WOOD - Materials for Civil and Construction Engineers - page 414

and temperature largely govern the internal friction in wood. At normal am-
bient temperatures, an increase in moisture content produces a proportion-
al increase in internal friction up to the fiber-saturation point. Under normal
conditions of temperature and moisture content, the internal friction in
wood (parallel to the grain) is 10 times that of structural metals. Because of
these qualities, wood structures dampen vibrations more quickly than metal
structures of similar design.

10.9 Testing to Determine Mechanical

Properties

Standard mechanical testing methods for wood are designed almost exclu-
sively to obtain data for predicting performance. To achieve reproducibility
in the testing environment, specifications include methods of material se-
lection and preparation, testing equipment and techniques, and computa-
tional methods for data reduction. Standards for testing wood and wood
composites are published by ASTM, the U.S. Department of Commerce, the
National Standard Institute (NSI), and various other trade associations, such
as the Western Woods Product Association.
Due to the many variables affecting the test results, it is of primary im-
portance to correctly select the specimen and type of test. There are two main
testing techniques for establishing strength parameters: the testing of repre-
sentative, small, clear specimens and the testing of timbers of structural sizes.
The primary purposes for testing small, clear specimens are to obtain the
mechanical properties of various species and to provide a means of control
and comparison in production activities. The testing of structural timbers
provides relationships among mechanical and physical properties, working
stress data, correlations between environmental conditions, wood imperfec-
tions, and mechanical properties. ASTM D 143 presents the complete testing
standards for small, clear wood specimens. This standard gives full descrip-
tions of sample collection, preparation, and testing techniques. Mechanical
tests included in this standard are the following:

Static bending Compression parallel to the grain

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Impact bending Toughness

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Compression perpendicular Hardness

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to the grain Cleavage

■

Shear parallel to the grain Tension perpendicular to the grain

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Tension parallel to the grain Specific gravity and shrinkage in

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Nail withdrawal volume

■

Radial and tangential Moisture determination.

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shrinkage

Figure 10.10 shows a schematic of test specimens of wood tested in tension,
compression, bending, and hardness. Static and impact bending, compression