Magnetic Particle Testing (MPT), also known as Magnetic Particle Inspection (MPI), is a non-destructive testing technique used to locate surface and near-surface discontinuities in ferromagnetic materials (such as carbon steel, low-alloy steel, cast iron, and magnetic alloys).

The process involves magnetizing the component using an AC or DC electromagnetic yoke or bench unit. If a surface-breaking flaw or shallow subsurface crack is oriented transverse to the magnetic flux lines, a magnetic leakage field (north and south pole gap) forms at the flaw site. Finely divided iron particles applied to the surface in dry powder or wet suspension form are drawn into the leakage field, producing sharp visual indication lines.