Magnetic Particle Inspection (MPI)
Magnetic Particle Inspection (MPI) also sometimes called as Magnetic Test (MT) is a non-destructive test method for the detection of surface and sub-surface discontinuities in ferrous materials. The test method involves application of magnetic field externally or applying electric current through the material which in turn produces magnetic flux in the material. Simultaneously, visible ferrous particles on sprinkled or sprayed on the test surface. The presence of a surface or near surface discontinuities in the material causes distortion in the magnetic flux which in turn causes leakage of the magnetic fields at the discontinuity. The magnetic particles are attracted by the surface field in the area of the discontinuity and adhere to the edges of the discontinuity appearing the shape of the discontinuity.
A current flow method through contact heads, encircling coil magnetising, threaded bard magnetising are the examples of magnetising methods on a magnetic particle bench. The most common method utilised in general industries is the magnetic flow method using electromagnetic yoke. The particles are often colored and usually coated with fluorescent dyes that are made visible with a hand-held ultraviolet (UV) light (black light). The test method using fluorescent coated particles is called as Fluorescent Magnetic Particle Inspection or test (FMPI) and the usage of other coloured particles is termed as colour contrast Magnetic Particle Inspection or test (MPI).
Magnetic Particle Inspection (MPI) is the economical and comparative faster non-destructive test method used widely in Aerospace, Locomotive, automotive, power generation, nuclear, petrochemical industries. The most common examples are testing of crank shafts, cam shafts, connecting rods, engine gears, landing gear, bearing caps, engine blocks, motor shafts, engine bolts, nuts, washers, threaded bars, studs, piping joints ( fabricated joints, welds) in power generation and petrochemical industries, etc.
Magnetic particle test (MT) is very sensitive test method. It can detect tight in-service fatigue cracks in rotating parts or creep cracks on steam piping. Magnetic Particle Inspection cannot be used for non-ferrous materials and non-magnetic ferrous materials such as austenitic stainless steels.