Non Destructive Testing: Magnetic Particle Inspection

Magnetic particle inspection is used to detect surface/subsurface defects within ferromagnetic material (materials that are easily magnetized). Magnetic fields and small magnetic particles are used to detect flaws in components. This method of non destructive material testing is used to inspect a variety of product forms, including castings, forgings, and weldments.

The basic steps to performing this non destructive material testing are as follows:

  1. Pre-clean the area to be inspected.
    • The part must be dry and free of rust, dirt, scale, and oil.
  2. Introduce the magnetic field.
    • Use permanent magnets, an electromagnetic yoke, prods, a coil, or other means to establish the necessary magnetic flux.
  3. Apply magnetic particles.
    • Magnetic particles can be suspended in an oil/water solution or can be dry. They can also be in a visible dye form or fluorescent with the use of a black light.
  4. Visually inspect the area.
    • A skilled inspector must interpret indications that are formed after applying the magnetic field. This requires the person to distinguish between relevant and irrelevant indications.

Advantages of Magnetic Particle Inspection

  • Can detect both surface and near subsurface indications.
  • Can easily inspect parts with irregular shapes.
  • Pre-cleaning is not as critical as for some other material inspection methods.
  • Fast method of inspection and indications are visible directly on the specimen surface.
  • Considered low cost compared to many other NDT techniques.
  • Very portable inspection, especially when used with battery-powered equipment.

Disadvantages of Magnetic Particle Inspection

  • Only effective for the examination of ferromagnetic materials.
  • Discontinuity detection is limited to those at or near the surface.
  • Demagnetization may be required before, between, and after inspections.
  • Discontinuities will only be detected when the major axis interrupts the primary flux lines. This necessitates inspection in more than one direction to assure discontinuity detection regardless of orientation.
  • Paint and/or coating removal is necessary.
Contact DBI for a Magnetic Particle Inspection