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Wood Handbook--Chapter 4--Mechanical Properties of Wood

4 1 Chapter 4 mechanical Properties of WoodDavid W. Green, Jerrold E. Winandy, and David E. KretschmannContentsOrthotropic Nature of Wood 4 1 Elastic Properties 4 2 Modulus of Elasticity 4 2 Poisson s Ratio 4 2 Modulus of Rigidity 4 3 Strength Properties 4 3 Common Properties 4 3 Less Common Properties 4 24 Vibration Properties 4 25 Speed of Sound 4 25 Internal Friction 4 26 mechanical Properties of Clear Straight-Grained Wood 4 26 Natural Characteristics Affecting mechanical Properties 4 27 Specific Gravity 4 27 Knots 4 27 Slope of Grain 4 28 Annual Ring Orientation 4 30 Reaction Wood 4 31 Juvenile Wood 4 32 Compression Failures 4 33 Pitch Pockets 4 33 Bird Peck 4 33 Extractives 4 33 Properties of Timber From Dead Trees 4 33 Effects of Manufacturing and Service Environments 4 34 Moisture Content 4 34 Temperature 4 35 Time Under Load 4 37

4–1 Chapter 4 Mechanical Properties of Wood David W. Green, Jerrold E. Winandy, and David E. Kretschmann Contents Orthotropic Nature of Wood 4–1

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Transcription of Wood Handbook--Chapter 4--Mechanical Properties of Wood

1 4 1 Chapter 4 mechanical Properties of WoodDavid W. Green, Jerrold E. Winandy, and David E. KretschmannContentsOrthotropic Nature of Wood 4 1 Elastic Properties 4 2 Modulus of Elasticity 4 2 Poisson s Ratio 4 2 Modulus of Rigidity 4 3 Strength Properties 4 3 Common Properties 4 3 Less Common Properties 4 24 Vibration Properties 4 25 Speed of Sound 4 25 Internal Friction 4 26 mechanical Properties of Clear Straight-Grained Wood 4 26 Natural Characteristics Affecting mechanical Properties 4 27 Specific Gravity 4 27 Knots 4 27 Slope of Grain 4 28 Annual Ring Orientation 4 30 Reaction Wood 4 31 Juvenile Wood 4 32 Compression Failures 4 33 Pitch Pockets 4 33 Bird Peck 4 33 Extractives 4 33 Properties of Timber From Dead Trees 4 33 Effects of Manufacturing and Service Environments 4 34 Moisture Content 4 34 Temperature 4 35 Time Under Load 4 37

2 Aging 4 41 Exposure to Chemicals 4 41 Chemical Treatment 4 41 Nuclear Radiation 4 43 Mold and Stain Fungi 4 43 Decay 4 43 Insect Damage 4 43 References 4 44he mechanical Properties presented in this chapterwere obtained from tests of small pieces of woodtermed clear and straight grained because theydid not contain characteristics such as knots, cross grain,checks, and splits. These test pieces did have anatomicalcharacteristics such as growth rings that occurred in consis-tent patterns within each piece. Clear wood specimens areusually considered homogeneous in wood of the mechanical Properties of wood tabulated in thischapter were derived from extensive sampling and analysisprocedures. These Properties are represented as the averagemechanical Properties of the species.

3 Some Properties , suchas tension parallel to the grain, and all Properties for someimported species are based on a more limited number ofspecimens that were not subjected to the same sampling andanalysis procedures. The appropriateness of these latter prop-erties to represent the average Properties of a species is uncer-tain; nevertheless, the Properties represent the best informa-tion , or variation in Properties , is common to allmaterials. Because wood is a natural material and the tree issubject to many constantly changing influences (such asmoisture, soil conditions, and growing space), wood proper-ties vary considerably, even in clear material. This chapterprovides information, where possible, on the nature andmagnitude of variability in chapter also includes a discussion of the effect of growthfeatures, such as knots and slope of grain, on clear woodproperties.

4 The effects of manufacturing and service environ-ments on mechanical Properties are discussed, and theireffects on clear wood and material containing growth featuresare compared. Chapter 6 discusses how these research resultshave been implemented in engineering Nature of WoodWood may be described as an orthotropic material; that is, ithas unique and independent mechanical Properties in thedirections of three mutually perpendicular axes: longitudinal,radial, and tangential. The longitudinal axis L is parallel tothe fiber (grain); the radial axis R is normal to the growthrings (perpendicular to the grain in the radial direction); and 4 2the tangential axis T is perpendicular to the grain but tangentto the growth rings. These axes are shown in Figure 4 PropertiesTwelve constants (nine are independent) are needed to de-scribe the elastic behavior of wood: three moduli of elasticityE, three moduli of rigidity G, and six Poisson s ratios.

5 The moduli of elasticity and Poisson s ratios are related byexpressions of the form ijijijEEi j i, j L,R,T= =, (4 1)General relations between stress and strain for a homogene-ous orthotropic material can be found in texts on of ElasticityElasticity implies that deformations produced by low stressare completely recoverable after loads are removed. Whenloaded to higher stress levels, plastic deformation or failureoccurs. The three moduli of elasticity, which are denoted byEL, ER, and ET, respectively, are the elastic moduli along thelongitudinal, radial, and tangential axes of wood. Thesemoduli are usually obtained from compression tests; how-ever, data for ER and ET are not extensive. Average values ofER and ET for samples from a few species are presented inTable 4 1 as ratios with EL; the Poisson s ratios are shownin Table 4 2.

6 The elastic ratios, as well as the elastic con-stants themselves, vary within and between species and withmoisture content and specific modulus of elasticity determined from bending, EL,rather than from an axial test, may be the only modulus ofelasticity available for a species. Average EL values obtainedfrom bending tests are given in Tables 4 3 to 4 5. Represen-tative coefficients of variation of EL determined with bendingtests for clear wood are reported in Table 4 6. As tabulated,EL includes an effect of shear deflection; EL from bending canbe increased by 10% to remove this effect adjusted bending EL can be used to determine ER and ETbased on the ratios in Table 4 s RatioWhen a member is loaded axially, the deformation perpen-dicular to the direction of the load is proportional to thedeformation parallel to the direction of the load.

7 The ratio ofthe transverse to axial strain is called Poisson s ratio. ThePoisson s ratios are denoted by LR, RL, LT, TL, RT, and TR. The first letter of the subscript refers to direction ofapplied stress and the second letter to direction of lateraldeformation. For example, LR is the Poisson s ratio fordeformation along the radial axis caused by stress along thelongitudinal axis. Average values of Poisson s ratios forsamples of a few species are given in Table 4 2. Values for RL and TL are less precisely determined than are those forthe other Poisson s ratios. Poisson s ratios vary within andbetween species and are affected by moisture content andspecific directionFigure 4 1. Three principal axes of wood withrespect to grain direction and growth 4 1. Elastic ratios for various species atapproximately 12% moisture contentaSpeciesET/ELER/ELGLR/ELGLT/ELGRT /ELHardwoodsAsh, Birch, , Cottonwood, Mahogany, , , Maple, Oak, Oak, Sweet , , northern , western.

8 May be approximated by increasing modulus of elasticity values in Table 4 3 by 10%.4 3 Modulus of RigidityThe modulus of rigidity, also called shear modulus, indi-cates the resistance to deflection of a member caused by shearstresses. The three moduli of rigidity denoted by GLR, GLT,and GRT are the elastic constants in the LR, LT, and RTplanes, respectively. For example, GLR is the modulus ofrigidity based on shear strain in the LR plane and shearstresses in the LT and RT planes. Average values of shearmoduli for samples of a few species expressed as ratios withEL are given in Table 4 1. As with moduli of elasticity, themoduli of rigidity vary within and between species and withmoisture content and specific PropertiesCommon PropertiesMechanical Properties most commonly measured and repre-sented as strength Properties for design include modulus ofrupture in bending, maximum stress in compression parallelto grain, compressive stress perpendicular to grain, and shearstrength parallel to grain.

9 Additional measurements are oftenmade to evaluate work to maximum load in bending, impactbending strength, tensile strength perpendicular to grain, andhardness. These Properties , grouped according to the broadforest tree categories of hardwood and softwood (not corre-lated with hardness or softness), are given in Tables 4 3 to4 5 for many of the commercially important species. Averagecoefficients of variation for these Properties from a limitedsampling of specimens are reported in Table 4 of rupture Reflects the maximum load-carrying capacity of a member in bending and is propor-tional to maximum moment borne by the of rupture is an accepted criterion of strength, al-though it is not a true stress because the formula by whichit is computed is valid only to the elastic to maximum load in bending Ability to absorbshock with some permanent deformation and more or lessinjury to a specimen.

10 Work to maximum load is a meas-ure of the combined strength and toughness of wood underbending strength parallel to grain Maximumstress sustained by a compression parallel-to-grain speci-men having a ratio of length to least dimension of lessthan stress perpendicular to grain Reportedas stress at proportional limit. There is no clearly definedultimate stress for this strength parallel to grain Ability to resist inter-nal slipping of one part upon another along the presented are average strength in radial and tangen-tial shear bending In the impact bending test, a hammerof given weight is dropped upon a beam from successivelyincreased heights until rupture occurs or the beam deflects152 mm (6 in.) or more. The height of the maximumdrop, or the drop that causes failure, is a comparative valuethat represents the ability of wood to absorb shocks thatcause stresses beyond the proportional strength perpendicular to grain Resistance ofwood to forces acting across the grain that tend to split amember.


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