The term fracture toughness is used in several ways. When used generically, it refers to the resistance of a crack to grow under stress. This definition is often used to describe the results of fracture toughness tests such as the Charpy impact test. Using the more rigorous definition, fracture toughness refers to strictly defined, mathematically determined results obtained from carefully pre-cracked test specimens that are then failed by the application of a load or force. Fracture toughness results, based on this more rigorous definition, are less common due to the relatively high costs of preparing and testing these specimens. In either case, fracture toughness measurements estimate the stress and the flaw, or crack, size that a structure will tolerate before it fails.