Magnetic Particle Inspection

Magnetic particle inspection (often abbreviated MT or MPI) is a nondestructive inspection method that provides detection of linear flaws located at or near the surface of ferromagnetic materials. It is viewed primarily as a surface examination method.
Basic principle
The part to be inspected is magnetized which produces lines of force or flux lines within the item. Flaws and imperfections in the item distort these flux lines causing them to ‘leak’ out. These areas of flux leakage create regions of magnetic polarity on the surface of the item, and the magnetic particles applied to the surface collect at these areas forming a visible indication seen by the inspector. Two separate magnetizations in different directions are necessary to locate flaws in all orientations. The magnetic particles are very small ferromagnetic oxides that are chemically dyed to provide contrast against the background of the part being inspected.

Magnetic Particle Inspection
MT has several advantages. The method is quick and simple to perform. The results are instantaneous and the indications appear directly on the surface of the part. This method of inspection may work through coatings such as cadmium and chrome plating and black oxide.
The single largest limitation to this method is that it is applicable only to ferromagnetic materials (steel). Nonferrous materials (aluminum, titanium, magnesium, copper, etc.) cannot be inspected using this method.
Demagnetization is a necessary step to remove residual magnetism that may interfere with subsequent processes (e.g., plating, welding). This is typically accomplished using an AC coil.

Magnetizing and inspecting parts

MT MPI MIL Surface Crack

Identifying surface crack of metal forging