Magnetic Particle

Magnetic Particle (MT, MPI) is an NDT process which detects surface and near-surface discontinuities in ferromagnetic parts by putting a magnetic field into the material being tested. Materials which are ferromagnetic, such as iron, are comprised of unpaired electrons, which each have a small magnetic field. These fields align with each other in the presence of an external magnetic field.

To understand how Magnetic Particle testing works, it is best to consider the basic principles of a bar magnet, which has both a north and south pole. A pole is any location on the magnet in which a magnetic line of force enters or exits. If this same magnet was broken in half, it would result in two new magnets, each with its own north and south poles. If the magnet was merely cracked, and not entirely broken in half, a north and south pole would form at the edges of the crack. The air gap between the north and south poles in the crack causes the magnetic field to spread out, since air cannot support the same amount of magnetic field as the magnet. This is known as a flux leakage field.

Ferrous particles are then applied to the surface of the part. These particles will adhere to the north and south poles at the ends of the magnet and also to the poles in the crack. The presence of the particles makes the crack visually apparent to the inspector.

The basic steps for Magnetic Particle inspection are: 

  1. Clean the product of oil and other contaminants
  2. Determine the amount of electrical current required to magnetize the part, according to the required specification 
  3. The current is applied for a minimum of .5 seconds, during which time the part is washed with the ferrous particles.
    (Orbit offers Magnetic Particle inspection using the wet continuous method and therefor applies the particles in an oil suspension) 
  4. UV-A lamp is applied while the inspector visually examines the part for defects 
  5. The product is accepted or rejected based on pre-determined criteria
  6. The part is demagnetized, which slowly reduces the magnetic field to zero

Typically the requirements of the test require the orientation of the magnetic field to be shifted 90 degrees to inspect for indications that could not be detected from steps 3-5

Orbit offers both fluorescent and non-fluorescent magnetic particle inspection at the following locations:

Orbit Industries, Inc.
6840 Lake Abram Drive, Middleburg Heights, OH 44130 

 Primary Inspection Services  Additional Services Available: 
 Wet Particle   Saw Cutting
      (1) 17' MPI Bed  Grinding
      (1) 8' MPI Bed   Indication Removal
   Level III Consulting
   Technical Services

OrbitNDT-Bedford Heights 
23800 Corbin Drive, Bedford Heights, OH 44128

 Primary Inspection Services  Additional Services Available: 
 Wet Particle   Shot Peening (via National Peening, same facility)
      (1) 17' MPI Bed  Blasting