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Magnetic Particle Inspection

Identifying Defects on Ferromagnetic Materials

Magnetic Particle InspectionMagnetic Particle Inspection, also called MPI Inspection, is a very sensitive nondestructive test method used to identify defects on the surface of ferromagnetic materials such as forgings, castings, weldments and machined or stamped parts.

Meeting Your Requirements

Laboratory Testing Inc. is accredited by PRI Nadcap and offers both Visible Dry and Wet Fluorescent Magnetic Particle Inspection methods.

  • Certified Level 2 and 3 inspectors evaluate the test results.
  • MPI Inspection is performed with highly reliable products from Magnaflux.
  • Materials to 12 ft. in length can be inspected in Magnaflux benches using a wet suspension.
  • A Certified Test Report is provided with each order.

Test Methods/Specifications

  • ASTM E1444
  • MIL-STD-1949
  • MIL-STD-271
  • MIL-STD-2132
  • AMS 2300 Series (Material Cleanliness)

The Magnetic Particle Inspection Process

In both the visible dry and wet fluorescent methods, the test is performed by spreading fine iron oxide particles with high magnetic permeability over the surface of a test piece as it is magnetized. The magnetic particles are applied in either of two ways:

  • Dry as a powder
  • Wet in a suspension, petroleum-based or water-based liquid or carrier

Discontinuities such as cracks, laps and seams in the material distort the surrounding magnetic field, causing the particles to be drawn to and collected in these areas for a visible indication of the defect. A trained and experienced nondestructive testing technician interprets any indications that are observed during the magnetic particle inspection.

Each test piece is inspected in two opposing directions to find defects in all locations (e.g. head shot, central conductor shot). Performing the test in each direction requires that the full magnetic particle inspection process (spray, magnetize, examine for defects) be performed multiple times to evaluate the entire part. After the MPI inspection is completed, all parts are run through a demagnetizer to remove or reduce the residual magnetism to within the allowable limits of the applicable specification. They are also post-cleaned in a solvent degreaser and coated with a light rust preventative oil.

Wet fluorescent magnetic particle inspection is performed most often at LTI because it is more reliable, more sensitive to very fine defects and more convenient than the visible dry method.

  • The wet suspension reduces the chance of adherence to surface contamination and allows for the use of smaller particles for better flow to small leakage fields
  • Test samples are viewed in a dark environment under a black light where the fluorescent colors provide better visibility of indications
  • The particles are contained in a liquid carrier for easier and more uniform spray application

Samples for wet fluorescent magnetic particle inspection are tested using Magnaflux benches that can handle products up to 12 ft. long and have a maximum output of 6,300 amps. Dry powder MPI inspection is used most often in the field for convenience or on products that do not fit the Magnaflux benches.

LTI Capabilities

  • Inspection Methods – visible dry and wet fluorescent
  • Products Tested – ferromagnetic materials and parts
  • Size Limitations – test materials to 12 feet in length w/wet suspension
  • Inspector Qualifications – Certified NDT Level 2 & Level 3 Inspectors

News & Articles

Magnetize to Find Discontinuities