Magnetic Particle Inspection processes are non-destructive methods used for the detection of defects in ferrous materials. The externally applied or generated magnetic field is used by applying current to the tested material, with the principle that the magnetic susceptibility of a defect is marked poorer (the magnetic resistance is greater) than the surrounding material.
The presence of a surface or near surface flaw (void) in the material causes distortion in the magnetic flux through it, which in turn causes leakage to the magnetic fields at the flaw. This deformation to the magnetic field is not limited to the immediate locality of the defect but extends for a considerable distance; even through the surface and air if the magnetism is intense enough. Thus the size of the distortion is much larger than that of the defect and is made visible at the surface of the part by means of tiny particles that are being attracted to the leakage fields.This magnetically held collection of particle forms an outline of discontinuity and generally indicates its location, size, shape and extent. Magnetic particles are applied over a surface as dry particles, or as wet particle in a liquid.