Which part of a ferromagnetic component is most susceptible to defects detectable by Magnetic Particle Inspection (MPI)?

The surface and just below the surface of ferromagnetic components are most susceptible to defects detectable by Magnetic Particle Inspection (MPI) due to the nature of how MPI works. This testing method relies on the magnetic field to identify surface and near-surface discontinuities.

When a ferromagnetic material is magnetized, any surface or near-surface flaws such as cracks, laps, or inclusions will disrupt the magnetic field. These disruptions result in a concentration of magnetic particles at the defect, making it visible during inspection. Since many manufacturing processes can lead to these types of defects, and because they often have implications for the integrity of the component, MPI is particularly effective at detecting such issues.

Defects deeper in the material, such as those found in the internal structure or the center of the component, typically do not affect the magnetic field as significantly and are therefore less reliably detected using MPI. The grain boundaries, while important, are not the primary focus for defect detection using this method, as they form part of the material’s overall structure rather than indicating flaws. Hence, the choice that highlights the surface and just below the surface accurately reflects the characteristics and limitations of MPI in identifying defects.