Magnetic particle Inspection (MPI) is a non-destructive testing (NDT) process for detecting surface and slightly subsurface discontinuities in ferromagnetic materials such as iron, nickel, cobalt, and some of their alloys. Testing is performed to international standards such as EN473 and ISO9712 by qualified personnel. The process generates magnetic field in the testing part of the job. 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 discontinuity on the surface or subsurface in the material allows the magnetic flux to leak, since air cannot support as much magnetic field per unit volume as metals. Ferrous iron particles are then applied to the testing part. The particles may be dry or in a wet suspension. The particles will be attracted to the flux leakage area and will build up at the maximum leakage across the defect and will form configuration over the defect. The indication can then be evaluated to determine the defect.
This process works on Magnetic materials only. The surface or sub surface flaws can only be detected by this method .Mostly dry iron powder is sprayed on the portion where yoke is put on the job .The powder gathers and forms the configuration of flaws (if any) in the job, after the current is passed through the yoke.