Compression testing wood
Wood has unique, independent properties in the three mutually perpendicular axes:
longitudinal, radial, and tangential. The longitudinal axis is parallel to the grain, the
tangential axis is perpendicular to the grain but tangent to the annual rings, and the radial
axis is normal to the annual rings (and perpendicular to the grain direction).
Modulus of elasticity
The modulus of elasticity in the longitudinal direction is the highest, and is between 1-2 x
106 psi (6.9 – 13.8 x 103 MPa) at 12% moisture content, depending on the species. The
modulus of elasticity of many hardwoods (e.g. Birch, Hickory, and Oak) is closer to or
higher than the high range, but that of many softwoods (e.g. Douglas Fir, Hemlock,
Redwood, and Loblolly Pine) is closer to the low range. The modulus of elasticity (both
parallel and perpendicular to the grain) depends on the moisture content, and decreases
(from dry condition) by 1-3% for every 1% increase in the moisture content. For
example, the modulus of elasticity (parallel to the grain) of Western Hemlock at 12%
moisture content is 1.63 x 106 psi, whereas it is 1.31 x 106 psi in green condition. The
average modulus of elasticity of higher grade lumber (Construction, No. 1, etc.) is nearly
the same as that of clear wood. In a particular species, the modulus of elasticity (as well
as other mechanical properties) increases with the amount of summerwood. The modulus
of elasticity in the radial direction is about 10% of that in the longitudinal direction, and
that in the tangential direction is about one-half of that in the radial direction.
Compressive strength
The compressive strength of lumber parallel to the grain is much higher than that
perpendicular to the grain. Columns, posts and members of a truss are subjected to axial
loads parallel to the grain of the wood. When a column rests on a beam, the load from the
column creates compressive (bearing) stress on the beam that is perpendicular to the
grain of the wood.
The compressive strength of most softwood lumber parallel to the grain is in the range of
2000-4000 psi. The stress at the proportional limit is about 80% of this strength. The
compressive strength perpendicular to the grain varies between 12-18% of that parallel to
the grain. The compressive stress at the proportional limit, for loading perpendicular to
the grain, is about 12-25% of that for loading parallel to the grain. In dry condition,
lumber can possess significantly higher compressive strength. For example, the
compressive strength (parallel to the grain) of Douglas fir in green condition is about
3700 psi, while that at 12% moisture content is about 7000 psi. Seasoning increases the
compressive strength. Typically, the compressive strength (parallel and perpendicular to
the grain) increases an average 4-6% for every 1% decrease in moisture content. Species
such as Oak, Maple, Douglas Fir, Southern Yellow Pine, and Western Larch possess very
high compressive strengths.
TEST
Purpose: To determine the parallel-to-grain compressive strength of a clear wood
specimens.
Related Standard: ASTM D143
Equipment: Universal testing machine, compressometer.
Sample: Clear wood specimen of Alder, Walnut, Maple, Pine and Fir will be tested. Six
samples of each, with nominal dimensions ¾” x ¾” x 3” will be tested.
Procedure:
1.
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5.
The ends of the specimen should be level
Measure the distance between grain, and the angle of the grain.
Position the specimen under the crosshead of the testing machine, and apply load
continuously at 0.024 in. /min.
Continue the loading until the proportional limit is well passed.
Get the deformation readings to the nearest 0.001 in.
Report:
1. Calculate the compressive strength and elastic modulus of each wood
specimen.
2. Calculate the average compressive strength and elastic modulus, and the
standard deviation of each.
3. Make a table listing the compressive strength and elastic modulus for each type
of wood tested.
4. For each type of wood (Maple, Oak, Fir, Pine), make a graph of breaking
strength vs. thickness of the grain, and breaking strength vs. angle of the grain
with respect to the longitudinal axis. Discuss how the strength varies with each of
these variables.
5. Compare the failure strength of each wood, and comment on the variation of
compressive strength with respect to wood type (e.g., compare the strength of all
the fir samples, and compare differences between fir and walnut and…).
Comment on how variation of properties should be allowed for in the design
process.