General information
Wood is a natural material that consists of tightly packed plant cells. Due to its physical characteristics, the substance has an elastic, dense structure. Thanks to the cellulose it contains, the wood is very strong and durable.
Wood retains heat well. There are voids between the cells that accumulate energy. The resource resists dynamic and static loads, while remaining lightweight. When compressed along the fibers, the strength of the substance is not inferior to cement.
Substance in boards Source stroy-podskazka.ru
Wood is easy to process. Raw materials can be cut, sawn, glued and pressed under pressure. The material holds solid fasteners (staples, nails). Wood is used to create both durable structures and decorative items.
Due to its cellular structure, wood can “breathe”. Constant air exchange allows you to maintain a favorable indoor microclimate. Due to the presence of oils and resins in the composition, the raw materials have a specific aroma. The substances help the tree fight tissue destruction.
The decorative value and value of wood often depend on the plant species. All varieties are divided into two large groups - coniferous and deciduous. Additionally, monocotyledonous crops are distinguished, which include bamboo and palm trees.
Sawn wood Source zen.yandex.ru
Due to its structural features, wood burns easily and rots in humid conditions. The material has many defects that reduce its grade and limit its use. Due to its high hygroscopicity, the substance swells, warps or cracks. The consequences of deficiencies are minimized by changing operating conditions and special protection during processing.
Lesson: “Wood is a natural structural material.”
Lesson No. _____ Date:_________________
Topic:
Wood is a natural structural material.
Goals
: create conditions for students to develop: the concepts of “wood”, “wood structure” to develop the ability to distinguish tree species by their characteristics; create conditions for the development of memory, logical thinking, and imagination in students; create conditions for the formation of self- and mutual control.
Lesson type:
combined
.
Forms of work:
independent, individual, group
.
DURING THE CLASSES.
I. Organizational moment.
II. Updating basic knowledge.
Conversation
- Remember what material is called structural.
- What raw materials are paper and cardboard made from?
- Name the construction materials that are used to make cars, airplanes, build houses, and make home furniture. Where are these materials made and what raw materials are used for this?
III. Learning new material.
The development of modern equipment and technologies depends on the production and use of various structural materials: wood, metal, plastics, glass, etc.
The use of wood has become widespread. Products made from it are used in almost all areas of our lives. Paper, cardboard, artificial silk, plastic, furniture, building elements, musical instruments and souvenirs and many other necessary things are made from this material.
All tree species are divided into two groups: coniferous and deciduous (Fig. 13).
Conifers have needle-shaped leaves. These include: spruce, pine, cedar, larch, fir, etc. Deciduous species are alder, linden, oak, beech, hornbeam and others (Fig. 14). Trees are used to make structural wood products.
Wood materials can easily be processed with various cutting tools: saws, knives, chisels, drills, files and others. Structural elements made of wood materials are reliably and firmly connected with nails, screws, and gluing.
Trees are the tallest of all plants, although there are also dwarfs among them, up to several centimeters in height.
Wood as a natural structural material is obtained from tree trunks by sawing them into pieces.
The tree trunk has a thicker (butt) part at the base and a thinner part at the top. The surface of the trunk is covered with bark. The bark is like clothing for a tree and consists of an outer cork layer and an inner bast layer. The cork layer of bark is dead. The bast layer serves as a conductor for the juices that nourish the tree. The main interior of a tree trunk is made up of wood. In turn, the wood of the trunk consists of many layers, which are visible in the section as growth rings. The age of the tree is determined by the number of growth rings.
The loose and soft center of the tree is called the heartwood. The medullary rays extend from the core to the bark in the form of light shiny lines. They have different colors and serve to conduct water, air and nutrients inside the tree. The core rays create the pattern (texture) of wood.
Cambium is a thin layer of living cells located between the bark and wood. Only from the cambium does the formation of new cells and the annual increase in thickness of the tree occur. “Cambium” comes from the Latin “exchange” (nutrients).
Main sections of the trunk.
1 - end section;
2 - radial section;
3 - tangential section
To study the structure of wood, three main sections of the trunk are distinguished. A cut that runs perpendicular to the core of the trunk is called an end cut. It is perpendicular to the grain. Cut 2 passing through the core of the trunk is called radial. It is parallel to the fibers. Tangential cut 3 runs parallel to the core of the trunk and is some distance away from it. These cuts reveal various properties and patterns of wood.
Wood species are determined by their characteristic features: texture, smell, hardness, color.
Wood/species/
Hardness
Color
Application
Pine
/coniferous/
Soft. Impregnated with resinous substances
Wood of light red color with a pronounced texture
Used for the manufacture of windows and doors, floors and ceilings, furniture, in the construction of ships, carriages, bridges
Spruce
/coniferous/
Soft. Impregnated with resinous substances
Color white with a yellowish tint
Used for making musical instruments, furniture, windows and doors
Birch
/hardwood/
Solid
Color white with a brownish tint
Used for making plywood, furniture, dishes, gun stocks, tool handles, skis
Aspen
/hardwood/
Soft. Prone to rotting.
The color is white with a greenish tint.
Used to make matches, dishes, toys, paper.
Linden
/hardwood/
Soft.
The color is white with a soft pink tint.
It is used for making dishes, drawing boards, pencils, and products with artistic carvings.
Alder
/hardwood/
Soft.
The color is white, turns red in the air.
Serves as a raw material for the manufacture of plywood, hollowed-out dishes, and packing boxes.
Oak
/hardwood/
Solid. On a radial section, the medullary rays in the form of shiny stripes are clearly visible.
The color is light yellow with a brown-gray tint and a pronounced texture
It is used for the manufacture of furniture, parquet, cladding of valuable products, as well as in the construction of bridges and carriages.
The pattern on the surface of wood formed as a result of cutting growth rings and fibers is called wood texture. The beautiful surface of wood is said to have a rich texture. For example, walnut wood has brown and gray colors in a wide variety of shades; it is highly valued for making furniture and hunting rifle stocks. Oak, ash, as well as mahogany species growing in Africa, America and Australia, which produce red wood of various shades, have beautiful textures. Such valuable types of wood are planed into thin sheets (veneer), which are glued onto valuable products.
To make useful things, a variety of construction materials are used: metal, plastic, plexiglass, silk, textile and other materials. The use of wood and materials made from it has become widespread. All structural materials have certain properties that must be taken into account when manufacturing products. These include the color and texture of wood that you already know. In addition, you also need to know how easily a certain type of wood and materials made from it can be processed, what tool needs to be used for this, whether nails, screws and other fastening parts will be held in it, how humidity and changes in ambient temperature will affect wood materials and so on. It is also necessary to consider what type of wood or materials made from it must be used so that the structure, for example a bridge or multi-story building, does not collapse if it is used under heavy loads, etc.
of mechanical properties will help answer these questions.
construction materials.
The main ones primarily include: strength, hardness, elasticity .
Strength
is
a property that characterizes the resistance of wood against external mechanical forces, that is, its ability to withstand heavy loads and not collapse. It is worth making structural elements from high-strength wood, that is, those that are subject to significant loads. The most durable wood is oak, followed by ash, hornbeam, maple, birch, pine, spruce, alder, aspen, and linden.
Hardness
- the ability of a material to resist the penetration of another solid body into it, for example, processing tools (knives, files, chisels, drills and other cutting tools). Knowing the hardness of wood is important. Cutting tools for wood processing are used taking this property into account. The harder the wood, the harder it is to process and the greater the sharpening angle the tool must have.
Based on hardness, wood can be arranged in the following order: hornbeam, oak, ash, maple, birch, pine, alder, spruce, linden. That is, hornbeam has the greatest hardness. Therefore, it is difficult to process with cutting tools. Linden is the easiest to process of all wood materials. Therefore, it is used mainly for making souvenirs, household items, etc.
Elasticity
is
the property of a material to change its shape (and not collapse) under the influence of a load and resume it after the cessation of this action. Wood bends (deforms) under the influence of force and straightens out again, or springs back, after the load is removed. The wood of ash, oak, larch, pine and other species has high elasticity.
IV. Consolidation of the studied material.
PRACTICAL WORK
Determination of wood species from samples.
- Study the table of tree species.
- Write down in your notebook the main characteristics by which wood species are determined.
- Identify the types of wood using the samples given by the teacher.
V. Summing up.
Conversation:
1. What types of wood are classified as coniferous? To deciduous?
2. What wood materials are produced at woodworking enterprises?
3. What is wood texture called?
4. What is the structure of a tree?
5. What types of lumber do you know?
6. Describe the role of forests in human life.
7. How do green spaces influence the improvement of the natural environment?
VI. Homework.
____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
Learn the notes.
Material structure
Wood is the inner part of the trunk, located under the bark and freed from roots and branches. The porous fibrous layered resource consists of numerous cells. Due to their tight fit, small particles create a continuous layer of fabric.
The inner walls of cells contain organic compounds. Among the main components are:
- Pulp. The building element of the fibrous structure gives the material elasticity and strength.
- Lignin. The carbohydrate-free substance is located in the intercellular space, connects fibers to each other, causing rigidity and woodiness.
- Bast, cambium. Thin mucous layers are composed of cells. Division actively occurs from spring to autumn, but stops in winter. The elements can be seen on the cut tree in the form of annual rings.
- The core. Located inside the first cyclic circle, slightly offset from the center. It participates in the life of trees only in the early years, then it turns into a soft, fragile material.
During the growth process, minerals enter the wood through the roots, which after combustion become ash. Extractive elements are part of essential oils, give the material aroma, taste and color, and prevent rotting. Wood cells are assembled into pieces, creating the structure of the raw material.
In the transverse elements of deciduous species there are medullary rays, along which nutrients move and accumulate. Substances are colored lighter or darker than the base material. Different tree species have their own unique texture patterns.
Structure of the material on the saw cut Source twimg.com
Coniferous species have horizontal and vertical passages in their raw material through which the resin moves. Resin components protect wood from rotting and reduce moisture absorption. The parameters protect the wood from destruction without affecting the appearance and strength.
Properties of wood
To correctly determine the scope of use of wood in construction, you need to know its properties.
Physical properties:
- Density - 400-700 kg/m3 (fir 390, pine 540, larch 790). True density 1.54 g/cm3. Density determined at a certain wood moisture content is usually recalculated at a standard wood moisture content of 12%.
- Hygroscopicity. The hygroscopic limit (fiber saturation limit) is 30%.
- Humidity W. Freshly cut has a moisture content of 50-100%; in air - 15-20%; room drying - 8-12%. Water in wood causes a decrease in its strength and swelling deformation, reaching 6% (in the radial plane) and 12% (in the tangential plane). When drying wood, shrinkage and warping are observed: along the fibers - 0.1%; in the radial plane - 3-6%; tangential - 6-12%.
- Thermal conductivity of dry wood along the fibers is 0.17 W/m 0K. Humidity is determined by drying or electric moisture meters. The range of wood moisture measurement with an electric moisture meter (EV-2M) is from 7 to 30%.
- The strength of wood, especially coniferous species, is very high (solutions of salts, alkalis, organic and mineral weak acids). Wood is poorly preserved in seawater. It is also destroyed by concentrated solutions of mineral acids (nitric acid of any concentration). Resinous larch wood is most resistant to aggressive environments.
- The color of wood is determined by tannins. The texture (pattern on the surface) is very diverse. Wood, unlike other building materials, has a pleasant aromatic smell (resins, essential oils).
Mechanical properties. The strength of wood is maximum at zero moisture content and quickly decreases with increasing humidity to the limit of hygroscopicity.
Compressive strength of wood: along the fibers - 50 MPa; horizontally - 20 MPa. Bending strength - 100 MPa. In a stretched state - 130 MPa (steel - 15 times, fiberglass - 3 times). Modulus of elasticity in bending - 104 MPa.
Defects in wood greatly reduce strength. Defects: knots (healthy, rotten, fused, unfused); cracks (metal, frost); defects in the shape of the body (curvature, consistency); defects in wood structure (inclination of fibers, double core); chemical dyes; fungal infections; wormholes; mechanical damage.
Properties of matter
The characteristics of the material depend on the physical and chemical characteristics. Within a single breed, parameters vary due to growing conditions, moisture levels and processing methods. Among the main indicators, there are 6 properties of wood.
Texture
The natural pattern consists of the anatomical components of the tree, which can be seen when sawing. The more complex the structure of the plant, the more luxurious the texture. Patterns determine the decorative properties of the material, which is important when creating building finishes and furniture facades.
Different texture of the material Source rubankom.com
Color
The parameter helps determine the type of wood and varies from almost white to black. Tanning and coloring components located in the intercellular space of the raw material are responsible for the variety of shades. The intensity of the color depends on the age and period of felling. Changes in tone may be associated with disease and developmental conditions.
Color scale of the substance Source guns.allzip.org
Changed wood properties
Physical and mechanical properties
Modified wood is a material with predetermined properties. Thus, at the customer’s request, it is possible to manufacture modified products with the required set of properties.
When acetylation and treatment with ammonia, the properties of wood change the least: density increases to 800 kg/m3; compressive strength increases by 1.5 times; the wood darkens.
Ammonia-treated and pressed wood (lignamon) compacts to a greater extent: = 1400 kg/m3; the compressive strength along the fibers increases by almost an order of magnitude; compressive strength along the fibers Rс = 180 MPa (for natural wood 70 MPa).
When modified, the wear resistance of wood across the grain increases: birch (lignamon) - 12% weight loss; natural oak 22%, beech 21%.
Resistance to rotting increases 2-3 times. Lignamon is non-toxic and flammable.
WPC wood modified with polymers is characterized by the following properties: color - brown - phenolic alcohols; volumetric swelling is reduced from 15% to 3-4%; strength increases by 4-6 times (except for strength to static bending and shear along fibers - 1.5 times); impact strength may even decrease from 70 to 40 kJ/m2 because most modifiers have reduced impact strength; to increase deformability, plasticizers are introduced into them; The wear resistance of WPC is 2-3 times higher than that of natural oak; increased resistance and durability in aggressive environments (sea water, solutions of mineral fertilizers, solutions of acids, alkalis).
Fire resistance. Modified wood also becomes fire resistant. Natural wood in a muffle furnace at 500 o lights up in 1-1.5 minutes, and burns out in 5 minutes. Modified wood (FS) ignites in 9 minutes and burns for 8 minutes.
The fire retardant properties of modified wood depend on the impregnating polymers. For example, for wood modified with MMA, the fire retardants are acrylic acid (inhibits oxidative reactions) and urea (promotes self-extinguishing). During fire tests there was no spontaneous combustion, weight loss was 4.4%.
Biostability. A polymer that fills wood cavities increases its biostability. Already at 5% content of phenolic alcohols, modified wood acquires almost absolute resistance to wood-destroying fungi. This is due to the effect of phenol and the difficulty of the mycelium penetrating deep into the wood. After aging the samples for two months on a film-like house fungus culture, the wood modified with phenolic alcohols and urea resin lost 2-3% in weight, while the untreated wood lost 16-23%.
Chemical resistance. The increased chemical resistance of modified wood is due to the slow diffusion of the aggressive agent into the wood (clogging - the clogging effect of the polymer); increased polymer strength.
The compressive strength of modified wood (FC) in 10% sulfuric acid is reduced by 1.3 times, and of natural wood by 4 times. Modified wood is also characterized by increased resistance to solutions of salts and alkalis. Modified wood is preferably used in particularly difficult operating conditions.
Weather resistant. The slow diffusion of moisture in modified wood and the reduced level of humidity lead to its increased weather resistance and dimensional stability.
Outdoors, the humidity of natural wood throughout the year varied from 23%, modified - 5-8%. Thus, wet deformations and stresses in modified wood were 4-5 times less than in natural wood.
Defects of raw materials
Physical characteristics or deficiencies of raw materials impair performance characteristics. Specific natural defects arise at the stage of plant development or due to the actions of pests and diseases. Defects reduce the quality of wood and limit the possibility of exploitation.
Knots are the remains of branches enclosed in fabric. The defects have their own system of cyclic circles that are harder and darker than the rest of the material. Flaws disrupt the natural integrity and uniformity of the wood, distorting the grain and degrading the appearance of the grain. Due to the destruction of the structure, strength decreases and processing becomes more complicated.
Remains of knots in raw materials Source fonstola.ru
