MAGNETIC PARTICLE TESTING (MT)

MT uses one or more magnetic fields to locate surface and near-surface discontinuities in ferromagnetic materials. The process puts a magnetic field into the test piece. The piece can be magnetized by direct or indirect magnetization. Direct magnetization occurs when the electric current is passed through the test object and a magnetic field is formed in the material. Indirect magnetization occurs when no electric current is passed through the test object, but a magnetic field is applied from an outside source. The magnetic lines of force are perpendicular to the direction of the electric current, which may be either alternating current (AC) or some form of direct current (DC) (rectified AC). The presence of a surface or subsurface discontinuity in the material allows the magnetic flux to leak, since air cannot support as much magnetic field per unit volume as metals. When the magnetic field encounters a discontinuity transverse to the direction of the magnetic field, the flux lines produce a magnetic flux leakage field of their own. To identify a leak, ferrous particles, either dry or in a wet suspension, are applied to a part. These are attracted to an area offlux leakage and form what is known as an indication, which is evaluated to determine its nature, cause, and course of action, if any.