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Strength
/ Mechanics
of Materials Table of Content
The quality known as toughness
describes the way a material
reacts under sudden impacts. It is defined as the
work required to deform one cubic inch of metal until
it fractures. Toughness is measured by the Charpy
test or the Izod test.
Both of these tests use a notched sample. The
location and shape of the notch
are standard. The points
of support of the sample, as well as the impact of
the hammer, must bear a constant relationship to the
location of the notch.
The tests are conducted by mounting the
samples as shown in
Figure 8 and allowing a pendulum of a known
weight to fall from a set height. The maximum
energy developed by the hammer is 120 ft-lb in the Izod test
and 240 ft-lb in the Charpy
test. By properly calibrating the machine, the energy
absorbed by the specimen may be measured
from the upward swing of the pendulum after it has fractured
the material specimen as
shown in Figure 9. The greater the amount of energy absorbed
by the specimen, the smaller the
upward swing of the pendulum will be and the tougher the
material is.
Indication of toughness is relative and
applicable only to cases involving
exactly this type of sample
and method of loading. A sample
of a different shape will yield
an entirely different result. Notches
confine the deformation to a
small volume of metal that reduces
toughness. In effect, it is the
shape of the metal in addition to
the material composition that determines
the toughness of the material.
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