The Charpy test provides a measure of the energy required to break a material under impact loading. It was first standardized some 60 years ago to allow comparison between different types of steel made from a variety of manufacturing processes.

The test consists essentially of a hammer with a given amount of energy striking a notched test piece of fixed dimensions and recording the energy required to fracture the test piece at a specific temperature and recording whether the fracture mode was ductile or brittle.

Before carrying out a Charpy impact test, you might wish to review some historical evidence associated with brittle fracture behavior and the effects of a notch on stresses in a tensile test.

Recall that a shear stress across a crystal will cause dislocations present in the material to move along slip planes. On the other hand cleavage of crystal planes is favored by tensile stress, fracture of mica or glass, for example.

Ductile fracture is usually a consequence of plastic deformation. A ductile crack is one which spreads as a result of intense localized plastic deformation at the crack tip.

The most common form is a 'cup-and-cone' appearance in a tensile test. Brittle fracture in contrast is often preceded by little or no plastic deformation. Intercrystalline fracture occurs by separation (or loss of cohesion) along grain-boundaries.