What is Magnetic Particle Inspection?

Magnetic particle inspection (MPI) or also known as Magnetic Particle Testing (MT) in another part of the inspection world, is a non-destructive testing (NDT) process for detecting surface and low subsurface discontinuities in ferromagnetic materials such as iron, nickel, cobalt, and some of their alloys. The process sets a magnetic field into the part to be magnetized by direct or indirect magnetization.

How Does It Work?

When high levels of magnetic flux are applied to ferromagneticmaterials, surface breaking or low subsurface discontinuities will create leakage fields. These leakage fields attractmagnetic particles (which is actually finely divided magnetite) to themselvesleading to the formation of an indication.

The magnetic particles may be visibly or fluorescently pigmented to provide contrast with the substrate. On the contrary, the substrate may be lightly coated with a white background lacquer to contrast with the particles.

Fluorescent magnetic particles normally provide the greatest sensitivity in a liquid suspension, usually applied by spraying. In certain cases, dry particles may be applied by a gentle jet of air. The technique is applicable only to ferromagnetic materials at a temperature below the Curie point (about 650°C).

The leakage field will be highest for linear fluorescent pigmented the right angles to the magnetic field. Therefore, for a comprehensive test, the magnetic field must normally be white background and in two perpendicular directions.

Magnetic Particle Inspection is economical to use in terms of equipment cost and the swiftness of inspection.  Moreover, the level of operator training required is relatively low.

Advantages and Limitations

The advantages of Magnetic Particle Inspection are listed below;

  • Inexpensive equipment
  • Direct location of the defect
  • Surface conditions not critical
  • Can be applied without power
  • Low skill level
  • Sub-surface defects found 1-2mm
  • Quick, instant results
  • Hot testing (using dry powder)
  • Can be used in the dark (UV light)

The limitations are;

  • Only magnetic materials
  • May need to demagnetize components
  • Access may be a problem for the yoke
  • Need power if using a yoke
  • No permanent record
  • Calibration of equipment
  • Testing in two directions required
  • Need good lighting – 500 lux minimum

Here’s an interesting video explaining MPI;

There you have it, a short info about Magnetic particle inspection. If you have anything to add, please let us know in in the comment section below.

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