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 examples of brittle materials.  Sometimes the word brash is used to describe this condition in wood.  A brittle wood breaks suddenly with a clean instead of a splintery fracture and without warning.  Such woods are unfitted to resist shock or sudden application of load.

The measure of the stiffness of wood is termed the modulus of elasticity (or coefficient of elasticity).  It is the ratio of stress per unit of area to the deformation per unit of
        { unit stress }
length. { E = ------------- } It is a number indicative of
        { unit strain }
stiffness, not of strength, and only applies to conditions within the elastic limit.  It is nearly the same whether derived from compression tests or from tension tests.