Magnetic Particle Inspection is used on Ferromagnetic materials in order to detect surface breaking flaws and occasionally sub-surface flaws.
It can be carried out in visable light or in darkened conditions using an ultra violet light. The method of inspection is usually determined by the the inspection code.
In it's simplest form an electical current or permanent magnet creates flux lines between the poles of the magnet. Iron filings are then applied between the poles and if there are defects present they are shown up as continuous lines.
The most serious defects in any engineering object are surface breaking cracks. Magnetic particle inspection is a fast, extremely sensitive, easy to apply method that can be applied to ferritic components over a wide range of surface temperatures (up to 315?C with no lower limit). It is relatively unaffected by poor surface conditions.