. Density can vary within an annual growth ring. The percentage of earlywood and latewood in each growth ring determines the overall density of a piece of wood. Permeability is the property of wood, which allows chemicals (liquid or gas) to pass through it under pressure. Permeability is influenced by the structure of the wood cells. Permeability also depends on the wood species. The treatment by chemicals is important to fire-, insect-, and decay resistance. Thermal properties of wood are conductivity, specific heat and coefficient of thermal

expansion. The conductivity is determined by density, moisture content, and direction of conduction. The knowledge of thermal conductivity is important to wood processing because heating of wood materials – in drying or in pressing – is a necessary step.

While using wood the following properties are also taken into consideration: heat insulation, odour, colour.

Mechanical properties. The mechanical properties are elasticity, strength and vibration characteristics. These properties depend on wood species, fibre orientation, moisture content, loading rate, and size and location of natural characteristics such as knots. Wood is both, an elastic and plastic material. Elasticity is the property of wood material to recover its size and shape after the loading is removed. When a loading is applied to a piece of lumber, this piece of lumber undergoes different kinds of stresses. The loading causes the lumber to bend, producing tension and compression along the faces and also horizontal shear at the ends of the piece of lumber. Strength of wood is the ability of wood to resist these stresses, or, in other words, to resist changes in size or shape and stay strong when it is affected by mechanical forces.

Load

Fig. 1. Fiber Stresses

These properties are: compression strength, tensile strength, bending strength and shearing strength. The strength of wood is influenced greatly by the combined effect of high temperatures and moisture content. The higher temperature and moisture content are, the lower wood strength is.

Horizontal Shear

Compression

Tension

Damping and sound velocity are two most important vibration characteristics. Damping is a property of wood to dissipate mechanical energy as heat. The velocity of a sound wave through wood can be used to measure mechanical strength and stiffness: the higher the velocity, the higher the stiffness and strength. Like other properties of wood, the velocity of sound along the three principal axes is different.

Technological properties of wood. In many cases of wood processing, the properties, such as hardness, plasticity (or toughness), the wood property of holding steel details (screws, nails) must be taken into consideration. These properties of wood are called technological. Plasticity is a valuable property of wood which makes it possible to use wood for manufacturing bentwood furniture. The ability to be deformed is different in various wood species. The hardwoods such as ash, elm, oak, beech are easy to bend while softwoods are not very good for bending.