What limit does MPI have in terms of defect detection depth?

Magnetic Particle Inspection (MPI) is a nondestructive testing method that is particularly effective in identifying surface and near-surface defects in ferromagnetic materials. The ability of MPI to detect defects is significantly influenced by the depth of the defect relative to the surface of the material being inspected.

When an external magnetic field is applied to the material, any surface or near-surface discontinuities that interrupt the magnetic field will cause leakage flux, which can be detected by the magnetic particles used in the process. However, as the depth of the defect increases, the magnetic field strength diminishes, making it difficult for MPI to identify defects that are embedded deeper within the material. This is because the magnetic particles may not receive adequate magnetic influence to gather and indicate the presence of such deep defects effectively.

Therefore, the core limitation of MPI is its reliance on detecting defects that are situated at or very close to the material's surface, rather than those that are deep within it. This understanding is fundamental for effective application and interpretation of results in Magnetic Particle Inspection.