A notch in mechanical and materials engineering refers to a deliberately introduced v-shaped, u-shaped or circular defect in a planar material whereby stress is concentrated. Notch geometries play an important role in fracture mechanics and materials characterisation. Notches are most often found in impact testing of materials where the presence of a crack origin of controlled morphology is necessary in order to achieve standardised characterisation of fracture toughness. This is most commonly realized using the Charpy impact test, in which the impact of a pendulum hammer (striker) with a horizontal notched specimen and the height of its subsequent swing-through is used to infer the energy absorbed in fracture of the specimen. Impact testing of notched bars is also applied in the Izod impact strength test, typically applied to a cantilever configuration with a vertically positioned specimen. Charpy testing is conducting with U- or V-notches whereby the striker contacts the specimen directly behind the notch, whereas the now largely obsolete Izod method involves a circular notch facing the striker. Notched specimens find use in other characterization protocols, including tensile testing and fatigue.