the limit beyond which it is
impossible to carry the distortion of a body without producing a
permanent alteration in shape. After this limit has been
exceeded, the size and shape of the specimen after removal of
the load will not be the same as before, and the difference or
amount of change is known as the ~permanent set~.

Elastic limit as measured in tests and used in design may be
defined as that unit stress at which the deformation begins to
increase in a faster ratio than the applied load. In practice
the elastic limit of a material under test is determined from
the stress-strain diagram. It is that point in the line where
the diagram begins perceptibly to curve.[2] (See Fig. 1.)

[Footnote 2: If the straight portion does not pass through the
origin, a parallel line should be drawn through the origin, and
the load at elastic limit taken from this line. (See Fig. 32.)]

~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. This may be graphically represented by a
diagram in which the abscissae represent the amount of deflection
and the ordinates the force acting. The area included between
the stress-strain curve and the initial line (which is zero)
represents the work done. (See Fig. 1.) If the unit of space is
in inches and the unit of force is in pounds the result is
inch-pounds. If the elastic limit is taken as the apex of the
triangle the area of the triangle will represent the ~elastic
resilience~ of the specimen. This amount of work can be applied
repeatedly and is perhaps the best measure of the toughness of
the wood as a working quality, though it is not synonymous with
toughness.

Permanent set is due to the ~plasticity~ of the material. A
perfectly plastic substance would have no elasticity and the
smallest forces would cause a set. Lead and moist clay are
nearly plastic and wood possesses this property to a greater or
less extent. The plasticity of wood is increased by wetting,
heating, and especially by steaming and boiling. Were it not for
this property it would be impossible to dry wood without
destroying completely its cohesion, due to the irregularity of
shrinkage.

A substance that can undergo little change in shape without
breaking or rupturing is ~brittle~. Chalk and glass are common