Magnetic Particle Testing Method
Magnetic particle Inspection (MPI) detects surface and slightly subsurface discontinuities in ferromagnetic materials such as iron, nickel, cobalt, and others made from these alloys. A high-amperage, low-voltage current is passed through the part, which establishes a magnetic field. Magnetic flux is introduced to the part. Since cracks and defects have different magnetic properties than those of the surrounding material, their presence causes distortion by interrupting the magnetic field, thereby indicating the shape and position of the crack or void.
“Mag” particle testing puts a magnetic field into the part and the part can be magnetized by direct or indirect magnetization. Direct magnetization passes electric current 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 or some form of direct current.
The presence of a surface or subsurface discontinuity in the material allows magnetic flux to leak, since air cannot support as much magnetic field per unit volume as metals. Ferrous particles are then applied to the part. The particles may be dry or in a wet suspension. If an area of flux leakage is present, the particles will be attracted to this area. The particles will build up at the area of leakage and form what is known as an indication. The indication can then be evaluated to determine what it is, what may have caused it, and what action should be taken, if any.
Contact American Metal Testing
Contact our knowledagebe staff on your Magnetic Particle inspection needs today.