NDT : Liquid Penetrant Testing

The liquid penetrant test is one of the oldest methods of nondestructive testing.  The method relies on a penetrant seeping into a discontinuity. It is applicable only to surface defects or to subsurface defects with surface openings.  It is applicable to all homogeneous materials except those of a generally porous nature where penetrant would seep into and leak from the entire surface.  Penetrant testing is therefore best adapted to inspection of all types of surface cracks, porosity, laminations, and lack of bond at exposed edges of joined materials, and of leaks in tubing, tanks, welds and the like.  It has been used with excellent success on ferrous and nonferrous metals, ceramics, powdered metal products, ceramics and glass, as well as on some plastic and synthetic materials.

Penetrant inspection is basically a very simple process.  First a liquid penetrant is applied to the surface of a part.  It is permitted to remain on the surface for a period of time, during which it penetrates into any defects open at the surface.  After the penetrating period, the excess penetrant that remains on the surface is removed.  Then an absorbent, light colored, powdered material called a developer is applied to the surface.  This developer acts as a blotter and draws out a portion of the penetrant which has previously seeped into the surface openings.  As the penetrant is drawn out, into diffuse into the coating of the developer, forming indications that are much wider than the surface openings with which they are associated.  An inspector then views the part and looks for these colored indications against the background of the developing powder.

source : PCA Engineering