Magnetic Particle InspectionMagnetic Particle Testing | Magnetic Particle Inspection | Wet Fluorescent Magnetic Particle Testing (WFMPT)
For detection of surface and near-surface flaws in ferromagnetic material, Magnetic Particle Testing or MPT is used. Hand-held prods, flexible cables, electromagnet or a permanent magnet is used to apply a magnetic field to the specimen, which is local or overall. Most of the magnetic flux thus generated is concentrated just below the surface of the material if the material is sound. However in the event of the material being flawed, the flux generated interacts with the magnetic field and is distorted locally. The flux then â€œleaksâ€ from the surface of the region and is accumulated in the region of the flaw. A visible indication of the flaw is created by the fine magnetic particles, which are applied to the surface of the specimen, when they are attracted to the area of flux leakage. However, deeper subsurface flaws cannot be detected satisfactorily by this process. This is due to the fact that influence of the distorted lines of magnetic flux on the magnetic particles spread over the surface of the specimen become weaker with distance. Thus, the sensitivity falls away rapidly with the increasing depth of the flaw.
The most commonly used material for this purpose are black iron particles and red or yellow iron oxides. To view the material under UV lamp in darkened condition, the iron particles are coated with fluorescent material. The main advantages of this inspection are: Simple and easy technique Free from restrictions with respect to size, shape, composition and heat treatment of the ferromagnetic specimen. The prime application areas of MPT inspection are in structural steel, automotive, petrochemical, power generation and aerospace industries in addition to castings, forgings and welding among others. The most commonly revealed defects using this technique are quenching cracks, thermal cracks, seams, laps, grinding cracks, overlaps, non-metallic inclusions, fatigue cracks and hot tears among others.