rom another. An external force is always
balanced by the internal stresses when the body is in
equilibrium.

If no external forces act upon a body its particles assume
certain relative positions, and it has what is called its
_natural shape and size_. If sufficient external force is
applied the natural shape and size will be changed. This
distortion or deformation of the material is known as the
~strain~. Every stress produces a corresponding strain, and
within a certain limit (see _elastic limit_, in FUNDAMENTAL
CONSIDERATIONS AND DEFINITIONS, above) the strain is directly
proportional to the stress producing it.[1] The same intensity
of stress, however, does not produce the same strain in
different materials or in different qualities of the same
material. No strain would be produced in a perfectly rigid body,
but such is not known to exist.

[Footnote 1: This is in accordance with the discovery made in
1678 by Robert Hooke, and is known as _Hooke's law_.]

Stress is measured in pounds (or other unit of weight or force).
A ~unit stress~ is the stress on a unit of the sectional
{ P }
area. { Unit stress = --- } For instance, if a load (P) of one
{ A }
hundred pounds is uniformly supported by a vertical post with a
cross-sectional area (A) of ten square inches, the unit
compressive stress is ten pounds per square inch.

Strain is measured in inches (or other linear unit). A ~unit
strain~ is the strain per unit of length. Thus if a post 10
inches long before compression is 9.9 inches long under the
compressive stress, the total strain is 0.1 inch, and the unit
l 0.1
strain is --- = ----- = 0.01 inch per inch of length.
L 10

As the stress increases there is a corresponding increase in the
strain. This ratio may be graphically shown by means of a
diagram or curve plotted with the increments of load or stress
as ordinates and the increments of strain as abscissae. This is
known as the ~stress-strain diagram~. Within the limit mentioned
above the diagram is a straight line. (See Fig. 1.) If the
results of similar experiments on different specimens are
plotted to the same scales, the diagrams furnish a ready means
for comparison. The greater the resistance a material offers to
deformation the steeper or nearer the vertical axis will be the
line.

[Illustration: FIG. 1.--Stress-strain diagrams of two longleaf
pine beams. E.L. = elastic limi