Wood Handbook, Wood as an Engineering Material

CHAPTER 5

Mechanical Properties of Wood

David E. Kretschmann, Research General Engineer

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 characteris-

tics 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 “homoge-

neous” 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 aver-

age 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 proper-

ties 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: longitudi-

nal, 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.

Contents

Orthotropic Nature of Wood 5–1

Elastic Properties 5–

Modulus of Elasticity 5–2

Poisson’s Ratio 5–2

Modulus of Rigidity 5–3

Strength Properties 5–

Common Properties 5–3

Less Common Properties 5–15

Vibration Properties 5–

Speed of Sound 5–17

Internal Friction 5–17

Mechanical Properties of Clear Straight-Grained

Wood 5–21

Properties 5– 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 11–

Additional References 5–44