Magnetic Particle Inspection (MPI or MT) is a nondestructive testing (NDT) method used to detect surface and nearsurface flaws (cracks, seams, laps, etc.) in ferromagnetic materials (iron, nickel, cobalt, and some steels).
How MPI Works
- Magnetization – The test object is magnetized using:
Direct current (DC) → Detects subsurface flaws.
Alternating current (AC) → Best for surface cracks.
Yoke, prod, or coil methods (depending on part geometry).
- Application of Magnetic Particles
Dry method (colored powder) – Used for rough surfaces.
Wet method (fluorescent or visible particles in liquid) – More sensitive for fine cracks.
- Inspection Under Proper Lighting
Visible particles (black/red) inspected under white light.
Fluorescent particles inspected under UVA (black light).
- Flaw Indication
Discontinuities disrupt the magnetic field, causing particles to cluster at defect sites.
- Demagnetization (If Required)
Some parts must be demagnetized after testing to prevent interference with further operations.
Advantages of MPI
Fast & costeffective compared to dye penetrant or ultrasonic testing.
Detects fine surface & slightly subsurface defects (up to ~6mm deep).
Works on complex shapes (unlike Xray).
Immediate results (no lengthy processing).
Limitations of MPI
Only works on ferromagnetic materials (not for aluminum, copper, austenitic stainless steel).
Surface preparation required (clean, dry, free of coatings).
Limited to nearsurface flaws (not for deep internal defects).
Directional sensitivity – Flaws must be roughly perpendicular to the magnetic field.
Applications of MPI
Welding inspection (cracks, lack of fusion).
Aerospace & automotive components (crankshafts, landing gear).
Pressure vessels & pipelines.
Structural steel & forgings.
Standards for MPI
ASTM E1444 (Standard Practice for MPI)
ASME BPVC Section V (Boiler & Pressure Vessel Code)
ISO 9934 (Magnetic Particle Testing)
AWS D1.1 (Structural Welding Code)
When to Use MPI?
For ferromagnetic materials needing quick, reliable crack detection.
When surface defects are critical (e.g., fatigue cracks in welds).
For field inspections (portable MPI kits available).
