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In: Arntzen, Charles J., ed. Encyclopedia of Agricultural Science. Orlando, FL: Academic Press: 549-561. Vol. 4. October 1994.
Wood Properties
JERROLD E. WINANDY, USDA-Forest Service, Forest Products Laboratory, 1 Wisconsin
Wood Structure Physical Properties Mechanical Properties Factors Affecting Properties of Wood Properties and Grades of Sawn Lumber
Glossary
Allowable property Value of a property normally published for design use; allowable properties are identified with grade descriptions and standards, and they reflect the orthotropic structure of wood and anticipated end uses Anisotropic Exhibiting different properties along different axes; in general, fibrous materia]s such as wood are anisotropic Annual growth ring Layer of wood growth put on a tree during a single growing season. In the temperate zone, the annual growth rings of many species (e. g., oaks and pines) are readily distinguished because of differences in the cells formed during the early and late parts of the season; in some temperate zone species (e. g., black gum and sweetgum) and many tropical species, annual growth rings are not easily recognized Diffuse-porous wood Certain hardwoods in which the pores tend to be uniformly sized and distributed throughout each annual ring or to decrease in size slightly and gradually toward the outer border of the ring Earlywood Portion of the annual growth ring that is formed during the early part of the growing season; it is usually less dense and mechanically weaker than latewood Hardwoods General botanical group of trees that has broad leaves in contrast to the conifers or soft- (^1) The Forest Products Laboratory is maintained in cooperation with the University of Wisconsin. This article was written and prepared by U.S. Government employees on offical time, and it is therefore in the public domain and not subject to copyright.
Encyclopedia of Agricultural Science, Volume 4
woods; term has no reference to the actual hardness of the wood Latewood Portion of the annual growth ring that is formed after the earlywood formation has ceased; it is usually denser and mechanically stronger than ear- lywood Lumber Product of the saw and planing mill manu- factured from a log through the process of sawing, resawing to width, passing lengthwise through a stan- dard planing machine, and crosscutting to length Orthotropic Having unique and independent prop- erties in three mutually orthogonal (perpendicular) planes of symmetry; 3 special case of anisotropy Ring-porous woods Group of hardwoods in which the pores are comparatively large at the beginning of each annual ring and decrease in size more or less abruptly toward the outer portion of the ring, thus forming a distinct inner zone of pores, the earlywood, and an outer zone with smaller pores, the latewood Softwoods General botanical group of trees that in most cases has needlelike or scalelike leaves (the coni- fers); term has no reference to the actual hardness of the wood
Wood is an extremely versatile material with a wide range of physical and mechanical properties among the many species of wood. It is also a renewable re- source with an exceptional strength-to-weight ratio. Wood is a desirable construction material because the energy requirements of wood for producing a usable end-product are much lower than those of competi- tive materials, such as steel, concrete, or plastic.
|. Wood Structure
A. Microstructure The primary structural building block of wood is the
WOOD PROPERTIES
cubic centimeter of wood could contain more than 1.5 million wood cells. When packed together they form a strong composite. Each individual wood cell is even more structurally advanced because it is actu- ally a multilayered, fdament-reinforced, closed-end tube (Fig. 1) rather than just a homogeneous-walled, nonreinforced straw. Each individual cell has four distinct cell wall layers (Primary, S 1 , S 2 , and S 3 ). Each layer is composed of a combination of three chemical polymers: cellulose, hemicellulose, and lignin (Fig. 1). The cellulose and hemicellulose are linear poly- saccharides (i.e., hydrophilic multiple-sugars), and the lignin is an amorphous phenolic (i. e., a three- dimensional hydrophobic adhesive). Cellulose forms long unbranched chains. and hemicellulose forms short branched chains. Lignin encrusts and stiffens these polymers. Because carbohydrate and phenolic components of wood are assembled in a layered tubular or cellular manner with a large cell cavity, specific gravity of wood can vary immensely. Wood excels as a viable building material because the layered tubular structure provides a large volume of voids (void volume), it has an advantageous strength-to-weight ratio, and it has other inherent advantages, such as corrosion resistance, fatigue resistance, low cost, and ease-of- modification at the job site.
layer of the bark. Usually, it is 1 to 10 cells wide
B. Macrostructure
The cross-section of a tree is divided into three broad categories consisting of the bark, wood, and cambium
FIGURE 1 Microfibril orientation for each cell wall layer of Scotch pine with chemical composition as percentage of total weight. Cell wall layers are primary (P), S 1 , S 2 , and S 3.
(Fig. 2). Bark is the outer layer and is composed of a dead outer phloem of dry corky material and a thin inner phloem of living cells. Its primary functions are protection and nutrient conduction. The thickness and appearance of bark vary substantially depending on the species and age of the tree. Wood, or xylem, is composed of the inner sections of the trunk. The primary functions of wood are support and nutrient conduction and storage. Wood can be divided into two general classes: sapwood and
It functions primarily in food storage and the mechan- ical transport of sap. The radial thickness of sapwood is commonly 35 to 50 mm but may be 75 to 150 mm for some species. Heartwood consists of an inner core of wood cells that have changed, both chemically and physically, from the cells of the outer sapwood. The cell cavities of heartwood may also contain deposits of various materials that frequently give heartwood a much darker color. Extractive deposits formed during the conversion of living sapwood to dead heartwood often make the heartwood of some species more dura- ble in conditions that may induce decay. The cambium is a continuous ring of reproductive tissue located between the sapwood and the inner
depending on the season. All wood and bark cells are aligned or stacked radially because each cell in a radial line originated from the same cambial cell.
- Growth Growth in trees is affected by the soil and environ- mental conditions with which the tree must exist and
FIGURE 2 Elements of microstructure normally visible without magnification.
WOOD PROPERTIES
FIGURE 3 Three principal axes of wood with respect to grain direction and growth rings.
longitudinal axis. Property values tabulated for struc- tural applications arc often given only for axis direc- tions parallel to groin (longitudinal) and perpendicular to grain (radial or tangential).
B. Moisture Content
The moisture content of wood is defined as the weight of water in wood given as a percentage of ovendry weight. In equation form, moisture content (MC) is expressed as follows:
MC =
moist weight – dry weight dry weight
x 100%. (1)
Water is required for the growth and development of living trees and constitutes a major portion of green wood anatomy. In living trees, moisture content de- pends on the species and the type of-wood, and may range from approximately 25% to more than 250% (two and a half times the weight of the dry wood material). In most species, the moisture content of sapwood is higher than that of heartwood. Water exists in wood either as bound water (in the cell wall) or free water (in the cell cavity). As bound water, it is bonded (via secondary or hydrogen bonds) within the wood cell walls. As free water, it is simply present in the cell cavities. When wood dries, most free water separates at a faster rate than bound water because of accessibility and the absence of secondary bonding. The moisture content at which the cell walls are still saturated but virtually no water exists in the cell cavities is called the fiber saturation point. The fiber saturation point usually varies between 21 and 28%.
Wood is a hydroscopic material that absorbs mois- ture in a humid environment and loses moisture in a dry environment. As a result, the moisture content of wood is a function of atmospheric conditions and depends on the relative humidity and temperature of the surrounding air. (^) Under constant conditions of temperature and humidity, wood reaches an equilib- rium moisture content (EMC) at which it is neither gaining nor losing moisture. The EMC represents a balance point where the wood is in equilibrium with its environment. In structural applications, the moisture content of wood is almost always undergoing some changes as temperature and humidity conditions vary. These changes are usually gradual and short-term fluctua- tions that influence only the surface of the wood. The time required for wood to reach the EMC depends on the size and permeability of the member, the tem- perature, and the difference between the moisture content of the member and the EMC potential of that environment. Changes in moisture content cannot be entirely stopped but can be retarded by coatings or treatments applied to the wood surface.
C. Dimensional Stability Above the fiber saturation point, wood will not shrink or swell from changes in moisture content because free water is found only in the cell cavity and is not associated within the cell walls. However, wood changes in dimension as moisture content varies be- low the fiber saturation point. Wood shrinks as it loses moisture below the fiber saturation point and swells as it gains moisture up to the fiber saturation point. These dimensional changes may result in split- ting, checking, and warping. The phenomena of di- mensional stability and EMC must be understood,
Dimensional stability of wood is one of the few properties that significantly differs in each of the three axis directions. Dimensional changes in the longitudi- nal direction between the fiber saturation point and ovendry are between 0.1 and 0.2% and are of no practical significance; however, in reaction or juvenile wood, these percentages may be significantly higher. The combined effects of shrinkage in the tangential and radial axes can distort the shape of wood pieces because of the difference in shrinkage and the curva- ture of the annual rings (Fig. 4). Generally, tangential shrinkage (varying from 4.4 to 7.8% depending on species) is twice that of radial shrinkage (from 2.2 to 5.6%).
WOOD PROPERTIES
FIGURE 4 Characteristic shrinkage and distortion of wood as affected by direction of growth rings. Such distortion can result in warp, generally classified as bow, twist, crook, and cup.
D. Thermal Expansion
Thermal expansion of dry wood is positive in all directions; wood expands when heated and contracts when cooled. Wood that contains moisture reacts to temperature changes differently than dry wood. The linear expansion coefficients of dry wood par- allel to grain are generally independent of specific gravity and species and range from approximately 3 x 10 -6 to 4.5 x 10 -6 per o C. The linear expansion coefficients across the grain (tangential and radial) are in proportion to density and range from approxi- mately 5 to 10 times greater than parallel to grain coefficients. When moist wood is heated, it tends to expand because of normal thermal expansion and shrink be- cause of moisture loss from increased temperature. Unless the initial moisture content of the wood is very low (3 to 4%), the net dimensional change on heating is negative. Wood at intermediate moisture contents of approximately 8 to 20% will expand when first heated, then gradually shrink to a volume smaller than the initial volume as moisture is lost in the heated condition.
E. Pyrolytic Properties
Under appropriate conditions, wood will undergo thermal degradation or pyrolysis. The by-products
of pyrolysis may burn, and if enough heat is generated and retained by the wood, the wood can be set on fire. In the presence of a pilot flame (independent source of ignition), the minimum rate of heating nec- essary for ignition is of the order of 0.3 calorie per square centimeter. In the absence of a pilot flame, the minimum rate of heating necessary for ignition is of the order of 0.6 calorie per square centimeter, nearly double the rate of the pilot flame situation. Still, heavy timber construction deserves an ex- tremely favorable fire-insurance rating because it will generally not produce sufficient heat energy to main- tain combustion unless an external heat source is pres- ent. Timber will gradually produce a char layer from the residue of wood combustion. This char acts as a thermal insulator. On heavy timbers, this char layer will eventually inhibit combustion by establishing a thermal barrier between the uncharred wood (interior to char) and the heat of the fire (exterior to char). Heavy timber is virtually self-extinguishing, but steel, which has a thermal conductivity 100 times that of wood, will absorb heat until it reaches a temperature at which it yields under structural load without actu- ally burning.
F. Density and Specific Gravity The density of a material is the mass per unit volume at some specified condition. For a hydroscopic mate- rial such as wood, density depends on two factors: the weight of the wood structure and moisture retained in the wood. Wood density at various moisture contents can vary significantly and must be given relative to a specific condition to have practical meaning. Specific gravity provides a relative measure of the amount of wood substance contained in a sample of wood. It is a dimensionless ratio of the weight of an ovendry volume of wood to the weight of an identical volume of water. In research activities, specific grav- ity may be reported on the basis of both weight and volume ovendry. For many engineering applications, the basis for specific gravity is generally the ovendry weight and volume at a moisture content of 12%. For example, a volume of wood at some specified moisture content with a specific gravity of 0.50 would have a density of 500 kg/m 3.
G. Electrical Resistance Wood is a good electrical insulator. However, sig- nificant variations in conductivity do exist. These variations in electrical resistance can be related to vari-
WOOD PROPERTIES
material, wood is usually assumed to behave as an elastic material for most engineering applications. For an isotropic material with equal property values in all directions, elastic properties are measured by three elastic constants: modulus of elasticity (E), mod-
lowing equation shows the relationship:
where i, j, and k represent the three principal axes. Because wood is orthotropic, 12 constants are re- quired to measure elastic behavior: three moduli of elasticity, three moduli of rigidity, and six Poisson’s ratios.
1. Modulus of Elasticity Modulus of elasticity relates the stress applied along one axis to the strain occurring on the same axis. The three moduli of elasticity for wood are denoted E (^) L , ER, and E (^) T to reflect the elastic moduli in the longitu- dinal, radial, and tangential directions, respectively. For example, EL relates the stress in the longitudinal direction to the strain in the longitudinal direction. Elastic constants vary within and between species and with moisture content and specific gravity. The only constant that has been extensively derived from test data is E (^) L. Other constants may be available from limited test data but are most frequently developed from material relationships or by regression equations that predict behavior as a function of density. Relative values of elastic constants for clearwood of several common wood species are given in Table I. 2. Shear Modulus Shear modulus relates shear stress to shear strain. The three shear moduli for wood are denoted G (^) LR, GLT , and G (^) RT for the longitudinal-radial. longitudinal-tangential, and radial-tangential planes. respectively. For example, G (^) LR is the modulus of ri- gidity based on the shear strain in the LR plane and the shear stress in the LT and RT planes. The modulus of rigidity for several wood species and for each plane are given in Table 1. 3. Poisson’s Ratio Poisson’s ratio relates the strain parallel to an ap- plied stress to the accompanying strain occurring lat- erally. For wood, the six Poisson’s ratios are denoted
refers to the direction of applied stress; the second subscript refers to the direction of the accompanying lateral strain. For example, ~~~ is the Poisson’s ratio for stress along the longitudinal axis and strain along
the radial axis. Estimates of Poisson’s ratios for several wood species and for each orientation are given in Table I.
B. Strength Properties Strength properties me the ultimate resistance of a material to applied loads. With wood, strength varies significantly depending on species, loading condition, load duration, and a number of assorted material and environmental factors. Because wood is anisotropic, mechanical properties also vary in the three principal axes. Property values in the longitudinal axis are generally significantly higher than those in the tangential or radial axes. Strength- related properties in the longitudinal axis are usually referred to as parallel-to-grain properties. For most engineering design purposes, simply differentiating between parallel- and perpendicular-to-grain proper- ties is sufficient because the relative tangential and radial directions are randomized by the primary saw- ing process (i. e., conversion from logs to boards).
1. Compression When a compression load is applied parallel to grain, it produces stress that deforms (shortens) wood cells along their longitudinal axis. When wood is stressed in compression parallel to grain, failure ini- tially begins as the microfibrils begin to fold within the cell wall, thereby creating planes of weakness or instability within the cell wall. As stress in compres- sion parallel to grain continues to increase, the wood- cells themselves fold into S shapes. forming visible wrinkles on the surface. Large deformations occur from the internal crushing of the complex cellular structure. The average strength of green clear wood specimens of Douglas-fir and loblolly pine in com- pression parallel to grain is approximately 26.1 and 24.2 MPa, respectively. When a compression load is applied perpendicular to grain, it produces stress that deforms the wood cells perpendicular to their length. Once the hollow cell cavities are collapsed, wood is quite strong be- cause no void space exists. In practice, compressive strength of wood perpendicular to grain is usually assumed to be exceeded when deformation exceeds 4%. of the proportional limit stress. Using this con- vention, the average strength of green clear wood specimens of Douglas-fir and loblolly pine in com- pression perpendicular to grain is approximately 4. and 4.6 MPa, respectively. Compression applied at an angle to the grain pro- duces stresses that act both parallel and perpendicular
WOOD PROPERTIES
immediate and, for the most part, reversible for short heating durations. However, if wood is exposed to elevated temperatures for an extended time, strength is permanently reduced because of wood substance degradation and a corresponding loss in weight. The magnitude of these permanent effects depends on moisture content, heating medium, temperature, ex- posure period, and to a lesser extent, species and speci- men size. As a general rule, wood should not be exposed to temperatures above 65 O C. The immediate effect of temperature interacts with the effect of mois- ture content so that neither effect can be completely understood without consideration of the other.
3. Decay and Insect Damage Wood is conducive to decay and insect damage in moist, warm conditions. Decay within a structure cannot be tolerated because strength is rapidly reduced in even the early stages of decay. It has been esti- mated that a 5% weight loss from decay can result in strength losses as high as 50%. If the warm, moist conditions required for decay cannot be controlled, then the use of natural]y decay resistant wood species or chemical treatments are required to impede decay. Insects, such as termites and certain types of beetles, can be just as damaging to mechanical performance. Insect infestation can be controlled via mechanical barriers, naturally durable species, or chemical treat- ments.
V. Properties and G r a d e s o f Sawn Lumber
At first, the highest quality level of sawn 1umber might seem desirable for all uses, and indeed it is needed for several uses. However, in most situations, such material would be prohibitively expensive and a wasteful use of our timber resource. In practice, the quality level needed for a function can be easily specified because lumber and timber are graded in an orderly system developed to serve the interests of the users and the producers. The grading system is actually several systems, each designed for specific products. Hardwood lumber is mostly graded for remanufacture, with only small amounts graded for construction. Softwood is also graded for both remanufacture and construction, but primarily for construction. In practice, an orderly, voluntary but circuitous system of responsibilities has evolved in the United
States for the development, manufacture, and mer- chandising of most stress-graded lumber and tim- ber. In general, stress-grading principles are de- veloped from research findings and engineering concepts, often within committees and subcommit- tees of the American Society for Testing and Mate- rials. For lumber, the National Institute for Standards and Technology cooperates with producers, dis- tributors, users, and regional grade-rules-writing agencies through the American Lumber Standard Committee (ALSC). The ALSC has assembled a vol- untary softwood standard of manufacture, called the American Softwood Lumber Standard. The Ameri- can Softwood Lumber Standard and its related Na- tional Grading Rule prescribe the ways in which stress-grading principles can be used to formulate grading rules for dimension lumber (nominal 2 to 4 in. thick). This lumber standard is the basis for commercially marketing structural lumber in the United States. For timbers (more than 5 in. nominal), the National Grading Rule does not apply. Thus, each regional grade-rules-writing agency publishes grade rules for timbers following the general principles of the Na- tional Grading Rule, but each differs slightly in even- tual grade requirements and names. For further spe- cifics on the various characteristics for the individual species-grade combinations, contact the individual grade-rules-writing organizations directly. In North America, those agencies are National Lumber Grades Authority (Vancouver, BC, Canada), Northeastern Lumber Manufacturers Association (Cumberland, ME), Redwood Inspection Service (Mill Valley, CA), Southern Pine Inspection Bureau (Pensacola, FL), West Coast Lumber Inspection Bureau (Portland, OR), and Western Wood Products Association (Port- land, OR). [See FOREST T REE , GENETIC IMPROVEMENT. ]
Bibliography American Society for Testing and Materials (1991). “An- nual Book of Standards, ” Vol. D.09 Wood. Philadel- phia, PA. Forest Products Laboratory (1987). “Wood Handbook: Wood as an Engineering Material. ” Agric. Handb. 72. U.S. Department of Agriculture, Forest Service, Wash- ington, DC. Panshin, A. J., and deZeeuw, C. (1980). “Textbook of Wood Technology, ” 4th ed., p. 705. McGraw-Hill, New York. U,S. Department of Commerce. (1986). American Lumber Standard PS20-70. Washington, DC.
