A crack 0.5 mm deep and 3 mm long on a crankshaft fillet radius is invisible to the naked eye under shop lighting — but under magnetic particle inspection (MPI), it explodes into a vivid fluorescent line that no inspector can miss. MPI is the oldest and most widely used NDT method for surface and near-surface crack detection in ferromagnetic materials, and it works on a principle that has not changed in 90 years: a magnetic field distorts around a discontinuity, and iron particles cluster at the distortion. What has changed is how the field is generated, how the particles are applied, and how the results are documented. This article explains the physics, the method choices, and where MPI fits in a modern quality program.
How does the physics actually work?
When a ferromagnetic part — steel, iron, nickel alloys — is magnetized, magnetic flux lines flow through the material. If the flux encounters a discontinuity (crack, seam, lap, inclusion) that is perpendicular to the flux direction, it cannot jump the gap cleanly. The flux leaks out of the surface at the discontinuity edges, creating localized north and south poles. Finely divided ferromagnetic particles — dry powder or fluorescent particles suspended in oil or water — are applied to the surface. These particles are attracted to the leakage field and accumulate at the crack edges, forming a visible indication that is wider than the actual crack by a factor of 10–100×. The key constraint: the crack must be at least 45° to the flux lines to produce a detectable leakage field. A crack perfectly parallel to the flux direction is invisible. That is why every MPI procedure requires magnetization in at least two directions, typically 90° apart.
Wet vs dry vs fluorescent: which method for which application?
| Method | Best For | Crack Sensitivity |
|---|---|---|
| Dry powder (red/black/gray) | Rough surfaces, field inspection, welds above 50 °C | ~0.5 mm minimum crack length |
| Wet visible (oil-suspended black particles) | Machined surfaces, production-line inspection | ~0.25 mm |
| Wet fluorescent (oil-suspended, UV-A illuminated) | Aerospace, automotive safety parts, highest sensitivity | ~0.1 mm — fine as a human hair |
Fluorescent MPI under UV-A (365 nm) in a darkened booth provides the highest contrast ratio — the green-yellow fluorescent particles against the dark part surface produce indications visible at 10× lower particle concentration than visible methods. This is the standard for aircraft engine components, automotive steering and suspension parts, and pressure vessel welds governed by ASME Section V.
How do you choose the magnetization technique?
Five magnetization methods exist, and choosing wrong means the crack is not found:
- Headshot (direct contact). Current flows directly through the part between two contact heads. Best for long parts like shafts, axles, and bolts. Produces a circular magnetic field that detects longitudinal cracks.
- Central conductor. A copper bar passes through a hollow part (bearing races, nuts, pipe fittings) and carries the current. The circular field detects cracks on the inner and outer diameters simultaneously.
- Coil shot. The part is placed inside a coil energized with AC or DC. Produces a longitudinal field that detects transverse cracks. The most common method for general weld inspection.
- Yoke (portable electromagnet). A handheld horseshoe electromagnet placed on the part surface. AC yoke for surface cracks, DC yoke for near-surface. Used in field inspection of pipelines, storage tanks, and structural welds where the part cannot be moved.
- Multidirectional. Two fields at 90° applied simultaneously — typically a headshot plus a coil shot. Detects cracks in all orientations in a single cycle. Standard for high-volume production of safety-critical parts.
What does demagnetization and post-cleaning require?
After inspection, the part retains residual magnetism — enough to attract grinding swarf, machining chips, or bearing debris in service, accelerating wear and creating new failure modes. Demagnetization passes the part through an AC field of decreasing amplitude, randomizing the magnetic domains. A field strength meter confirms residual field below 3 gauss (0.3 mT), which is the aerospace and nuclear industry standard. Post-cleaning removes all particle residue — critical for parts entering paint, plating, or assembly operations. Fluorescent particles in particular leave a thin oil film that can interfere with adhesive bonding or welding.
While MPI detects surface cracks on ferromagnetic parts, aerospace and power generation manufacturers often pair it with complementary NDT methods. Ultrasonic inspection covers subsurface flaws that MPI cannot reach. Browse our ultrasonic thickness gauge and visual inspection equipment for a complete NDT toolkit. For production-line environments, the Waygate USM 100 ultrasonic flaw detector provides digital documentation that complements MPI's surface sensitivity with volumetric flaw detection.
