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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