Strength Properties
Strength refers to the maximum stress that can be developed
in a member due to applied loads prior to failure. Mechanical properties most commonly measured and represented as
“strength properties” for design include modulus of rupture
in bending, tension strength parallel-to-surface, tension
strength perpendicular-to-surface, compression strength
parallel-to-surface, shear strength, fastener holding strength,
and hardness. Strength tests are typically made on specimens at moisture equilibrium under prescribed conditions
or after soaking. The procedures to determine strengths for
wood-based composites are described in ASTM D 1037,
ASTM D 3044, ASTM D 5456, ASTM D 3737, and
ASTM D 7031.
Modulus of rupture reflects the maximum load-carrying
capacity of a member in bending and is proportional to maximum moment borne by the specimen. Modulus of rupture
is an accepted measure of strength, although it is not a true
stress because the formula by which it is computed is valid
only to the elastic limit (McNatt 1973).
Tension strength parallel-to-surface is the maximum stress
sustained by a specimen from a test with tension forces applied parallel to the surface. Tests are made with the long dimension of the specimen cut both parallel and perpendicular
to the long dimension of the board to determine the strength
in each of the primary panel directions.
Tension strength perpendicular-to-surface (internal bond
strength) is the maximum stress sustained by a specimen
from a test with tension forces applied perpendicular to the
surface. Tests are made on specimens in the dry condition
to determine the resistance of the specimen to delamination
or splitting in the direction perpendicular to the plane of the
board.
Compression strength parallel-to-surface is the maximum
stress sustained by a specimen from a test with compression forces applied parallel to the surface. Tests are made
with the long dimension of the specimen cut both parallel
and perpendicular to the long dimension of the board to determine the material’s resistance to crushing in each of the
primary panel directions.
Interlaminar shear (planar shear) indicates the ability to
resist internal slipping of one layer upon another within the
panel. It is used to describe the glue line or bonding performance inside or between the test materials.
Hardness is measured as resistance to indentation using a
modified Janka hardness test, measured by the load required
to embed an 11.3-mm (0.444-in.) diameter ball to one-half
its diameter.
Fastener holding strength is the maximum resistance to
separate or withdraw a fastener in a plane normal to the
testing face. It usually contains three tests: nail withdrawal,
nail-head pull-through, and direct screw withdrawal.
