Saturday, March 19, 2011

Impact Testing of Metals

The impact test is designed to determine the toughness of a material and the two most commonly used methods are those using the ‘Charpy’ and ‘Izod’ impact-testing machines.

The Izod test  uses a notched specimen supported on one side only. The pendulum strikes the specimen that is not held by the machine. The Charpy test also uses a notched sample, but this is supported on both sides with the pendulum hammer striking the middle of the specimen.

Both tests use specimens of standard dimensions, which record the energy absorbed by the test piece on impact to give a measure of toughness. A brittle material will break easily and will absorb little energy, so the swing of the pendulum (which is recorded against a calibrated scale) will not be reduced significantly. A tough material will absorb considerably more energy, and greatly reduce the recorded pendulum swing.

Most materials show a drop in toughness with a reduction in temperature, though some materials show a rapid drop as the temperature is progressively reduced. This temperature range is called the Transition Zone, and components, which are designed for use at low temperature, should be operated above the material’s Transition Temperature.


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