considerably below its ultimate strength in tension,
compression, or shear. For example, you can break a
determine the amount of elongation that will occur
thin rod with your hands after it has been bent back
when a given stress is developed in the material. For
and forth several times in the same place, although you
this purpose, you divide the stress by the modulus of
could not possibly cause an identical rod to fail in
elasticity to obtain the elongation (inch per inch) that
tension, compression, or shear merely from force
will occur.
applied by hand. This tendency of a material to fail
Closely related to the elastic limit of a material is
after repeated stressing at the same point is known as
the YIELD POINT. The yield point is the stress at
FATIGUE.
which deformation of the material first increases
markedly without any increase in the applied load. The
METAL FATIGUE
yield point is always somewhat above the elastic limit.
When the stresses developed in a material are greater
Metal fatigue is the tendency for a metal to break
than the yield point (or, as it is sometimes called, the
under the action of repeated cyclic stresses. Fatigue
yield strength), the material is permanently deformed.
may occur for values of cyclic stress considerably less
than the ultimate tensile strength of the material. This
STRENGTH
phenomenon applies to certain fractures in metals that
are caused by repeated stresses of a low enough value
Strength is the property that enables a material to
that a single application of the stress apparently does
resist deformation. ULTIMATE STRENGTH is the
nothing detrimental to the structure. When enough of
maximum stress that a material is capable of
these seemingly harmless stresses are applied in a
withstanding in tension, compression, or shear. The
cyclic manner, however, they bring about a small
COMPRESSIVE STRENGTH of a metal is a measure
of how much squeezing force it can withstand before
it fails. The metal to be tested is mounted in a tensile
tester, but instead of pulling on the metal, a squeezing
( c o m p r e s s i o n ) force is applied. TENSILE
STRENGTH, or the ultimate strength of a material in
tension, is the term most frequently used to describe
the strength of a material. Tensile strength is the
ability of a metal to resist being pulled apart. This
property may be measured on a tensile testing
machine, which puts a stretching load on the metal.
Figure 6-9 illustrates the types of loads imposed on
structures.
Table 6-2 shows how the tensile strength,
elongation (explained below), and yield point are
affected by the carbon content of steel. As the carbon
content increases, the tensile strength and yield point
first increase then decrease.
Some materials are equally strong in compression,
tension, and shear. However, many materials show
marked differences. For example, cured portland
cement has an ultimate strength of 2,000 psi in
compression, but only 600 psi in tension. Carbon steel
has an ultimate strength of 56,000 psi in tension and
in compression, but an ultimate strength in shear of
only 62,000 psi. In dealing with ultimate strength,
therefore, the kind of loading (tension, compression,
or shear) should always be stated.
If a material is stressed repeatedly, in a cyclical
Figure 6-9.--Types of stresses or loads imposed on structures:
manner it will probably fail at a loading that is
compression, tension, shear, torsion, bending, and fatigue.
6-8