Physical and mechanical properties of wood

WOODEN MATERIALS

Free of a bark fiber fabric of tree trunk is meant by wood. The wood is widely applied in construction as saw-timber, plywood, joinery, glued structures, prefabricated wooden houses, elements from the wastes of woodworking and sawmilling. Wood is applied mainly as a roundwood and boards.

Wooden materials are distinguished from others by the row of positive features: comparatively high mechanical strength at a small average density, high processability, elasticity, low thermal-conductivity, considerable resistance to the alternate freezing and thawing and some to other corrosive actions.

The hygroscopic property, ability to decay, casting, swelling and cracking under cyclic moisture conditions, heterogeneity of physical-mechanical properties in various directions (anisotropy), flammability belong to the factors limiting application of materials from wood in construction. Inherent failings of wood are largely removed by its modification with polymeric substances, antiseptics, and fire-retarding additives.

The wood of coniferous trees is most widely used in construction. According to the scales of application in construction the softwood (coniferous wood) takes a place in the following row (after diminishing): pine-tree, fir-tree, larch, silver fir, cedar. Larch, which wood is valued due to the high density, strength and resistance against decay has the best physical-mechanical properties.

The wood of oak among the hard wood for responsible constructions on air and under water, for parquet, millwork is widely used.

The problem of the thrifty use of the wood resources become more important with every year, they are the sources of not only a construction wood but also of many various chemical and other products, and also as one of main natural factors.

With development of production of the precast concrete and other modern construction materials, the application of wood is limited by the only indeed the rational area, where its technical and economic advantages become sensible.

The problem of utilization of wastes of wood procurement and processing, production of various materials on their basis acquires all of greater value.

Structure of wood

Composition and structure. Wood is characterized by the stratified fibrous structure and consists of cells, which have a different form, size and assignment. Thus, 90-95% of wood of coniferous trees consists of tracheides - the stretched cells of wood in the line of a tree trunk with the length 2-5 mm and width 30-70 mm which conduct water and solutions from roots to the head in the time of tree life. The cells shell is formed mainly by the cellulose (С₆H₁₀O₅), which is a main component of bearing frame of tree. The polysaccharides- lignin and hemicelluloses are complex organic compounds. According to the composition they are close to the cellulose and also belong to the components of cell walls and intercellular substance

Usually wood includes 40-50% of celluloses, 20-30% of lignin and 15-30% of hemicelluloses, 1-3% are the concomitant components (resins, oils, tannin and other).

The atomic average chemistry of wood is practically identical for all of woods: 49.5% of carbon, 44.08% of oxygen, 0.12% of nitrogen and 6.3% of hydrogen. Mineral substances which give ash at combustion of wood are 0.2-1.7%. The salts of alkali-earth metals are mainly included into composition of the ash.

Wood is the main and the most capacious of mass part of trunk. Except of it, there is a core tube which has usually a diameter 2-5 mm approximately in a trunk center (Fig. 19.1). It is the weakest part of trunk that is easily subjected to the decay.

Wood is outwardly crusty, which protects a tree from atmospheric and external mechanical actions. Bark includes two zones: external - the cork which carries out the protective function and internal - bast that actively takes part in the nutrients moving in a tree.

On the border between the bast and wood there is a skim of cells, fissionable and capable to grow which is called the cambium. The cambium predetermines the increase of wood and bark.

Wood depending on the features of macrostructure is divided into three groups - heartwood, ripewood and sapwood. Wood of heartwood wood species (pine-tree, cedar, larch, oak, ashwood, poplar and other) has darker coloring of central part of core and lighter peripheral part - sapwood. The wood of all of trees consists only of sap-wood in the early age. A core appears, for example, in pine-trees in age of 30-35 years, in oak 8-12 years. It consists of dying off cells, impregnated with and stopped up by the stratums of resin, calcite, tannic and other substances. A core has an enhanced density and resistance against decay.

If the central part of wood has an identical color with peripheral and differs only with less humidity, it is called not a core, but ripewood. The ripewood as a core is denser part of the trunk.

Sapwood consists of younger cells and is intended for movement of moisture mineral permeates in it. With the age sapwood gradually passes to the heart or ripewood. At identical humidity many mechanical properties of the sap-wood correspond to the heartwood. It’s resistance to decay is less, but it is easier saturated by the antirots. To the sapwood forest trees, which practically have identical wood according to the coloring and humidity both in a center and on periphery, a lot of wood species belong (cedar, alder, hornbeam and other).

Wood consists of separate annual layers which differ with a naked eye especially in coniferous trees. On the transversal cut of the trunk these layers have the appearance of concentric rings surrounding a core. Annual layers include two parts - summerwood and latewood. The summerwood appears in spring, it is lighter and softer than late, that appears only at the end of summer. This difference is especially strongly imaged in coniferous trees.

Composition of latewood largely determines the physical-mechanical properties of wood integrally. The resin ducts are concentrated in late softwood. Resin which fills them diminishes the water absorption of wood, increases resistance to decay. The presence of rays is characteristic for all woods-lines which radially diverge to the bark directly from a core or at some distance from it. They serve for conducting of water solutions of nutrients in horizontal direction in wood. Wood splits on primary rays easily and gives cracks at the shrinkage, because cells, which are included in these areas, are bonds between them are comparatively weak.

In the hardwood, vessels- tubular formations of cells with a diameter 0.1-0.4 mm and to 10 cm of length, directed in the line of a trunkare - are the weakest elements of structure, except of primary rays.

Defects of wood. The defects of structure, violation of wholeness, damages and illness that means deficiencies which reduce quality of commercial timbers are taken to the defects of woods (Fig. 19.2). In obedience to operating classification all the defects are divided in ten groups: knottiness, fungi paints and rots, chemical paints, insect damage, deformations, checking, defects of trunk form, defect of wood structure, wounds, undue laying in wood, mechanical damages and defects of treatment.

The basic defect that determines the type of wood is knottiness; knots are the basis of branches, located in wood of the trunk. Negative influence of knots consists in worsening of mechanical properties of wood as a result of violation of homogeneity and curving of fibers.

The knots hamper also the woodworking and in some cases are accompanied by internal rottenness. The type of knots (form, degree of growth, state of wood), their sizes and number, is specified in description of the knottiness.

Decay of wood appears in the fading of its color, diminishing of average density and strength.

Rots are caused by the development of the simplest vegetable organisms in the wood –fungi. The fungi which sit around the wood do not contain chlorophyll and can not synthesize the organic substances. Hereupon they are forced to feed the ready organic compounds and that is why they are settled on a tree.

Development of fungi in the wood takes place only at certain humidity (usually 25-70%) and temperature of air upon the average from 5 to 25°C. The decay does not take place in water, because access of oxygen, required for the vital functions of fungi is halted. Development of fungi is halted also at a temperature below 0°C and higher 40-45°C. The chemism of wood decay consists in its decomposition with the outburst of free carbon dioxide and water.

There are distinguished destructive and corrosive rottenness. The first one is generated by the fungi which destroy the cellulose of dead wood; the forest fungi, which parasitize on living wood and which use, mainly, a lignin, are the reasons of the second one.

A destructive rottenness is characterized by the prismatic splitting and darkening of wood, and the corrosive is accompanied with the stratification of wood by annual rings with its coloring in brown colors.

Along with the wood-destroying there is a group of wood fungi which give the various paints to wood and does not worsen almost its physical-mechanical properties. The variety of wood paints is mildew, which meets on the raw sap-wood of all the trees and caused by the carpophores of the mold fungi.

The insect damage of wood is called wormholes. The main masses of insect- vermin of wood are the different types of beetles.

There is a group of strong destroyers of wood from the class of shellfishes or crustaceous in the sea, they do not inhabite in the rivers and lakes.

The variety of worm-holes according to the degree of damage of wood is set (superficial to 3 mm, shallow to 5 mm, profound (rotten) is more than 5 mm) - and count up the number of openings.

Deformations and crackings - group of defects which are the consequence of change of form or violation of wood density. They arise up under the action of considerable internal tensions which appear in the process of tree growing, at the sharp change of temperatures, uneven deleting of moisture etc.

The number, character and sizes of cracks, and also their direction in relation to operating forces influence on the mechanical properties of wood. So, the most negative influence at a bend is shown by the crack of neutral area, which is in the plane, perpendicular to the force. Crack area of which coincides with directing effort has the least influence.

Physical and mechanical properties of wood

Physical-mechanical properties. The wood humidity influences on the physical and mechanical properties, and also its availability. The humidity hesitates from 30 (oak) to 45% (fir-tree) for greenwood. The air-dry wood which for a long time has been laid on the air has the humidity 15-20%.

There are distinguished the hygroscopic (inherent) and free moisture in the wood. The hygroscopic moisture impregnates the cells and is retained by the physical-chemical bounds. The maximal amount of hygroscopic moisture, which can be imbibed by wood after its conditioning on the air, saturated with the aqueous vapor, is called the saturation point of cell walls or limit of hygroscopic property. Maximal humidity of cell walls of the greenwood or wetted by conditioning in water is called the limit of saturation. The humidity of wood which equals to the limits saturation and hygroscopic property at a temperature 15-20° C is practically identical and upon the average for all types of wood equals to 30%.

Unlike bound water the free one fills canals of vessels and intercellular space and is retained by the physical-mechanical links with the wood. The removal of free water requires less power consumption that is why its influence on wood properties is considerably less substantial. At first the free water mainly is removed, and then bound at the wood drying. The process of wood drying is halted for achievement of equilibrium humidity that means the humidity of ambient air. It is possible to find the magnitude of equilibrium humidity with the help of the special diagrams.

The shrinkage takes place at removal bound moisture from wood that means reduction of timber sizes. Opposite, at the wood humidification the cell walls are thickened, that causes swelling. The cross-grained moisture deformations are the most substantial. So, a complete linear shrinkage of wood in tangential direction is 6-10%, and longitudinally to fibers - only 0.1-0.63%. The value of shrinkage and swelling grows also with the increase of average density of wood.

The moisture deformations can be calculated by coefficients of shrinkage (K_shr) and swelling (K_sw) that characterize the proper deformations according to a decline or increase of content of bound moisture in wood at 1%.

Ratio between K_shr and K_sw is determined by the formula:

. (19.1)

The coefficients of volumetric shrinkage of some widespread woods and other physical-mechanical properties are resulted in Table19.1.

At drying, as a result of nonuniformity of distribution of humidity in cross-section of wood and anisotropy, the internal tensions appear in it. The development of these tensions can cause cracking and casting of timber.

Таble19.1

Physical-mechanical properties of wood

For prevention of these defects the special value has the mode of wood drying. Drying is one of the most responsible and labour- intensive operations in technology of woodworking. The humidity should not exceed 8-10% for millwork, and for external structures – 15-18%.

At the calculation of drying processes, impregnation and other, the thermal properties of wood should be known. As a result of porous structure, wood conducts the warmth badly. Low thermal conductivity of wood, especially cross-grained, predetermines its wide application in the non-load-bearing structures of buildings that are heated. The timber, with thickness up to 15 cm, is equivalent by a thermal conductivity to the wall of brick with the thickness in 2.5 bricks.

Coefficient of linear expansion of wood along fibers is only (3-5) 10^{-6 0}С^-1, that means in 3-10 times less than for metal, concrete and glass, due to what it is possible not to arrange expansion joints in wooden buildings. In transversal direction of fibers a change of linear sizes is in 7-10 times are more than in longitudinal.

Dry wood has very small conductivity, approximately the same, as well as good electric insulating materials. However with the increase of humidity, conductivity grows. At humidity which equals to the satiation limit, it is in ten of millions times larger than the conductivity of seasoned wood.

The density of wood is determined by the complex of substances which are the component parts of cell walls. As these substances have practically identical composition for all of wood, the real density of wood (density of wood substance) hesitates in narrow limits - from 1.49 to 1.56 g/cm³ and equals to the average 1.53 g/cm³.

The average density of wood depends on its humidity and porosity. The value of average density is specified for the standard 12%-th humidity ( ). In a range from zero to 30%-th humidity the next formula can be applied:

, (19.2)

where K_shr - coefficient of volumetric shrinkage; - humidity.

At humidity of wood more than 30% it is possible to use a formula for the calculation of average density :

, (19.3)

where А - coefficient equals 1.222 for a birch, beech, larch, robinia and 1.203 for other wood.

According to the average density all types of wood are divided into three groups: light-weight ( <550 kg/m³), middle-weight ( =550-750 kg/m³) and heavy-weight ( > 750 kg/m³).

Mechanical properties. The indexes of mechanical properties of wood species, as its physical properties, depend on humidity; moreover only the bound water, which is in cell walls, influences. The increase of bound moisture content diminishes the indexes of all of mechanical properties sharply. The strength of wood can be calculated by a formula:

, (19.4)

where and - ultimate strength of wood at 12% humidity and at humidity ; a - coefficient of the strength decrease of wood at the growth of its humidity at 1% (for the ultimate compressive strength lengthwise fibers and static bend a =0.04; at tension lengthwise fibers a=0.01).

As a result of structural features, the mechanical properties of wood depend, also, on an angle between direction of operating effort and direction of fibers.

The most essential and characteristic mechanical property of wood is compressive strength lengthwise fibers. This property of wood is determining for piles, farms, columns and other structural timber.

In the most cases it is impossible to find out destruction at the action of compressive forces across the fibers; therefore it is limited by determination of proportional limit which is taken as conditional limit of the strength. Conditional compressive strength across the fibers upon the average for all wood is approximately in 10 times smaller than the compressive strength lengthwise the fibers.

The compressive strength across the fibers has a practical value in the places of angle joints or connections of timber details with metallic, for railway sleepers, etc.

The ultimate tensile strength lengthwise the fiber is in 2 and more times higher than at a compression (Table 19.1). For pine-tree and fir-tree, for example, it is equal on the average about 100 MPa. At tension across the grain, the ultimate strength is in 10-40 times less. Here the strength in a radial plane of all wood is higher than at tension in tangential area. It is caused that the break of weak pith rays passes in last case, while in a radial area it goes on early and dense late zone. The tensile strength especially strongly goes down at presence of knots and curly grain.

Wood rarely works on tension in constructions and elements. It is predefined with difficulty to prevent the destruction of details in the places of fixing. The indexes of the tensile strength of wood across the fibers are taken into account for prevention of its cracking at the intensive modes of drying.

Wood is widely used for structures which work on cross bending: in the floors, in bridge truss, trestles, platforms, stair, and others like that. The strength of wood at a static cross-bending is middle between the tensile strength and compressive along the fibers. On the average, it can be accepted equal approximately 90 MPa for different wood.

The strength of wood in some occasions is important at a shear and twisting for the calculation of structural timber. The most widespread type of tests on shear is splitting off along the fibers, resistance to which is approximately 0.15 of the compressive strength. The strength at twisting for the basic wood is almost in 1.5 times higher than resistance to splitting off.

The wood hardness is an important at treatment with machining tools and at deterioration actions. This property is determined on standards-cubes by indentation method. The most hardness (50-90 MPa) is inherent to an ash, beech, elm, larch.

The creep that results in noticeable deformations of constructions of the protracted loading is characteristic for the wood especially, during work in the moist conditions,

Wood working in dry premises, outdoors, and also in underground and underwater structures, in conditions which eliminate formation of fungi, is characterized with the high resistance. Mechanical properties of wood change considerably after staying in the river water during for a few hundreds of years. Salt water already through comparatively short time significantly worsens the properties of wood.

At the action of acids and alkalis, the mechanical properties of wood are worsened to the extent of their concentration increasing. Corrosive resistance of hard wood is lower than coniferous one.

Within the limits of one type of wood, the resistance depends on its density. Resistance is increased with the age of tree, at a movement from a sap-wood to the kernel and from the underbody of trunk to overhead. Protecting of wood from decay is carried out mainly by the chemical treatment with antisepsics, and from ignition - by the fire-retardants.

The directed change of the wood properties is arrived at its modification due to pressing after a previous steaming-out or heating, and also treatment of synthetic polymers. The modified wood has in few times larger strength, hardness, impact resistance, decreased hygroscopicity and water absorption.

Antiseptics are the toxic compounds which give resistance against the wood fungus, insects, etc.

It is possible to divide antisepsics into three groups depending on chemical and physical properties: oils and soluble in oils; soluble in organic solvents; water-soluble. Carboniferous and shale penetrating oils are mainly included in the first group of antiseptics; in the second - pentachlorophenol and copper naphthenate soluble in organic solvents. The basic representatives of the third group are sodium fluoride, hydrochloric zinc and other.

Substances which increase the fire-resistance of wood are called fire-retarding agents. The protective action of fire-retardant additives can be predefined by the extraction at heating of crystallization water as steam or other noncombustible gases which displace the air from the surface of wood and attenuate combustible gases (sulfuric and acid phosphorous ammonium, galloon). There are many fire-retarding additives (for example, borer, boric acid, sodium silicate, and hydrochloric zinc), that melt at heating and form protective dense tape, which covers the surface of wood and interferes to the oxygen access. Such fire-retarding additives as hydrate of potassium, some glues, facilitates at a high temperature creation of foam heat-insulating layer.

The mixtures of different fire-retarding additives are applied usually in practice. Wood is saturated with fireproof mixtures at the action of flame smolders, but does not burn. After the removing of fire a smoldering is halted. The various paints can also protect the wood from ignition.

Date: 2015-12-18; view: 1404