environ.science

2. DEFINITION
 The mechanical properties of wood are its
fitness and ability to resist applied or
external forces
 The mechanical properties of wood
considered are
 (1) stiffness and elasticity, (2) tensile
strength, (3) compressive or crushing
strength, (4) shearing strength, (5)
transverse or bending strength, (6)
toughness, (7) hardness, (8)
cleavability, (9) resilience.

3. STIFFNESS

 The property by means of which a body acted upon by
external forces tends to retain its natural size and shape, or
resists deformation.
 Thus a material that is difficult to bend or otherwise
deform is stiff; one that is easily bent or otherwise
deformed is flexible. Flexibility is not the exact counterpart
of stiffness, as it also involves toughness and pliability.

4. TENSILE STRENGTH

The tensile strength of wood parallel to the
grain depends upon the strength of the fibers
and is affected not only by the nature and
dimensions of the wood elements but also by
their arrangement.

5. COMPRESSIVE OR
CRUSHING STRENGTH

 Is very closely related to hardness and
transverse shear.
 There are two ways in which wood is
subjected to stress of this kind, namely, (1)
with the load acting over the entire area of
the specimen, and (2) with a load
concentrated over a portion of the area.

6. SHEARING STRENGTH

 Whenever forces act upon a body in such a way that
one portion tends to slide upon another adjacent to it
the action is called a shear. In wood this shearing
action may be (1) along the grain, or (2) across the
grain.

7. TRANSVERSE OR BENDING
STRENGTH: BEAMS

 When external forces acting in the same plane are
applied at right angles to the axis of a bar so as to
cause it to bend, they occasion a shortening of the
longitudinal fibers on the concave side and an
elongation of those on the convex side.

8. FAILURES IN BEAMS
 (1) Simple tension, in which there is a direct pulling in two
of the wood on the underside of the beam due to a tensile
stress parallel to the grain
 (2) Cross-grained tension, in which the fracture is caused
by a tensile force acting oblique to the grain. This is a common
form of failure where the beam has diagonal, spiral or other
form of cross grain on its lower side.
 (3) Splintering tension, in which the failure consists of a
considerable number of slight tension failures, producing a
ragged or splintery break on the under surface of the beam.
This is common in tough woods.

9.  (4) Brittle tension, in which the beam fails by a clean break extending
entirely through it. It is characteristic of a brittle wood which gives way
suddenly without warning, like a piece of chalk.
 (5) Compression failure has few variations except that it appears at
various distances from the neutral plane of the beam. It is very common
in green timbers. The compressive stress parallel to the fibers causes
them to buckle or bend as in an endwise compressive test.
 (6) Horizontal shear failure, in which the upper and lower portions of
the beam slide along each other for a portion of their length either at one
or at both ends is fairly common in air-dry material and in green material
when the ratio of the height of the beam to the span is relatively large.

11. TOUGHNESS:
TORSION

 Wood that is difficult to split is said to be tough
 Toughness includes flexibility and is the reverse of
brittleness, in that tough woods break gradually and give
warning of failure.
 Toughness is dependent upon the strength, cohesion,
quality, length, and arrangement of fiber, and the
pliability of the wood

12. HARDNESS

The term hardness is used in two senses, namely:
(1) resistance to indentation, and (2) resistance to
abrasion or scratching
In the latter sense hardness combined with
toughness is a measure of the wearing ability of
wood and is an important consideration in the
use of wood for floors, paving blocks, bearings,
and rollers.

13. CLEAVABILITY

 Cleavability is the term
used to denote the facility
with which wood is split.
A splitting stress is one in
which the forces act
normally like a wedge.

14. RESILIENCE

 Is the amount of work done upon a body in
deforming it. Within the elastic limit it is also a
measure of the potential energy stored in the
material and represents the amount of work the
material would do upon being released from a
state of stress