Brief overview of the microscopic structure of wood

Structure and composition of wood

Wood is a complex composite material created by nature. When considering the structure of wood, it is customary to distinguish between the macrostructure, visible to the naked eye, and the microstructure, visible using optical and electron microscopy.

The macrostructure of wood is the structure of wood visible to the naked eye. Three main sections of the trunk are considered: transverse - end and two longitudinal - radial, passing through the axis of the trunk, and tangential, passing tangentially to the annual rings (Fig. 3.1).

A cross-section of the wood of the trunk shows concentric annual rings located around the core. Each annual ring has two layers: early (spring) and late (summer) wood. The early wood is light in color and consists of large thin-walled cells.

Rice. 3.1. Tree trunk structure:

Late wood is darker in color and consists of small cells with thick walls; therefore, it is less porous and has greater strength than spring.

As the tree grows, the cell walls of the wood of the inner part of the trunk, adjacent to the core, gradually change their composition, become woody and are impregnated with resin in coniferous species, and with tannins in deciduous species. The movement of moisture in the wood of this part of the trunk stops, and it becomes stronger, harder and less susceptible to rotting. This part of the trunk is called core or mature wood in different species.

Microstructure of wood. Studying the structure of wood under a microscope, you can see that the bulk of the wood consists of mechanical tissue cells, spindle-shaped and elongated along the trunk.

Felled wood consists of dead cells, i.e., only cell membranes (Fig. 3.2). Cell walls are composed of several layers of very thin fibers called microfibrils, which are compactly arranged and directed in a spiral in each layer at a different angle to the cell axis (like individual strands in a rope). This provides high strength to the wood.

Chemical composition of wood. Microfibrils consist of long, chain-like cellulose macromolecules (from the Latin cellula - cell). These chains are built from a large number (several hundred) glucose units (therefore cellulose can be called a polysaccharide):

Due to the presence of highly polar groups, cellulose macromolecules are rigidly bound to each other, which explains the absence of a highly elastic state in wood, which occurs when heated in most linear polymers (for example, polyethylene). These same hydroxyl groups explain the hygroscopicity of wood and the accompanying swelling and shrinkage (see section 3.4). The mechanism of hygroscopicity consists in the formation of an electrostatic bond between polar groups of cellulose and water dipoles: depending on their type, location, size, and also on the purpose of wood products. The same defect in some types of products makes wood unsuitable, while in others it lowers its grade or is not of significant importance. Therefore, the standards for specific types of forest products contain instructions on acceptable defects.

Wood defects can be divided into several groups: defects in the shape of the trunk, defects in the structure of the wood, knots, cracks, chemical stains and fungal infections and warping. The main types of defects are discussed below.

Defects in the shape of the trunk are easily determined on a growing tree, so the trunks of such trees can be rejected at the cutting site. This group of defects includes camber, rootiness and curvature of the trunk (Fig. 3.3).

Tapering is a significant reduction in diameter along the length of the trunk. A normal runoff is considered to be a decrease in diameter by 1 cm per 1 m of trunk length. This defect reduces the yield of edged lumber. In addition, the material contains a lot of cut fibers, which reduces its strength.

Buttiness is a sharp increase in the diameter of the butt (lower) part of the trunk. The backbone can be round or ribbed. In any case, it increases the amount of waste and artificially causes cross-cutting in the finished product.

Trunk curvature is the curvature of a tree trunk in one or more places. Strong curvature makes the wood unsuitable for construction purposes.

Defects in the structure of wood are deviations from the normal arrangement of fibers in the tree trunk: inclination of fibers, twisting, tilting, double core, etc. (Fig. 3.4).

Rice. 3.3. Trunk shape defects:

The inclination of the fibers (oblique) is the non-parallelism of the wood fibers to the longitudinal axis of the lumber. This phenomenon (especially at large angles of inclination of the fibers) causes a sharp decrease in the strength of the wood and makes it difficult to process.

Rice. 3.4. Defects in the structure of wood: a - inclination of the fibers; b - curliness; c - heel; g - double core

Cross-grained lumber has an increased tendency to warp when humidity changes.

Curling is an extreme manifestation of cross-layering, when the wood fibers are arranged in the form of waves or curls.

Curling in some species (walnut, Karelian birch) gives a beautiful texture to the wood; Such rocks are used in finishing work.

Heeling is a change in the structure of wood when the annual rings have different thicknesses and densities on different sides of the core. The heel disrupts the uniformity of the wood.

Knots are the most common and inevitable wood defect, representing the base of branches embedded in the wood. They disrupt the homogeneity of the wood structure and cause fibers to bend (kink). Knots reduce the working cross-section of lumber, reducing their strength by 1.5...2 times (and more in thin boards and bars).

Based on the degree of fusion of knots with the wood of the trunk, knots are distinguished between fused, partially fused, and unfused (falling out) knots. Branched (fingered) knots are especially dangerous (Fig. 3.5).

Rice. 3.5. Different types of knots: a - fused healthy; 6 - drop-down; c - stitched; g - branched (palmated)

Healthy knots have hard and dense wood with no signs of rot. Often the knots rot until they turn into a loose powdery mass - these are the so-called tobacco knots.

For the manufacture of load-bearing wooden structures, wood that has only healthy fused knots is used. The number and placement of knots determine the grade of the material.

Cracks can appear both on a growing tree and when felled wood and lumber dry out. They violate the integrity of timber, reduce the yield of high-grade products, reduce strength and even make them unsuitable for construction purposes. In addition, cracks contribute to wood rotting.

The following types of cracks are distinguished: metic, frost and crack, which form on a growing tree, and shrinkage cracks, which form on felled wood (Fig. 3.6).

Metik - internal cracks running along the trunk from the center to the periphery; There can be several cracks, either located in the same plane or crosswise.

Rice. 3.6. Types of cracks: a, b - metic simple and complex; c, d — frost hole open and closed; d, f - ring and partial peel

A frost hole is an external open longitudinal crack that tapers towards the center. Such cracks occur when moisture in the trunk freezes during severe frosts.

Shooting is a complete or partial separation of the central part of the trunk from the peripheral part as a result of shrinkage of the former. Such cracks are located along the annual rings.

Shrinkage cracks are very common in wood of all species; they arise as a result of stresses caused by uneven shrinkage when wood is rapidly dried in air. These cracks are directed from the periphery to the center along the wood fibers.

Fungal lesions and chemical stains are caused by the simplest living organisms - fungi that develop from spores and use wood as a nutrient medium, or microorganisms. For the development of mushrooms, air oxygen, certain humidity and positive temperature are required. There are fungi that infect trees growing in the forest, freshly cut wood, and fungi that develop on wooden structures.

Wood-staining fungi can develop on growing trees. They feed on the contents of cells without affecting their walls. Therefore, the strength of such wood changes slightly, but colored spots and stripes appear on the wood.

A change in the color of wood without changing its mechanical properties can occur due to the biochemical oxidation of tannins provoked by microorganisms.

Wood-decaying fungi are much more dangerous. They feed on the material of cell walls - cellulose, decomposing it with the help of enzymes to glucose.

This is only possible if the wood has sufficient moisture. Glucose in the body of the mushroom is used in the process of its life and, ultimately, turns into carbon dioxide and water:

Rotting is essentially the same as burning, but at a very low speed.

A large number of wood-decaying fungi are known. Among them, the most common are the so-called house mushrooms. When affected by such fungi, the wood becomes rotten and light, and a coating of mold appears on its surface in the form of soft pads. House fungus can destroy wood very quickly (within a few months).

The rotting process stops when the wood moisture content decreases to 18...20% (dry wood does not rot), the temperature drops below 0 ° C or the supply of oxygen is excluded.

Insect damage (wormholes) are passages and holes made in wood by insects (bark beetles, borers) that live in it and feed on it. Borer beetles can develop in dry wood and even furniture.

Rice. 3.7. Longitudinal warping

Surface wormholes do not affect the mechanical properties of wood, since they go into the slab when sawing. Deep wormholes damage the integrity of the wood and reduce its strength.

Warping is a violation of the shape of lumber when its humidity changes during drying and storage or under the influence of internal stresses when longitudinally sawing large elements into smaller ones. Warping can be transverse, longitudinal (simple and complex) and helical (winged) (Fig. 3.7).

Read more: Types of materials and formwork made of wood Structure and properties of wood Protection of wood from rotting and fire Timber and wood products Main wood species used in construction The most important properties of wood Lumber for repair Wood materials and methods of their processing Types of lumber Types of lumber

Density

Wood density (bulk density) is the ratio of wood mass to its volume, expressed in kg/m3. The density of wood depends on its moisture content. All indicators of the physical and mechanical properties of wood are determined at a humidity of 12%.

There is a close relationship between strength and density . Heavier wood is generally more durable. Density is determined by the amount of wood substance per unit volume.

Based on density at a humidity of 12%, wood can be divided into 3 groups:

• light density - up to 550 kg/m3 (balsa, Siberian fir, spruce, willow, pine, aspen, linden, alder);
• medium density – 551–700 kg/m3 (horse chestnut, walnut, birch, cherry, larch, teak, beech, oak, sweetener, sycamore, maple);
• dense species - from 771 kg/m3 and above (ash, plum, pecan, boxwood, persimmon, apple, olive).

The density of wood is of great practical importance. Denser wood is less saturated with antiseptics and is less susceptible to abrasion in places such as floors, stairs, and railings.

Types of lumber

When building wooden structures and choosing lumber, you will come across names of types of lumber, the meaning of which you need to know:

• ridge - a thick tree trunk, freed from bark, or a segment of it of sufficiently long length (but not more than 25 cm);
• garter - the same ridge, but with a diameter of less than 25 cm;
• pole - a thin tree trunk (less than 9 cm in diameter), cleared of bark;
• plate - a tree trunk sawn along the grain;
• quarter - ½ plate, which is sawn along the grain;
• a log is a log in a horizontal position, hewn on both sides;
• timber - a log that is hewn on all four sides with a cross-section of at least 100x100 mm.
• whetstone - the same as timber, but smaller in size.
• Treated wood

In the construction industry, so-called refined wood has recently become popular - wood plastics obtained from processed wood materials.

Types of wood plastics:

- plywood;

— Fiberboard (fibreboard);

— Chipboard (chipboard);

— OSB (oriented strand board);

- wood-laminated plastic.

Advantages

Wood has a low thermal conductivity value. It can be successfully used as a heat-insulating material. Ease of processing allows the use of a wide range of tools. It is impossible to imagine any orchestra without musical instruments made of wood. The enchanting sounds of a violin are the result of such a property of wood as the ability to resonate. Wood bends easily, which opens up a large selection for the manufacture of various bent structures. Wooden products also have good sound-absorbing characteristics. A beautiful surface opens up space for imagination when designing rooms.

What parts does a fruit tree consist of (with photo and diagram)

The root collar is the place where the aerial part transitions into the root part. This main part of the tree is located approximately 5-7 cm below the grafting site and 3-4 cm above the first upper branch of the lateral root. In recent years, nurseries have mastered winter grafting directly into the root collar.

The trunk is the entire central part of a fruit tree or any other tree from the surface of the soil to the top.

The trunk is the part of the trunk from the root collar to the lower skeletal branch. In home gardens, trees should be planted with a lower standard of 40-60 cm, which is easier to protect from winter damage.

The central conductor or leader is the part of the trunk from the lower skeletal branch to the top of the tree.

Branches develop on the central conductor, from which skeletal branches of the first order are selected to form the crown (the branch directly extending from the conductor

A shoot is a growing leafy stem part that develops in the current growing season from the buds of last year's growth. After the leaves fall and before new growth begins, this part of the tree is called a branch, annual growth, or growth of the previous growing season. Starting next spring, the branches are already called branches.

Thinner branches, semi-skeletal (second-order branches), branch off from the skeletal branches, and overgrown branches subsequently appear on them. The set of all large and small branches located on the central conductor is called the crown of the tree.

The competitor is a strong shoot growing from a lateral bud adjacent to the top. Usually this part of the growing tree extends at an acute angle to the conductor and develops as strongly as the conductor, and sometimes more strongly. The fusion of the competitor with the conductor in the forks is usually fragile, so breakage can easily occur. It is better to cut out competitors in the year of their development in the first half of summer, but no later than the spring of next year.

Overgrowing branches are small, relatively young branches covering the skeletal branches. These are the main fruiting organs of the tree (generative), on which flower buds and the main fruit crop are formed.

Ringlets are the shortest branches, sometimes only a few millimeters long, have one well-developed bud at the apex and wrinkled bark.

As you can see in the photo, these parts of the tree extend at right angles to the branch on which they formed, they are cylindrical in shape and very fragile:

Warped

Warping of wood, that is, a change in its shape, occurs during its processing, drying, and storage.

Methods for measuring warp in lumber:

1. The longitudinal types of warping and the transverse type of warping are determined by the magnitude of the deflection of the material.
2. Winginess is determined by the highest deviation of the surface from the general plane.
3. Complex warping is determined by the magnitude of the deflection of the largest curvature entering it.

Some wood defects are determined visually, but most accurately and correctly they are detected and measured using hardware methods. Currently, gamma flaw detection, photoelectric, magnetic, X-ray, and acoustic methods are used for these purposes.

Depending on application

The properties of wood of various species are considered and compared depending on the type of product being manufactured.

Construction elements. The most popular types of building wood products are rounded logs, solid timber, laminated veneer lumber, structural beams, roof and ceiling building elements (boards and beams), frame elements, solid wood wall panels. For this category of products, the properties of wood such as strength and resistance to biological damage are important.

Floor coverings. Floor coverings are placed in a separate category, since today most enterprises are trying to ensure product quality in accordance with the requirements of European standards, which have a gradation not only in quality, but also in the appearance of the product (by the direction of the fibers - radial, tangential, mixed; by the presence of a nucleus, medullary rays, etc.). Floor coverings include: floor boards, parquet strips, parquet boards, deck boards.

This category also includes staircase elements (steps and railings). For this category of products, such properties of wood as hardness, density, shape stability, and abrasion are important.

Finishing materials, furniture and carpentry elements. Wood is widely used for interior and exterior decoration. For cladding a house, a board is used that imitates a rounded log or timber. For interior decoration, finishing boards (lining and eurolining), furniture panels for the manufacture of furniture, window sills, doors, wall panels and other elements of carpentry are widely used. For this category of products, stability of shape and wood density are important.

Other types of products. These are various board materials: chipboard, chipboard, fibreboard, MDF, OSB, etc.

In the next issue we will talk about the areas of application of coniferous wood.

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Wood protection

How and from what should wood be protected? What new technologies can specialists offer?

Assortment

Today you can find a large number of varieties of products on the lumber market.

Plates

These are parts of wood that are made by one longitudinal cut through the center into 2 equal halves. They have only 1 sawn side and are used for the production of the entire range of lumber.

Quarters

These are parts of wood that are obtained by two longitudinal cuts of logs through the center into 4 equal parts. They have 2 sawn, mutually perpendicular sides and are used for the production of some lumber, joinery and moldings.

timber

This is lumber whose cross-sectional dimensions along both axes are more than 100 mm. Made from hard wood. Used to create beams for supporting structures in the construction of bridges, ships, log houses and interfloor ceilings.

Bar

Lumber, the dimensions of which in cross-section along both axes are less than 100 mm, and the width is not more than twice the thickness. When creating them, the same standards are applied that are used in the production of boards.

Board

This is lumber, the width of which in cross-section exceeds the thickness by 2 times or more. It has wide sides called faces, and narrow sides called edges. Plates are always processed by planing and grinding. Edges can sometimes be tongue-and-groove by creating tongue-and-groove or tongue-and-groove joints.

There are several types of boards.

Lining is a long board of small width and thickness. Used for interior decorative finishing of a room, attached to the sheathing end-to-end or through grooves. Lining is the cheapest and most common type of decorative boards.

Peculiarities

The degree of hardness of timber implies their ability to provide effective resistance at the moments of penetration of bodies that are more rigid and strong.
Such bodies mean nails and other fasteners. The hardness of different species may vary depending on the directions of the wood cuts. Currently, it is customary to distinguish several types of hardness.

• Tortsovaya. This is a subtype of hardness that can be determined by embedding a metal rod into the material. The latter is characterized by a hemispherical end with a diameter of 11.28 mm. The rod is pressed to a depth that corresponds to a radius of 5.64 mm. This is done gradually, over 2 minutes. The print size is 1 square centimeter. That is why the hardness index itself is measured in kg/m³.
• Radial.
• Tangential.

The lateral hardness level of coniferous species is in most cases 40% less than the end hardness. As for deciduous varieties, this figure reaches only 30%. The tangential hardness parameter, for example, of oak, palm or elm materials turns out to be higher than the end hardness by almost 5-10%. The lion's share of existing wood species is characterized by approximately equal tangential and radial hardness.

In order to make a high-quality and attractive product from a natural and environmentally friendly material, it is very important to determine the degree of its hardness in advance. Basically, to find out such information about wood, they use a special Brinnell method

The meaning of this method is that you need to press a metal ball with a diameter of 10 mm into the base of a certain wooden workpiece with a force of at least 100 kg. Based on the identified type of deformation and the diameter of the remaining depression, it is possible to easily determine the hardness of the material.

Hardwoods are in great demand today. It's no secret that denser and stronger boards are most preferable in construction work or the manufacture of furniture structures. Such materials have their own strengths and weaknesses.

Let's look at the main advantages of hardwood:

• such natural materials do not require additional protective impregnations;
• They make more durable and wear-resistant boards of high quality;
• Hardwood furniture has a very beautiful, unique texture.

The disadvantages include the following characteristics.

• Hardwood boards are not easy to work with. They are quite capricious in processing, so it is not always possible to cope with them with a simple file.
• Such materials are much more expensive.
• Not suitable for all types of furniture or floors.

Properties

Wood has a number of characteristic properties. The structural features are the reason for this. The strength of wood is quite high, and among building materials in this indicator it occupies an intermediate position. And given its low specific gravity, it is comparable in this regard to metal. The weak point of wood is that it is an anisotropic material. The ability to resist fracture depends on the direction of the force relative to the arrangement of the fibers. The best strength indicators are visible when the material is exposed along the fibers.

The rigidity of wood is low, the reason for this is its specific structure. Wood is a porous, flexible material. Beams are able to restore their shape after a short-term load. But residual deformations, due to prolonged exposure, remain forever. A wooden beam will not be able to regain its shape after long use.

The hardness of building materials is determined by the load required to press a steel ball of a certain size. For the hardest types of wood, it is only 1000 N. Moreover, low hardness is one of the main advantages of the material. Wood is easy to process and holds nails, screws, and self-tapping screws firmly.

Wood moisture content is determined by the specific moisture content in the pores. In a freshly cut tree it reaches 100%. Depending on the purpose, freshly cut wood is dried to the required levels of 40 to 15%.

Wood materials

In addition to lumber, wood materials, chipboards and fiberboards, veneer, plywood, etc. are also produced and used.

Particle boards (chipboards) are made on special machines by pressing chips mixed with synthetic resin. Fiberboard (fibreboard) is pressed into sheets of shredded wood. Chipboard and fiberboard are used for making furniture and in construction.

Veneer is thin layers of wood. It is produced on special machines: a special knife cuts a thin layer of wood from the surface of a rotating log. This process is called peeling (Fig. 7). Plywood is a wood material obtained by gluing together three or more thin sheets of veneer. To produce plywood, birch, alder, beech, and pine wood is used.

Rice. 7. Obtaining peeled veneer: 1 – log; 2 — peeling knife; 3 — veneer strip; 4 - pressure ruler

To make plywood (Fig. prepared and cut veneer sheets are smeared with glue. Then they are laid on top of each other so that the direction of the fibers in the sheets is mutually perpendicular, and compressed under a press until they are completely glued. This cross arrangement of fibers makes plywood a durable material. Plywood is widely used in furniture production, as well as in shipbuilding and aircraft manufacturing.

Rice. 8. Production of plywood from veneer sheets: a - arrangement of fibers on veneer sheets joined with glue; b - plywood

Laboratory and practical work No. 1

Recognition of wood and wood materials

1. Get wood samples from your teacher, study their texture, determine their color, smell, and hardness. Write down the studied properties of wood in the table in your workbook.
2. Carry out the following experiment: by pressing an awl into the surface of each sample (without rotating it), determine which rocks are hard and which are soft.

1. Consider the lumber available in training workshops, determine their name, find their main elements.
2. Study samples of wood materials: chipboard, fiberboard, veneer, plywood. Determine the number of layers in the plywood sample and measure its thickness with a ruler.

New words and concepts

Wood, deciduous and coniferous species, wood structure (bark, pith, growth rings), texture; lumber (boards, beams, beams, slabs), elements of lumber (face, rib, end, edge); wood materials (veneer, plywood).

Testing your knowledge

1. What types of trees grow in the area where you live?
2. List the advantages and disadvantages of wood.
3. What is wood grain?
4. What types of wood materials do you know?
5. What is stronger: plywood or a board of the same thickness?

Hardness

Hardness is the ability of wood to resist the penetration of harder bodies into it. The degree of hardness is influenced by the moisture content of the wood.

According to this parameter, all tree species at 12% humidity can be divided into three groups:

• soft - end hardness 40 MPa or less (pine, spruce, cedar, fir, juniper, poplar, linden, aspen, alder, chestnut);
• hard - end hardness 40.1–80 MPa (larch, Siberian birch, beech, oak, elm, plane tree, rowan, maple, hazel, walnut, persimmon, apple tree, ash);
• very hard - end hardness more than 80 MPa (white acacia, iron birch, hornbeam, dogwood, boxwood, pistachio, yew).

Hardwoods are more wear-resistant than softwoods. This is essential when processing with cutting tools: milling, sawing. And also when constructing floors, stairs, railings, since wood is subject to abrasion during use. In the production of parquet and solid wood parquet boards, rocks with a hardness of at least average are used.

How everything works

The properties and characteristics of the material cannot be understood without first studying the structure and composition of wood. The concept itself depends on who uses it. For an ordinary person and a builder, this is exclusively the part of the tree under the bark that can be used in everyday life or production. For a botanist, the structure of a tree and wood is the whole complex, including all elements from the roots to the crown.

The crown is used insignificantly in industry, and the branches are used as raw material for fiberboards and cardboard. The trunk is of primary importance. A cross-section reveals the structure of a wood trunk. The topmost layer, the cortex, protects living cells from external influences. Between the bark and the body of the trunk there is a layer of living cells - cadmium. In the very center, the core runs through the entire trunk. The loose fabrics of which it is composed make it unsuitable for utilitarian needs.

The core of the tree consists mainly of dead cells, resin deposits, coloring and tannins. The core is surrounded by sapwood, the part of the tree that is responsible for conducting water to the leaves from the roots. Accordingly, it contains a lot of moisture, it allows more water to pass through and is more susceptible to rotting. Not all trees have a clearly defined core. In some of them there is no difference between the central and peripheral parts of the trunk. Such species are called sapwood.