Method and apparatus for mapping stress within ferrromagnetic materials by analyzing Barkhausen noise formed by the introduction of magnetic fields

A system for identifying and measuring stress at specific locations within a ferromagnetic material by the production and detection of Barkhausen noise during magnetization of the material. The method involves introducing into the material a time varying magnetic field component and a spatially-varying magnetic field component that in combination create a singular zero magnetic flux element within the material that "sweeps" from one position within the material to another and back. By time-gating the detected acoustic Barkhausen signal that results as this zero flux element moves within the material, the signal is isolated as having been generated at a specific depth within the specimen. Since the level of Barkhausen noise varies with stress, a tomographic mapping of stress is obtained for the interior of the ferromagnetic specimen. Through the use of a computer, a three dimensional map of stress in the ferromagnetic material is obtained.