Magnetic Particle Testing - (MT)

What MT can do?
•Magnetic particle testing is an NDT method for detecting discontinuities that are primarily linearand located at or near the surfaceof ferromagneticcomponents and structures.
•Ferromagnetic metals:–Metals that are strongly attracted to a magnet and can become easily magnetized such as iron, steel, nickel, and cobalt•Onlyferromagnetic materials can be effectively inspected by MT.

Materials according to Magnetic Property
•Ferromagnetic
•Paramagnetic
–Paramagnetic materials are attractedto magnetic fields; do not retain any magnetization in the absence of an externally applied magnetic field; Al, Ca, Pt, Mg, U, O, some stainless steels, ..
•Diamagnetic
–Diamagnetic materials will be weakly repelledby a magnetic field; water, DNA, most organic compounds such as petroleum and some plastics, and many metals such as mercuryand gold
•Nonferromagnetic
–Do not react to a magnetic field; majority of metals and other nonmetallic materials

Magnetic particle inspection (MPI) is used for the detection of surface and near-surface flaws in ferromagnetic materials. A magnetic field is applied to the specimen, either locally or overall, using a permanent magnet, electromagnet, flexible cables or hand-held prods. If the material is sound, most of the magnetic flux is concentrated below the material's surface. However, if a flaw is present, such that it interacts with the magnetic field, the flux is distorted locally and 'leaks' from the surface of the specimen in the region of the flaw. Fine magnetic particles, applied to the surface of the specimen, are attracted to the area of flux leakage, creating a visible indication of the flaw.

The materials commonly used for this purpose are black iron particles and red or yellow iron oxides. In some cases, the iron particles are coated with a fluorescent material enabling them to be viewed under a UV lamp in darkened conditions.