Apart from visual inspection this is probably the oldest and most widely used of all the NDT methods. It can be used on any non-porous material. Its use is confined to the detection of surface breaking defects.
Liquid penetration inspection is used to reveal surface breaking flaws by bleedout of a colored or fluorescent dye from the flaw.
Test objects are coated with visible or fluorescent dye solution. Excess dye is then removed from the surface, and a developer is applied. The developer acts as blotter, drawing trapped penetrant out of imperfections open to the surface. With visible dyes, vivid color contrasts between the penetrant and developer make “bleedout” easy to see. With fluorescent dyes, ultraviolet light is used to make the bleedout fluoresce brightly, thus allowing imperfections to be readily seen.
Penetrant inspection can be used on any material and is most often used on materials clad in stainless steel, and stainless welded items which cannot be inspected by other methods.