increased a certain amount, which will be a percent-
age of the original length.
Stress occurs because molecular forces within the
material resist the change of shape that an applied
load tends to produce. In other words, stress results
from the resistance of the molecules to being shifted
around, pulled apart, or squeezed together. Because
stress involves molecular forces, a piece of metal that
is subjected to a load develops an enormous number
of stresses, rather than just one stress. If you had more
than a very few molecules, you would have to draw
thousands or perhaps millions of arrows to indicate all
the molecular forces involved. We often speak of
stress as though it were one internal force, acting in
one direction; that is, the direction opposite to the
direction of the applied load. In other words, we con-
sider the TOTAL EFFECT of all the molecular
stresses, rather than trying to consider each set of
molecular stresses separately.
Figure 6-1.--Tension forces and tension stresses.
The manner in which the load is applied deter-
mines the type of stress that will develop. Applied
forces are usually considered as being of three basic
kinds: tension (or tensile) forces, compression forces,
and shearing forces. The basic stresses, therefore, are
tension (or tensile) stresses, compression stresses, and
shearing stresses. Complex stresses such as bending
or more of the basic stresses.
Tension stresses develop when a material is sub-
jected to a pulling action. If, for example, a cable is
fastened to an overhead clamp and a weight is at-
tached to the free end, tension stresses develop within
the cable. The tension stresses resist the tension forces
that tend to pull the cable apart. Figure 6-1 shows
tension forces and the resulting "equal and opposite"
Figure 6-2.--Compression forces and compression stresses.
Compression stresses develop within a material to
oppose the forces that tend to compress or crush the
material. A column that supports an overhead weight
is said to be in compression, and the internal stresses
Shearing stresses develop within a material when
that develop within the column are compression
opposite external forces are applied along parallel
stresses. Figure 6-2 shows compression forces and
lines in such a way as to tend to cut the material.
Shearing forces tend to separate material by sliding