A specimen under test will exhibit different properties, depending on the rate at which the load is applied.
Because properties are strain-rate dependent, tests have been standardised to determine the energy required to break materials used sudden blows. These are termed impact tests.
The result is calculated based on the energy required to break the specimen. This can be calculated if one knows the initial energy and final energy or the initial angle and final angle of the object used to break the specimen (pendulum). The Izod and Charpy tests are commonly used to measure impact strength.
They differ only slightly, the configuration and specifications of the test specimen.