Magnetic Particle

Magnetic Particle Testing

Magnetic particle Testing (MPT) is a process for detecting surface and shallow subsurface discontinuities in ferromagnetic materials such as iron, nickel, cobalt, and some of their alloys. The process puts a magnetic field into the part.

The presence of a surface or subsurface discontinuity in the material allows the magnetic flux to leak since air cannot support as much magnetic field per unit volume as metals.

This NDT method is based on the principle of Magnetic Flux Leakage in air. MPT can only be carried out upon ferromagnetic material (Materials with high magnetic permeability).

During magnetic particle testing, a ferromagnetic test specimen is magnetized with a strong magnetic field created by a magnet or special equipment. If the specimen has a discontinuity, the discontinuity will interrupt the magnetic field flowing through the specimen and a leakage field will occur.

Finely milled iron particles coated with a dye pigment are applied to the test specimen with or without carrier fluid. These particles are attracted to leakage fields and will cluster to form an indication directly over the discontinuity. This indication can be visually detected under proper lighting conditions.

To identify a leak, ferrous particles, either dry or in a wet suspension, are applied to a part. These are attracted to an area of flux leakage and form an indication, which is evaluated to determine its nature, cause, and course of action if any.

  • Yoke – Longitudinal magnetic field
  • Prod method – Circular magnetic field
  • Headshot / Clamp method – Circular magnetic field
  • Cable Wrap Method – Longitudinal Magnetic field
  • Central Conductor Method – Circular magnetic field
  • MPT Test Bench – Circular + Longitudinal (Combined) field

Advantages

  • Can detect both surface and near sub-surface defects.
  • Can inspect parts with irregular shapes easily.
  • The fast method of inspection and indications are visible directly on the specimen surface.
  • Considered low cost compared to many other NDT methods.
  • Is a very portable inspection method especially when used with battery-powered equipment.
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