Schematic of flux lines flowing past both a perpendicular and parallel defect, and flux leakage occurring around the perpendicular flaw.
Magnetic Particle Inspection
In magnetic particle inspection (MPI), a magnetic field is induced in a test piece. Magnetic fields are composed of flux lines that flow through the magnetized specimen; they can deflect and leak out in the vicinity of a flaw. The magnetic flux leakage attracts particles, which cluster to form an indication.
Orientation of the magnetic field plays a greater role in defect detection. Test sensitivity is affected by the angle between the induced magnetic field and the defect orientation. Maximum flux leakage occurs when the magnetic field lies perpendicular to the discontinuity. Defects lying parallel to the magnetic flux will not form indications.
Depending on the application, this method can be performed using either dry powder or a powder suspended in a liquid. Particles of different colors are chosen to provide good visibility against the specimen surface. Dry powder particles may be gray, red, or black, and are viewed with white light. Wet fluorescent particles require an ultraviolet light (UVA) to view the indication.
Example of fluorescent magnetic particle inspection using UV light, with indications highlighted.
Magnetic Particle Inspection is performed in four steps:
- Induce a magnetic field in the specimen
- Apply magnetic particles to the specimen's surface
- View the surface, looking for particle groupings that are caused by defects
- Demagnetize and clean the specimen
Advantages of Magnetic Particle Inspection
- Can find both surface and near sub-surface defects
- Some inspection formats are extremely portable and low cost
- Rapid inspection with immediate results
- Indications are visible to the inspector directly on the specimen surface
- Can detect defects that have been smeared over
- Can inspect parts with irregular shapes (external splines, crankshafts, connecting rods, etc.)
Limitations of Magnetic Particle Inspection
- The specimen must be ferromagnetic (e.g. steel, cast iron)
- Paint thicker than about 0.005" must be removed before inspection
- Post cleaning and post demagnetization is often necessary
- Maximum depth sensitivity is typically quoted as 0.100" (deeper under perfect conditions)
- Alignment between magnetic flux and defect is important
Example of visible magnetic particle inspection using dry powder from a bulb.