Wood Handbook, Chapter 05: Mechanical Properties of Wood

Early stages of decay are virtually impossible to detect. For
example, brown-rot fungi may reduce mechanical properties
in excess of 10% before a measurable weight loss is
observed and before decay is visible. When weight loss
reaches 5% to 10%, mechanical properties are reduced from
20% to 80%. Decay has the greatest effect on toughness,
impact bending, and work to maximum load in bending, the
least effect on shear and hardness, and an intermediate effect
on other properties. Thus, when strength is important,
adequate measures should be taken to (a) prevent decay
before it occurs, (b) control incipient decay by remedial
measures (Chap. 14), or (c) replace any wood member in
which decay is evident or believed to exist in a critical section.
Decay can be prevented from starting or progressing if
wood is kept dry (below 20% moisture content).
No method is known for estimating the amount of reduction
in strength from the appearance of decayed wood.
Therefore, when strength is an important consideration, the
safe procedure is to discard every piece that contains even
a small amount of decay. An exception may be pieces in
which decay occurs in a knot but does not extend into the
surrounding wood.
Insect Damage
Insect damage may occur in standing trees, logs, and undried
(unseasoned) or dried (seasoned) lumber. Although
damage is difficult to control in the standing tree, insect
damage can be eliminated to a great extent by proper control
methods. Insect holes are generally classified as pinholes,
grub holes, and powderpost holes. Because of their
irregular burrows, powderpost larvae may destroy most of
a piece’s interior while only small holes appear on the surface,
and the strength of the piece may be reduced virtually
to zero. No method is known for estimating the reduction
in strength from the appearance of insect-damaged wood.
When strength is an important consideration, the safe procedure
is to eliminate pieces containing insect holes.
Literature Cited
Bendtsen, B.A.; Ethington, R.L.1975. Mechanical properties
of 23 species of eastern hardwoods. Res. Note FPL–RN–
0230. Madison, WI: U.S. Department of Agriculture, Forest
Service, Forest Products Laboratory. 12 p.
Bodig, J.; Jayne, B.A. 1982. Mechanics of wood and wood
composites. New York: Van Nostrand Reinhold Company.
Green, D.W.; Evans, J.W.; Craig, B.A. 2003. Durability of
structural lumber products at high temperatures I: 66°C at
75% RH and 82°C at 30% RH. Wood and Fiber Science.
35(4): 499–523.
Green, D.W.; Evans, J.W.; Hatfield, C.A.; Byrd, P.J. 2005.
Durability of structural lumber products after exposure at
82°C and 80% relative humidity. Res. Pap. FPL–RP–631.
Madison, WI: U.S. Department of Agriculture, Forest Service,
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General Technical Report FPL–GTR–190
Green, D.W.; Shelley, B.E.; Vokey, H.P., eds. 1989. In-grade
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WI: Forest Products Society.
Kingston, R.S.T. 1962. Creep, relaxation, and failure of
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Kretschmann, D.E. 2008. The influence of juvenile wood
content on shear parallel, compression and tension perpendicular
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Kretschmann, D.E.; Green, D.W. 1996. Modeling moisture
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Additional References
ASTM. [Current edition]. Standard methods for testing
small clear specimens of timber. ASTM D143–94. West
Conshohocken, PA: American Society for Testing and
Materials.
Bendtsen, B.A. 1976. Rolling shear characteristics of nine
structural softwoods. Forest Products Journal. 26(11):
51–56.
Bendtsen, B.A.; Freese, F.; Ethington, R.L. 1970. Methods
for sampling clear, straight-grained wood from the forest.
Forest Products Journal. 20(11): 38–47.
Bodig, J.; Goodman, J.R. 1973. Prediction of elastic
parameters for wood. Wood Science. 5(4): 249–264.
Boller, K.H. 1954. Wood at low temperatures. Modern
Packaging. 28(1): 153–157.
Chudnoff, M. 1984. Tropical timbers of the world. Agriculture
Handbook 607. Madison, WI: U.S. Department of Agriculture,
Forest Service, Forest Products Laboratory. 464 p.
Coffey, D.J. 1962. Effects of knots and holes on the fatigue
strength of quarter-scale timber bridge stringers. Madison,
WI: University of Wisconsin, Department of Civil Engineering.
M.S. thesis.
Gerhards, C.C. 1968. Effects of type of testing equipment
and specimen size on toughness of wood. Res. Pap.
FPL–RP–97. Madison, WI: U.S. Department of Agriculture,
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Gerhards, C.C. 1977. Effect of duration and rate of loading
on strength of wood and wood based materials. Res. Pap.