Magnetic particle inspection (MPI) is a non-destructive testing (NDT) process for detecting surface and subsurface discontinuities in ferrous materials. The process puts a magnetic field into the part that can be magnetized by direct or indirect magnetization. Direct magnetization occurs when the electrical current is passed through the test object and a magnetic field is formed in the material. Indirect magnetization occurs when no electrical current is passed through the test object but rather when a magnetic field is applied from an outside source. The magnetic lines of force are perpendicular to the direction of the electrical current, which may be either alternating current (AC) or some form of direct current (DC, or rectified AC).
The presence of a surface or subsurface discontinuity in the material allows the magnetic flux to leak. Ferrous iron particles are applied to the part being tested. The particles may be dry or in wet suspension. If an area of flux leakage is detected, the particles are attracted to this area.