Fracture toughness is defined as the resistance of a material to crack initiation. This characteristic of a material, which represents the fracture toughness, is described with the critical stress intensity factor KIc . From this point on, the material initiates and propagates cracks and, in the worst case, a forced rupture occurs. In other words, it describes the risk of crack propagation in a component if the component that has an existing crack is subjected to a given nominal stress. The fracture toughness is determined in a compact tensile test in accordance with DIN EN ISO 12737. An angular notch is introduced centrally in a tensile test. A crack is then created by means of vibration stress. The extent of the crack is measured after the sample has ruptured. Also noted is the prevailing stress at which the crack became unstable.