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Wood Handbook, Chapter 05: Mechanical Properties of Wood

Wood Handbook, Chapter 05: Mechanical Properties of Wood

Wood Handbook, Chapter 05: Mechanical Properties of

Mechanical Properties of Wood David E. Kretschmann, Research General Engineer Contents Orthotropic Nature of Wood 5–1 Elastic Properties 5–2 Modulus of Elasticity 5–2 Poisson’s Ratio 5–2 Modulus of Rigidity 5–3 Strength Properties 5–3 Common Properties 5–3 Less Common Properties 5–15 Vibration Properties 5–17 Speed of Sound 5–17 Internal Friction 5–17 Mechanical Properties of Clear Straight-Grained Wood 5–21 Natural Characteristics Affecting Mechanical Properties 5–26 Specific Gravity 5–26 Knots 5–26 Slope of Grain 5–28 Annual Ring Orientation 5–31 Reaction Wood 5–31 Juvenile Wood 5–32 Compression Failures 5–33 Pitch Pockets 5–33 Bird Peck 5–33 Extractives 5–34 Properties of Timber from Dead Trees 5–34 Effects of Manufacturing and Service Environments 5–34 Temperature 5–35 Time Under Load 5–38 Aging 5–41 Exposure to Chemicals 5–41 Chemical Treatment 5–41 Nuclear Radiation 5–43 Mold and Stain Fungi 5–43 Decay 5–43 Insect Damage 5–44 Literature Cited 5–44 Additional References 5–44 CHAPTER 5 The mechanical properties presented in this chapter were obtained from tests of pieces of wood termed “clear” and “straight grained” because they did not contain characteristics such as knots, cross grain, checks, and splits. These test pieces did have anatomical characteristics such as growth rings that occurred in consistent patterns within each piece. Clear wood specimens are usually considered “homogeneous” in wood mechanics. Many of the mechanical properties of wood tabulated in this chapter were derived from extensive sampling and analysis procedures. These properties are represented as the average mechanical properties of the species. Some properties, such as tension parallel to the grain, and all properties for some imported species are based on a more limited number of specimens that were not subjected to the same sampling and analysis procedures. The appropriateness of these latter properties to represent the average properties of a species is uncertain; nevertheless, the properties represent the best information available. Variability, or variation in properties, is common to all materials. Because wood is a natural material and the tree is subject to many constantly changing influences (such as moisture, soil conditions, and growing space), wood properties vary considerably, even in clear material. This chapter provides information, where possible, on the nature and magnitude of variability in properties. This chapter also includes a discussion of the effect of growth features, such as knots and slope of grain, on clear wood properties. The effects of manufacturing and service environments on mechanical properties are discussed, and their effects on clear wood and material containing growth features are compared. Chapter 7 discusses how these research results have been implemented in engineering standards. Orthotropic Nature of Wood Wood may be described as an orthotropic material; that is, it has unique and independent mechanical properties in the directions of three mutually perpendicular axes: longitudinal, radial, and tangential. The longitudinal axis L is parallel to the fiber (grain); the radial axis R is normal to the growth rings (perpendicular to the grain in the radial direction); and the tangential axis T is perpendicular to the grain but tangent to the growth rings. These axes are shown in Figure 5–1. 5–1

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