PHYSICAL AND MECHANICAL PROPERTIES OF CLEAR WOOD
Moisture Relations
Wood is a hygroscopic material which contains water in varying
amounts, depending upon the relative humidity and temperature of the
surrounding atmosphere. Equilibrium conditions are established as
shown in Table 6.7.1. The standard reference condition for wood is
oven-dry weight, which is determined by drying at 100 to 105°C until
there is no significant change in weight.
Moisture content is the amount of water contained in the wood, usually
expressed as a percentage of the mass of the oven-dry wood. Moisture
can exist in wood as free water in the cell cavities, as well as water
bound chemically within the intermolecular regions of the cell wall. The
moisture content at which cell walls are completely saturated but at
which no water exists in the cell cavities is called the fiber saturation
point. Below the fiber saturation point, the cell wall shrinks as moisture
is removed, and the physical and mechanical properties begin to change
as a function of moisture content. Air-dry wood has a moisture content of
12 to 15 percent. Green wood is wood with a moisture content above the
fiber saturation point. The moisture content of green wood typically
ranges from 40 to 250 percent.
Dimensional Changes
Shrinkage or swelling is a result of change in water content within the cell
wall. Wood is dimensionally stable when the moisture content is above
the fiber saturation point (about 28 percent for shrinkage estimates).
Shrinkage is expressed as a percentage of the dimensional change based
on the green wood size. Wood is an anisotropic material with respect to
shrinkage. Longitudinal shrinkage (along the grain) ranges from 0.1 to
0.3 percent as the wood dries from green to oven-dry and is usually
neglected. Wood shrinks most in the direction of the annual growth
rings (tangential shrinkage) and about half as much across the rings (radial
shrinkage). Average shrinkage values for a number of commercially
important species are shown in Table 6.7.2. Shrinkage to any moisture
condition can be estimated by assuming that the change is linear from
green to oven-dry and that about half occurs in drying to 12 percent.
Swelling in polar liquids other than water is inversely related to the size
of the molecule of the liquid. It has been shown that the tendency to
hydrogen bonding on the dielectric constant is a close, direct indicator
of the swelling power of water-free organic liquids. In general, the
strength values for wood swollen in any polar liquid are similar when
there is equal swelling of the wood.
Swelling in aqueous solutions of sulfuric and phosphoric acids, zinc
chloride, and sodium hydroxide above pH 8 may be as much as 25
percent greater in the transverse direction than in water. The transverse
swelling may be accompanied by longitudinal shrinkage up to 5 percent.
The swelling reflects a chemical change in the cell walls, and the
accompanying strength changes are related to the degradation of the
cellulose.
Dimensional stabilization of wood cannot be completely attained. Two
or three coats of varnish, enamel, or synthetic lacquer may be 50 to 85
percent efficient in preventing short-term dimensional changes. Metal
foil embedded in multiple coats of varnish may be 90 to 95 percent
efficient in short-term cycling. The best long-term stabilization results
from internal bulking of the cell wall by the use of materials such as
phenolic resins polymerized in situ or water solutions of polyethylene
glycol (PEG) on green wood. The presence of the bulking agents alters
the properties of the treated wood. Phenol increases electrical resistance,
hardness, compression strength, weight, and decay resistance but
lowers the impact strength. Polyethylene glycol maintains strength
values at the green-wood level, reduces electric resistance, and can be
finished only with polyurethane resins.
Mechanical Properties
Average mechanical properties determined from tests on clear, straightgrained
wood at 12 percent moisture content are given in Table 6.7.2.
Approximate standard deviation(s) can be estimated from the following
equation:
s 5 CX
where X 5 average value for species
0.10 for specific gravity
0.22 for modulus of elasticity
0.16 for modulus of rupture
C 5
ì
í
î
0.18 for maximum crushing strength parallel to grain
0.14 for compression strength perpendicular to grain
0.25 for tensile strength perpendicular to grain
0.25 for impact bending strength
0.10 for shear strength parallel to grain Relatively few data are available on tensile strength parallel to the grain.
The modulus of rupture is considered to be a conservative estimate for
tensile strength.
Mechanical properties remain constant as long as the moisture content
is above the fiber saturation point. Below the fiber saturation point,
properties generally increase with decreasing moisture content down to
about 8 percent. Below about 8 percent moisture content, some properties,
principally tensile strength parallel to the grain and shear strength,
may decrease with further drying. An approximate adjustment for clear
wood properties between about 8 percent moisture and green can be
obtained by using an annual compound-interest type of formula:

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