What is Magnetic Particle Inspection?

Overview

Magnetic Particle Inspection (MPI) magnetises a steel component and applies fine iron particles. Where a crack disrupts the magnetic field, particles gather and make the defect visible to the inspector.

MPI is a fast surface and near-surface method for suitable ferromagnetic components. Reliable coverage depends on field strength, field direction, surface condition and the ability to magnetise the relevant area; it is not a general test for non-ferromagnetic materials or deep internal defects.

What MPI Detects

• Surface-breaking cracks in steel
• Slightly sub-surface defects
• Weld and heat-affected-zone cracking
• Defects in castings and forgings

When MPI Is a Good Fit

MPI is often selected for crack detection in ferromagnetic steel welds, pins, hooks, forgings and other fatigue-critical details. Apply the magnetic field in a direction that crosses the expected crack orientation; a second direction may be required. See how MPI works.

Limitations and Interpretation

• Only suitable ferromagnetic materials can retain the required magnetic field
• Discontinuities aligned with the field may produce a weak response; field direction and coverage matter
• Paint, scale, roughness, geometry and poor contact can reduce sensitivity or create non-relevant indications
• MPI is primarily for surface and near-surface discontinuities, not a substitute for volumetric examination
• Residual magnetism may need to be removed where it could affect equipment or service; acceptance depends on the applicable criteria

Related Inspection Scope

For a commercial inspection scope, see magnetic particle inspection services, weld inspection or crack detection.