A shaft crack is a slowly growing fracture of the rotor. If undetected in an operating machine, a crack (also called a fatigue crack) will grow until the remaining, reduced cross section of the rotor is unable to withstand the dynamic loads that are applied to it. When this happens, the rotor will fail in a fast brittle fracture mode. The sudden failure will release the large amount of energy that is stored in the rotating system, and the rotor will fly apart. Shaft fractures have caused machine parts to penetrate the machine casing and even penetrate building walls. Damage due to this kind of failure is catastrophic and can cause serious injury or death to anyone unfortunate enough to be standing near the machine at the moment of failure. Obviously, shaft crack detection is a very serious matter, and machines that are suspected of having a crack must be treated with the utmost respect.