Liquid or dye Penetrant testing is a non –destructive method for finding discontinuities that are open to the surface of solid and essentially non-porous materials. Indications of flaws can be found regardless of the size, configuration, internal structure, or chemical composition of work piece being tested and regardless of flaw orientation. Liquid penetrant can seep into (and be drawn into) various types of minute surface openings (reportedly, as fine as 4micro inch in width) by capillary action. Because of this, the process is well suited for the detection of all types of surface cracks, laps, porosity, shrinkage areas, lamination and similar discontinuities in casting, forgings, welds and other product forms. Dye penetrant inspection is used extensively for the testing of wrought and cast products of ferrous and non-ferrous metals, powder metallurgy parts, and ceramics and glass objects.
In practice, the liquid penetrant inspection process is relatively simple. Equipment generally is simpler and less costly than that for most other NDT methods .When used on ferromagnetic steels, in some instances, the sensitivity of liquid penetrant test is better than that of magnetic particle testing.
The major limitation of liquid penetrant testing is that, it can detect only imperfections that are open to the surface. Another factor that may inhibit the effectiveness of liquid penetrant testing is the surface roughness of the object being tested .Rough or porous surfaces are likely to produce false indication.
Liquid penetrant testing depends mainly on a liquid's effectively wetting the surface of a solid work piece of specimen, flowing over the migrating into cavities that are open to the surface.Closely related to wetting ability is the phenomenon of capillary rise or depression.