It is the ability of a material to absorb energy at break after receiving an impact at a certain temperature. It measures the material resilience.
The Charpy test is the one that is made to measure the resistance to the flexion by shock; it consists of measuring the energy absorbed at the breaking of a notched specimen of normalized dimensions by striking it with a hammer (pendulum). The resilience values (KU or KV) are measured in Joules, and the pendulum energy difference before and after the fracture is measured. In USA it is more common to use the Izod test.
This feature is the toughness of the material. In materials such as steel, where temperature has great relevance to the toughness, it allows us to know the ductile-brittle transition curve that determines the functionality of the pieces in different work environments, of great importance in structural steels.