FUNDAMENTAL CONSIDERATIONS AND DEFINITIONS


Study of the mechanical properties of a material is concerned
mostly with its behavior in relation to stresses and strains,
and the factors affecting this behavior. A ~stress~ is a
distributed force and may be defined as the mutual action (1) of
one body upon another, or (2) of one part of a body upon another
part. In the first case the stress is _external_; in the other
_internal_. The same stress may be internal from one point of
view and external from 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