MT: Magnetic particle inspection (MPI) can be used for the detection of surface and near-surface flaws in ferromagnetic materials. Using a permanent magnet, electromagnet, flexible cables or hand-held prods a magnetic field is applied to the item under test. If a flaw is present the magnetic flux is distorted and 'leaks'. Fine magnetic particles, (normally in spray form in carrier fluid) can be applied to the surface of the specimen, are attracted to the area of flux leakage creating a visible flaw indication. It is recommended that the inspection surface is magnetised in at least two perpendicular directions at 90° to each other, due to lack of disturbance to the magnetic field if the crack runs parallel to the magnetic field
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