Wood and its properties = Древесина и ее свойства. Учебное пособие

property. Mechanical properties are usually the most important characteristics of wood products for structural applications. A structural application is any use where strength is one of the primary criteria for selection of the material. Structural uses of wood products include, among other things, floor joists and rafters in homes; power transmission and distribution poles; structural panel roof and wall sheathing and subflooring; glue - laminated glulam beams and decking in commercial buildings; particleboard flooring in mobile homes; and steps and rails of wood ladders, piling, and sailboat masts. Structural applications of wood and wood - based products are omnipresent in today’s society. A strong argument can be made that basic human evolution and expansion hunting, shelter, transportation, protection, etc. has been largely shaped by humankind ability to exploit the strength of wood. The term strength is often used in a general sense to refer to all mechanical properties. However, because there are many different types of strength and elastic properties, it is important to be specific about the mechanical property being discussed. A wood that is relatively strong with respect to one strength property may rank lower in a different property. The type of mechanical property most critical to any application is determined by the nature and type of loading to which that product will be subjected. For example, in a floor joist, the modulus of elasticity (MOE) is very important because it determines the amount the joist will bend or deflect under load and thus how solid the floor will seem. In the case of wood flooring, the surface hardness determines the resistance to denting when under a concentrated load. In general, it is not difficult for an engineer to design a structure, be it a house, bridge, skyscraper, or other that is strong enough, but it requires honed art and skill to design one that is just strong enough. To appreciate the meaning of the various strength properties of wood, it is necessary to first understand some basic engineering mechanics.

[From: Shmulsky R., Jones P. D. Forest Products and wood science / R. Shmulsky, P. D. Jones. – US : Willey-Blackwell, 2011. – P. 197]

3.Write out new words and word combinations from the text and translate them into Russian.

4.Answer the following questions using the words you’ve learned in

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exercise 3.

1.What does strength as ability mean?

2.What does the term «elastic deformation mean?

3.What happens if deformation develops slowly?

4.What properties are usually the most important characteristics of wood products for structural applications?

5.Where can structural application be used in?

6.Why is it important to be specific about the mechanical property being discussed?

7.Is it difficult for an engineer to design a structure?

8.What is the most critical to any application is determined by the nature

and type of loading to which that product will be subjected?

WOOD AND WATER

1. Read, translate and state to what parts of speech the following words belong.

Natural, stem, tree, satisfactory, immediately, wood, properties, dimensional, mechanical, stability, deterioration, weight, substance, equal, moisture, raw, physical, atmosphere, cell, remain, manufacture, understand, resistance, chapter, associate, products, microstructure.

2. Read and translate the text into Russian.

Water is a natural and necessary constituent of all parts of a living tree. In the xylem portion of the stem, water commonly makes up over half the total wet or green weight. Stated another way, the weight of water in green wood is commonly equal to or greater than the weight of dry wood substance. When a tree dies or a log is processed into lumber, veneer, chips, and so on, the wood immediately begins to lose some of its moisture to the surrounding atmosphere. As drying continues, the dimensions and the physical properties of the wood begin to change. Some water, however, generally remains within the structure of the cell walls even after wood has been manufactured into lumber or other wood - based products. The physical and mechanical properties, resistance to biological deterioration, and dimensional stability of any wood - based product are all affected by the amount of water present. Because almost all properties of wood and wood

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products are affected by water, it is important to understand the nature of water in wood and how it is associated with wood microstructure and properties. This chapter is devoted to this subject and, in addition, covers some practical aspects of wood drying and dimensional change. For satisfactory use of wood as a raw material, these relationships must be clearly understood.

[From: Shmulsky R., Jones P. D. Forest Products and wood science / R. Shmulsky, P. D. Jones. – US : Willey-Blackwell, 2011. – P. 141]

3.Write out new words and word combinations from the text and translate them into Russian.

4.Answer the following questions using the words you’ve learned in exercise 3.

1.How can water be important for a living tree?

2.What happens when a tree dies?

3.When do physical properties of the wood begin to change?

4.Which products are affected by the amount of water present?

5.What is it important to understand about the nature of water in wood?

6.Where can water remain even after the wood has been manufactured into lumber?

7.Which relationships must be clearly understood according to the text?

8.Which aspects of wood drying are shown in the text?

MOISTURE MOVEMENT DURING DRYING

1. Read, translate and state to what parts of speech the following words belong.

Movement, liquid, water vapor, diffusion, moisture concentration gradient, vapor pressure gradient, steepness, rate, limiting factor, tylose, aspirated pit, pit membrane, impermeable heartwood, surface layer, presteaming.

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2. Read and translate the text into Russian.

The movement of water in wood during drying takes place as mass movement of liquid water, water vapor, or diffusion of individual water molecules. Diffusion is a phenomenon that occurs as water moves from areas of higher concentration to those of lower concentration. Thus, for diffusion to occur there must be a moisture concentration gradient or a vapor pressure gradient across the cell walls. The rate of diffusion is related to the temperature, the steepness of the moisture gradient across the cells, and the characteristics of species that determine the ease with which diffusion can occur. The rate of diffusion in a species can be expressed as the diffusion coefficient. Diffusion through individual cells is noticeable only below the FSP. Above the FSP, free water moves out of wood as a result of surface drying, vapor diffusion, and capillary forces. At that stage of drying, wood can be thought of as a series of partially filled tubes, with water evaporating from its ends. The rate at which lumber dries is determined by the rate at which water is removed from the surfaces and the rates of internal liquid and vapor diffusion. In the initial drying stages, the rate is often controlled by surface evaporation and, in later stages, by the diffusion characteristics. For highly permeable woods such as the southern pines, surface evaporation is the primary limiting factor that controls the drying rate. In some species, the structure of wood inhibits the mass movement of liquid water. Such woods are referred to as impermeable or refractory. Tyloses, aspirated pits, and deposition of extractives on pit membranes are examples of wood features that inhibit movement of water. In woods with these characteristics, the movement of water must be principally by diffusion; thus, drying is an extremely slow process. Redwood, white oak, and walnut are a few of the species having relatively

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improve permeability by eliminating pit aspiration during drying. Incising or predrilling improves moisture transport by exposing additional end - grain area. Presteaming of wood has been found to be beneficial in some cases. Unfortunately, a universally effective means of improving liquid water movement and/or diffusion has not yet been found.

[From: Shmulsky R., Jones P. D. Forest Products and wood science / R. Shmulsky, P. D. Jones. – US : Willey-Blackwell, 2011. – P. 164]

3.Write out new words and word combinations from the text and translate them into Russian.

4.Answer the following questions using the words you’ve learned in exercise 3.

1.How can you explain the term diffusion?

2.What conditions are necessary for diffusion to occur?

3.What is the rate of diffusion related to?

4.How can the rate of diffusion be expressed?

5.What are the examples of species, which contain localized impermeable zones?

6.What can insufficient drying of such zones lead to?

7.What means are justified while drying these species?

8.Are there any universally effective means of improving liquid water movement

MOISTURE CONTENT OF GREEN WOOD

1. Read, translate and state to what parts of speech the following words belong.

Moisture, content, green wood, sapwood, heartwood, extractive, cell, lumber, pole, tie, log, pulpwood, increment core, a weight-to-volume ratio, conversion factor.

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2. Read and translate the text into Russian.

The moisture content of green wood is important because of its direct relation to the weight of logs and green lumber. Therefore, moisture content is of concern to those who design harvesting and transport equipment, purchase wood on a weight basis, or ship or transport green wood. The moisture content of green wood varies considerably among species. Note that among species shown in Table 1 the moisture content of heartwood ranges from 33 to 98 percent and that of sapwood from 44 to 249 percent. The values in Table 1 should be considered as only general indications. Within any species, there is considerable variation depending on the location, age, season of harvest, and tree size. In softwoods, the average green moisture content tends to decrease as a tree grows older. It is reported a 30 percent difference in the moisture content of southern pines over 45 years of age compared with trees of 25 years of age. Within a single tree, there is typically considerable variation in moisture content. The differences between sapwood and heartwood (Table 1) are one source of such variation. When wood in a tree changes from sapwood to heartwood, the amount of moisture in the cell wall often decreases, with the drop in moisture content preceded or accompanied by the deposition of extractives. These extractives tend to take the place of water molecules associated with cellulose and hemicellulose. Some extractives may also be left in solution or suspended in the free water in the lumina of the heartwood cells. Hardwoods generally have smaller differences in moisture content between sapwood and heartwood. This contrasts markedly to softwoods, where the moisture content of sapwood is usually much higher than heartwood, often by a factor of three to four. If you are lost in the woods and forced to build a fire from green wood, try burning the heartwood of softwood, not a hardwood! For most design or total weight estimates, the moisture contents of green wood given in Table 1 should be adequate. Many trade associations and railroads publish lists of weights, by species, for green and dried products such as lumber, poles, and ties. Such information is useful. If green weight is to be used as the basis for purchasing logs or pulpwood, it is advisable to conduct an on the site study of the green moisture content. The effect of log or bolt size and the season of the year should be determined. The goal of most weight scaling procedures is to pay a fixed amount per unit of dry wood. Occasionally, adjustments to the price per unit weight may be made for defects, for small diameters, or for the degree of seasoning of the wood. Little information has been published regarding seasonal variation of moisture content, although many firms and associations have compiled such data. In southern pine, the moisture content of increment cores is higher

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during midwinter than in summer. In cities, where aspen pulpwood is often purchased by weight, some mills use a weight - to - volume ratio (density) of approximately 905 kg m - 3 (4800 lb cord - 1) during the winter and 870 kg m - 3 (4600 lb cord - 1 ) as the conversion factor during the summer, which reflects a higher moisture content in the winter. For many species, higher moisture content is reported in the summer months.

[From: Shmulsky R., Jones P. D. Forest Products and wood science / R. Shmulsky, P. D. Jones. – US : Willey-Blackwell, 2011. – P. 152]

3.Write out new words and word combinations from the text and translate them into Russian.

4.Answer the following questions using the words you’ve learned in exercise 3.

1.Why is the determination of the green wood moisture content important?

2.Which spheres is the determination of the moisture content especially important for?

3.What does the variation of the moisture content within species depend on?

4.What does the tendency of moisture content decrease?

5.What sources of the moisture content variation can you name?

6.What is the drop in moisture content preceded or accompanied by?

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7.Where is the difference in moisture content between sapwood and heartwood smaller?

8.Why can the information of weight be useful?

BACTERIA

1. Read, translate and state to what parts of speech the following words belong.

Scope, fungi, thrive, anaerobic, saturation, log, utility, lumber, storage, sprinkler, damage, preservative, permeability, metabolic, deterioration, marine, anoxic, defense, splitting.

2. Read and translate the text into Russian.

Bacterial degradation of wood products is generally limited in scope due to the time required for bacterial degradation to occur. Unlike fungi, bacteria can survive and thrive in relatively anoxic or completely anaerobic environments; thus, high moisture levels even up to saturation are not an adequate defense. Bacterially infected wood can occur in the forest, commonly in bottomland trees; in the log yard, among water - stored logs; or in products such as utility poles and foundation, freshwater, and marine piling. Fortunately, bacterial degradation is generally slow.

Logs that are bacterially infected from the forest can usually be carefully processed into lumber. Special care should be taken during drying as excessive can occur. Logs that are ponded or stored under water sprinklers can become bacterially infected if storage time is prolonged beyond about 6 months. After that time, the bacteria begin to significantly degrade the pit and parenchyma structures, thereby causing increased permeability, creating a problem for treating, gluing, and finishing. Bacterial infection and damage in service is usually very slow, requiring decades. Bacteria can, however, cause costly damage to buried piles, poles, and other structural members. Wood preservatives slow down the deterioration but are not completely effective because strains of bacteria often adapt to the point at which they utilize the wood preservative as part of their metabolic processes.

[From: Shmulsky R., Jones P. D. Forest Products and wood science / R. Shmulsky, P. D. Jones. – US : Willey-Blackwell, 2011. – P. 238]

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3.Write out new words and word combinations from the text and translate them into Russian.

4.Answer the following questions using the words you’ve learned in exercise 3.

1.Where can bacteria survive and thrive?

2.Where can bacterially infected wood occur?

3. Is bacterial degradation generally slow?

4.How can bacterially infected logs be treated?

5.What can occur if storage time is prolonged beyond about 6 months?

6.What can bacteria damage?

7.What can slow down the deterioration?

CLASSIFICATION OF WOODY PLANTS

1. Read, translate, and state to what parts of speech the following words belong.

Wood, tree, seeds, cell, condition, vessels, stems, produce, lumber, fiber, occur, appearance, include, differ, ovaries, division, cover, conifers, species, needles.

2. Read and translate the text into Russian.

Woods, and the trees, are divided into two categories: hardwoods and softwoods. Hardwood and softwood trees are botanically quite different. Both are included in the botanical division spermatophytes (Picture 1), meaning they produce seeds. They are, however, in different botanical subdivisions. Hardwoods are in the subdivision Angiospermas, and softwoods are in the Gymnospermas subdivision. Angiosperms are characterized by production of seeds within ovaries, whereas gymnosperms produce seeds that lack a covering layer. Needlelike leaves characterize softwood trees. Such trees are commonly known as evergreens because most remain green the year around, annually losing only a portion of their

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