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.
Magnetic Particle Inspection (MPI) is the economical and comparative faster non-destructive test method. 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.
Advantages & Disadvantages
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.