Presentation on theme: "Chapter 12 Defects in wood 木材的缺陷 The term defect, as applied to wood, refers to any irregularity or deviation from the qualities that make wood suitable."— Presentation transcript:
Chapter 12 Defects in wood 木材的缺陷 The term defect, as applied to wood, refers to any irregularity or deviation from the qualities that make wood suitable for a particular purpose. Growth-related defects are imperfections in the wood of living trees, which arises from tree growth, or irregularities of growth. The defects, which develop in wood after it has cut, are the result of treatment or foreign organisms.
Deterioration of wood can be brought about: through mechanical wear, by decomposition caused by physical agencies, by chemical decomposition, by action of foreign biological agencie. This chapter is concerned primarily with wood deterioration caused by the biological agents and with the various types of stains.
Topical highlights: Ⅰ. Knots Ⅱ. Deterioration caused by fungi Ⅲ. Deterioration caused by bacteria Iv. Deterioration caused by insects V. Frost injuries Ⅵ. Grain direction in wood Ⅻ. Reaction wood Ⅷ. Pitch defects Ⅸ. Bark pockets X. Brashness Ⅺ. Growth stresses in trees Ⅻ. Weathering of wood
Ⅰ. Knots / 节子 1. Conception A knot is a branch base that is embedded in the wood of a tree trunk or of a larger limb or branch.
2.Structure of knots The organization of the branch is similar to that in the main stem, and the tissue systems of the two structures are interconnected.
3.All kinds of terms of knots According to the quality of knots — sound knots （健全节） — unsound knots （非健全节） — living knots （活节） — dead knots （死节） — loose knots （松节） — tight knots （紧节） — decayed knots （腐朽节） — lost knots （漏节）
According to the shape of knots — round knots （圆形节） — spike knots （条形节） — palm-shape knots （掌状节） — ring-shape knots （轮生节） According to the size of knots — pin knots （针眼节） — small knots （小节） — medium knots （中等节） — large knots （大节）
4. Effects of knots on wood value The presence of knots in lumber or timbers has a direct relationship to the mechanical behavior of the member. The knot itself is harder, more dense, often more resinous, and shrinks in a manner different from that of the surrounding tissue. increase the compression strength, hardness, and shear characteristics of the wood, and decrease the tensioning and bending strength. cause uneven wear on the surfaces give trouble because of the checking with moisture changes make difficulty in painting increase for cutting forces
Ⅱ. Deterioration caused by fungi / 真菌败坏 Definition of fungi : Plants or animals? — like a kind of plant in that they can not move, but with no chlorophyll and can not manufacture their own food 。 — like animal in that they have metabolizing action and cause wood deterioration, but can not move. — not plants, not animals, but microbes Three groups of wood inhabiting fungi — wood-destroying fungi （木腐菌） lead wood suffer brown rot （褐腐） or white rot （白腐） — soft rot （软腐菌） make wood lose strength and become soft — blue-staining fungi （木材变色真菌） causing discolorations on wood surface and interior Infection routes — spores — hyphae spread
Necessary conditions for fungi growth The growth of a wood-inhabiting fungus depends on: — favorable temperature, 5 - 40 o C — a supply of oxygen — an adequate amount of moisture, mc = 20 - 60% — the presence of a suitable food supply — pH of the substrate, pH = 4.5 - 5.5
1. Wood-destroying fungi / 木腐菌 On the basis of physical and chemical changes produced in wood and the resulting alterations in color of decaying wood, wood-destroying fungi are classified as: – brown rots （褐腐菌） - Basidiomycete fungi （担子菌纲） –white rots （白腐菌） -Basidiomycete fungi –soft rots （软腐菌） -Ascomycete fungi （子囊菌纲）
2. Characteristics of decayed wood by wood-destroying fungi 2-1 Color The incipient stages of decay may be accompanied by various changes in the natural color of the wood. These color changes are due to the follows. the color and concentration of the invading hyphae a chemical alteration of one or more of the principal cell wall components the pigmented materials in the wood the formation of distinctive coloring substances by fungi or bacteria
2-2 Odor The advanced stages of decay may be accompanied by characteristic odors. These odors are variable in intensity and character, with those of anise （茴芹） and wintergreen （鹿蹄草） being quite common. In some cases these odors may be of diagnostic value. In most instances they are only a nuisance, but in some forms of wood utilization they may be quite a serious problem. 2-3 Water conduction and moisture-holding capacity Decayed wood absorbs moisture more rapidly and can hold more water than sound wood.
2-4 Dimensional changes Wood containing decay in the advanced stages shrinks more on drying than sound wood. The differences in the rate of shrinkage are more pronounced in brown rots than in white rots at a comparable stage of decay. 2-5 Density Because of destruction of wood substance, wood becomes less dense as decay progresses. 2-6 Mechanical properties Wood with the incipient stages of brown rot suffers a considerable reduction in toughness, and becomes brash. Wood affected with white rot shows little weakening until the decay reaches more advanced stages.
2-7 Thermal properties Dry decayed wood ignites more readily than sound wood. However, the calorific value of decayed wood is reduced. 2-8 Resistance to insect attack Wood containing decay is more readily attacked by some insects and marine organisms.
3. Molds / 霉菌 3-1 Appearance of molds These fungi are characterized by cottony or downy （绒毛状） growth varying in color from white through shades of yellow, brown-red, purple-blue, and green to black. 3-2 Developing conditions Mild temperatures, an abundant supply of moisture, and still air, such as results from poor ventilation, favor their development. 3-3 Effects on wood Molds apparently do not affect the strength properties of wood. Heavy mold infection increases the permeability of wood due to the partial or complete breakdown of the ray parenchyma cells. Mold growth causes serious contamination of wood.
3-4 Control measures In air seasoning, molding can be controlled by adequate circulation of air. Molds on wood can be destroyed by steaming it at 170 ℉ and 100 percent humidity for about 1 hour. Dipping and spraying freshly cut stock in a chemical solution such as sodium pentachlorophenate （五氯酚钠 ） or ethyl mercury phosphate （磷酸乙基汞） are also effective in preventing mold infection.
4. Sap stain / 边材变色 4-1 Conception Discolorations in wood caused by fungi are by far the most important types of stains from the economic point of view. Since the activity of this class of fungi is almost exclusively confined to the sapwood, the resulting discolorations are called sap stains and the fungi causing them sap-stain fungi.
4-2 Differences of sap-stain fungi from wood destroying fungi They do not decompose the wood substance. Their hyphae are usually much larger. They do not make boreholes on the cell walls with enzyme. They penetrate from cell to cell mostly by passing through the pits in the walls, in contrast to those of wood-decaying fungus. In cases when hyphae of a stain fungus penetrate directly through the cell wall, the boreholes are extremely minute, several times narrower than the normal width of the hyphae. Another significant point of difference between these two classes of fungi is that perforation by stain fungi takes place mostly mechanically, without prior enzymatic action as is the case with the wood-destroying kinds.
4-3 Effects of sap-stain Do not decrease mechanical strength seriously. Contaminate wood surface and inside seriously. Increase the permeability of wood significantly. 4-4 Control measures storing logs under water or under continuous water spray dipping or spraying with a fungicide-insecticide solution rapid drying the lumber to reduce to the moisture content of the surface below the fiber saturation point before the spores can germinate
Ⅲ. Deterioration caused by bacteria Although bacterial deterioration of wood is less obvious than that caused by fungi, it has been recognized for some time that bacteria, alone or interacting with fungi, may play an important role, by causing changes in characteristics of wood in its natural state and when placed in service. These changes may be economically beneficial or detrimental, depending on the particular use of the affected wood.
Ⅳ. Deterioration caused by insects From the standpoint of wood utilization the insects that damage wood can be segregated roughly into those whose attacks are confined to wood before it is utilized and those whose damage is mainly restricted to wood in service. Pith flecks （髓状斑）, pinholes （针眼虫孔）, and grubholes （大虫孔） result from the activities of insects belonging to the first category; powder- post beetles and termites are the most important examples of insects that attack converted wood.
1. Insect damage in wood before it is utilized 1-1 Pith flecks Pith flecks, or medullary spots, are confined to hardwoods. On transverse surfaces they appear as small areas of wound tissue which are usually darker than the surrounding tissue and are wholly within the limits of a growth ring. his defect results from injury to the cambium by the larvae of flies. 1-2 Pinholes Pinholes are small round, and usually opened holes of 1/100 to ¼ inch in diameter, resulting from the mining of ambrosia beetles. 1-3 Grub holes Insect mines, exceeding ¼ inch in diameter are generally called grub holes.
2. Insect damage to wood in service 2-1 Powder--post beetle damage / 粉蠹甲虫危害 — Phenomenon Powder-post injury is practically limited to air-seasoned and kiln-dried lumber, though cases are on record where these beetles have invaded wood containing as much as 40 percent moisture. The term powder-post beetles was applied to these insects because the larvae bore through the wood and leave residual wood substance in a finely pulverized condition; the flour-like residue sifts out from the tunnels when the adults emerge during late spring and summer, leaving holes 1/16 to 1/12 inch in diameter. The interior of the wood may be riddled with tunnels, with little visual evidence on the outside of the piece to indicate the extent of destruction.
— Control measures Long-time storing logs to reduce the reserves of starch in parenchyma Heat sterilizing the lumber by steaming or drying at high temperature Coating thoroughly all the surfaces of wood products to seal the pores
2-2 Common furniture beetles / 普通家具甲虫 In spite of the name, the attack of this beetle (Anobium sp.) is not confined to furniture but extends to structural timber, plywood, and other types of seasoned wood products. Because of the size of the escape holes, the damage is frequently mistaken for that caused by powder- post beetles( 粉蠹甲虫 ). The same remedial treatment used in preventing powder-post-beetle damage is applicable for control of common-furniture-beetle infestations.
2-3 Termites / 白蚁 — Phenomenon Termites have a worldwide distribution through the tropical and temperate zones. They attack all species of wood, either sapwood or heartwood, and will infest sound or decayed wood. All termites use cellulose and other carbohydrates in the wood for food. Some species of termites found in the United States convert cellulose into a digestible form by the action of protozoa and bacteria inhabiting their intestinal tracts.
— Three groups of termites the subterranean termites the dry-wood termites the wet-wood termites — Control measures removal of all tree roots, stumps, and wood debris from the building site; provision for adequate soil drainage beneath and around the building; chemical treatment of the soil under the building and around the foundation; adequate ventilation under the building; keeping all wood in the building at least 8 inches from the ground; making all foundations as impervious to termites as possible; use of pressure-treated lumber, especially in the foundations.
3. Defects resulting from the activities of marine borers （海生钻木动物） Marine borers are animals belonging to the Mollusca (molluscs) and the Crustacea (crustaceans). These organisms inhabit salt and brackish water and inflict extensive damage of submerged wooden structures as well as to wood exposed at low tide. Control of marine borers is difficult. Many methods of protecting wood have been tried with varying degrees of success. The best results, so far, have been attained by thorough pressure impregnation of the wood with coal tar creosote and other suitable nonleaching preservatives.
Ⅴ. Frost injuries / 冻害 Two types of defects develop in wood of living trees, supposedly as the result of freezing temperatures. These are known as frost rings( 霜轮 ) and frost cracks( 冻裂 ). 1. Frost rings — Conception Frost rings appear to the naked eye as brownish lines within and parallel to the boundaries of growth rings; their general appearance simulates false rings. These zones of discoloration result from frost injury to the cambium and the immature xylem cells, after the cambium has become active in the spring, and before it becomes dormant in the autumn. — Effects There is collapse and distortion of cells that were immature at the time the freezing occurred ； Parenchyma appears in abnormal amounts ； The rays are wider at that point than normal.
2. Frost cracks — Conception Frost cracks develop as radial splits in the wood and bark near the base of the tree. They are found in all species growing in cold climates but are most frequent in hardwoods. Frost cracks are most common in old trees with stout primary roots and broad crowns and are absent in very young trees. — Formation One theory claims that when a tree is exposed to very low temperatures, the low heat conductivity of wood causes the outer region of the trunk to contract before the center of the trunk is affected by the temperature change. This differential shrinkage sets up a tensile stress on the outside of the trunk and results in radial cracks. Another theory considers that the mechanical action of wind on the frozen wood is the most important factor in frost-crack formation. Wetwood has also been advanced as the principal cause of frost-crack formation.
Ⅵ. Grain direction in wood Any form of deviation from the straight-grained condition is considered to be a defect in structural lumber because of the reduction of strength in the member in which it occurs. On the other hand, irregular forms of grain orientation, such as curly, wavy, and interlocked, may be an advantage when wood with such grain is used for decorative purposes because of its distinctive figure.
1. Terms for grain direction in wood 1-1 Spiral grain / 螺旋纹理 This term is applied to the helical orientation of the fibers in a tree stem, which gives a twisted appearance to the trunk after the bark has been removed.
1-2 Interlocked grain / 交错纹理 A regular reversal of right and left spirality in a tree stem produces the condition known as interlocked grain.
1-3 Diagonal grain / 对角纹理 When stock is sawn so that the grain of the wood intersects the surface at an angle, the piece has diagonal grain. The principal cause of diagonal grain is the practice of sawing lumber parallel to the pith. This defect can easily be eliminated in straight logs by sawing parallel to the bark surface. Diagonal grain can also originate from crooked logs, sawing irregular pieces from straight-grained stock, or resawing and ripping straight-grained material at an angle to the long axis.
1-4 Cross grain / 斜纹理 Grain deviations in the flat-sawn faces of boards or timbers, resulting from spiral grain in the tree, crook and sweep in the log, or localized grain disturbance around large knots. Grain deviation may be due to the manner of sawing lumber (diagonal grain). In sawn material, all these types of grain deviation appear the same and are simply designated as cross grain, regardless of the true origin of such grain. The term spiral grain should be limited in its application to grain direction in tree stems, logs, and poles.
2. Determination of grain direction Ways to determine the direction of the grain in wood: Splitting the wood piece observing the orientation of resin canals or vessels on the wood piece observing the seasoning checks on the tangential face observing the growth-increment boundaries on the radial
Ⅶ. Reaction wood / 应力木 A reaction is a response to a triggering event. Reaction wood was appropriately named. This special kind of wood may be formed if the main stem of a tree is tipped from the vertical. It can also arise following the deflection of a lateral stem (or branch) from its normal orientation. Reaction wood formed in hardwoods differs from that formed in softwoods. In softwoods, it is termed compression wood （应压木） and in hardwoods, tension wood （应拉木）. In both, however, the function of reaction wood is the same: to bring the stem or branch back to the original position. The formation mechanisms is still a mystery.
1.Conception Compression wood formed in softwood, on the under compression side of leaned steam Tension wood formed in hardwood, on the up tension side of tipped steam 2.Differences from normal wood in properties of compression wood Tracheids 30% shorter; Cellulose 10% less, hemicelluloses and lignin 8-9%more; Longitudinal shrinkage 10 times higher; Wood density much higher; Microfibril angle in S 2 layer much larger, going to 45 o.
3. Differences from normal wood in properties of tension wood Cellulose content much higher; Wood density much higher; No S 3 layer, but with a G-layer, and so causing fuzzy surface; Longitudinal shrinkage much higher, going to 1%; Collapse during wood drying much more severe.
Ⅷ. Pitch defects / 树脂缺陷 1. Formation A number of pitch defects are found in softwoods in which resin canals are normal in the wood. These defects develop through the accumulation of resin in excessive amounts in localized regions of the wood. 2. Types pitch: in irregular shape pitch streak: sharply outlined in a line Pitch pocket: planoconvex cavities, usually confined within the boundaries of a single growth increment.
Ⅸ. Bark pockets / 树皮囊 These are small patches of bark that are embedded in wood. They apparently develop from some injury to the tree, resulting in the death of a small area of the cambium.
X. Brashness / 脆性 Brashness is an abnormal condition that causes wood to break suddenly and completely across the grain at stress levels lower than expected. The surfaces of a brash break are relatively smooth and show the structure of the wood in cross section quite cleanly. In contrast, the normal type of failure results in a more jagged surface. The most objectionable feature of brash wood is the sudden failure without previous warning, especially when shock-loaded.
Ⅺ. Growth stresses in trees 树木中的生长应力 Growth stresses are present in most trees as a result of normal development of woody tissue. But in some tree species, e.g., eucalyptus, growth stress can go to very high level and cause serious problems during wood processing. 1.Phenomenon of growth stress log sawing, vegetable stalk splitting.
2. Effects of growth stress Growth stresses are a primary cause for shakes, brittle heart, and compression failure in standing trees and felled logs. The growth stresses also cause “spring” of sawn-wood products, i.e., bowing and crooking of pieces as they are sawn from the log. 3.Formation mechanism of growth stress Growth stresses arise from a deposition and polymerization of lignin within the secondary wall during the maturation of fibrous cells. The addition of lignin causes a small lateral expansion of the cross section of the fiber and a corresponding reduction in length according to Poisson's ratio. The shrinkage in length of each cell is small, but the cumulative effect is sufficient to produce considerable tensile stress longitudinally on the outside the woody cylinder of the trunk.
4.Distribution of growth stress Growth stresses are a maximum in tension at the surface of the log, and from here decrease inward to zero at about one-quarter of the radial distance to the pith. From this point inward to the pith the stresses are compressive in nature and increase in magnitude to a maximum at the pith.
5. Reduction of growth stresses by preventing growth of the tree in season prior to felling. by applying steel strapping at each side of the intended location of a crosscut which is be made for either felling or bucking by storage of logs in ponds or under water spray for months
Ⅻ. Weathering of wood / 木材风化 When wood, unprotected by paint or any other means, is exposed to the weather, its surface undergoes changes, in part physical and in part chemical, the cumulative effects of which are termed weathering. The first indication of weathering is change in color. Initially, the dark-colored woods tend to fade and the light-colored varieties to darken somewhat. However, as the weathering continues, all woods assume a silvery-gray color, with the gray layer extending 0.003 to 0.001 inch in depth. In humid climates the final appearance of weathered surfaces may be further modified by the surface growth of spores and mycelia of fungi, resulting in unsightly, blotchy discolorations, usually dark gray in color. At this stage the process of weathering is mainly photo-oxidative in nature. The surface appearance is greatly affected by the wavelength of light, with the ultraviolet light being most destructive, resulting in degradation of lignin and extractives, followed by removal of the extractive materials by the action of atmospheric moisture. As a result, the partially loosened wood fibers in the gray layer consist mostly of the more leach- resistant fractions of cellulose.
Since wood is hygroscopic in nature, its unprotected surface tends to absorb moisture and to swell during humid and rainy weather, and to lose moisture and shrink in periods of dry weather. This, coupled with the differential shrinkage of early and late woods, leads to formation of raised grain. Owing to the slow rate of moisture transfusion through wood, these changes in moisture content, and hence wood dimensions, are for the most part confined to the surface layers. The alternate compression and tension stresses set up in the shell of a piece of wood eventually result in formation of microscopic checks on the exposed surface.
If weathering is permitted to proceed further, the surface will develop larger and deeper checks, which become visible. This, combined with the abrasive effect of rain, hail, wind- borne particles, and freezing and thawing, causes the wearing away of surface layers and a roughened appearance of the surface. The differential dimensional changes between the surface and the interior of a board, combined with the surface abrasion, frequently lead to warping of weathered boards and the loosening of fasteners. Warping is more pronounced in the denser woods and in wide boards, the width of which exceeds their thickness by more than eight times.
The effect of normal weathering may be accelerated by polluted atmosphere containing sulfur dioxide, which may be responsible for partial hydrolysis of cellulose and disintegration of lignin. Surface molds may also contribute to weathering. Finally, it should be noted that the surface conditions commonly attributed to nonbiological weathering may in fact be due to the attack of soft-rot fungi. In the case of the more popular species used for siding e.g., redwood, western redcedar, or baldcypress, more uniformly attractive, weathered surfaces can be achieved by treating the exposed surfaces with a water-repellant preservative, which tends to retard leaching of wood extractives and formation of mildew on the surface. Weathered surfaces of wood have poor adhesion for finishing materials and should be carefully sanded before paint or other finishes are applied.
Reflection and practice: 1.Effects of knots on wood properties? 2.The main groups of wood fungi and their harm to wood? 3.The necessary conditions for fungi to grow on wood? 4.Principles of wood preservation? 5.Conceptions of reaction wood, compression wood, and tension wood? 6.Formation Mechanism of growth stress? 7.Distribution pattern of growth stress?