Mechanical properties of wood

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.

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.

STIFFNESS

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.

TENSILE 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.

COMPRESSIVE OR

CRUSHING 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.

SHEARING STRENGTH

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.

TRANSVERSE OR BENDING

STRENGTH: BEAMS

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.

(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.

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

TOUGHNESS:

TORSION

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.

HARDNESS

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.

CLEAVABILITY

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

RESILIENCE