The Charpy vee-notch impact test is the most common
fracture toughness test used by industry. A notched specimen is broken by a
swinging pendulum and the amount of energy required to break the specimen is recorded
in foot-pounds or joules. This is determined by measuring how far the pendulum
swings upwards after it fractures the specimen. If the specimen is tough, the pendulum
will only swing up a small distance since part of its energy has been absorbed
by the specimen. If the specimen is brittle it will absorb little energy thus
allowing the pendulum to swing up to almost its original height.
The amount of energy absorbed can be read directly
off of the dial indicator that is located on the machine. The specimen is
supported in place as shown and the pendulum strikes it from behind the notch.
ASTM E23 Standard Methods For Notched Bar Impact
Testing Of Metallic Materials).
Impact energy vs. temperature
The absorbed energy is the most common value reported;
however, the percent shear and the lateral expansion may also be noted. Metals
that exhibit a high Charpy vee notch value are typically those that are more
resistant to brittle fracture. It is important to remember that these tests are
comparative only and are no guarantee of ductile behavior in actual service.
The fractured ends of a specimen often reveal the
manner in which it fractured. If the specimen has fractured in a brittle manner
with low energy the faces will have a flat, crystalline and shiny surface. A tough
specimen will exhibit more deformation and will have a dull and fibrous surface.
Metals such as carbon and low alloy steels, exhibit a
change in failure mode with decreasing temperature. For this reason, it is
common to conduct impact tests over a range of specimen temperatures.
The performance of the material at different temperatures
can be observed and a conclusion made regarding the temperature below which the
material can no longer be used without a risk of brittle fracture. Them graph
shows the relationship between test specimen temperature and absorbed energy.
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