The basic principle of dye penetrant or liquid penetrant or simply penetrant testing (PT) is that when a very low viscosity (highly fluid) liquid (the penetrant) is applied to the surface of a part, it will penetrate into fissures and voids open to the surface, like surface cracks, porosity, shrinkage areas, laminations, and similar discontinuities. Once the excess penetrant is removed, the penetrant trapped in those voids will flow back out, creating an indication.
Penetrant testing can be performed on magnetic and non-magnetic materials, but does not work well on porous materials. Penetrants may be "visible", meaning they can be seen in ambient light, or fluorescent, requiring the use of a "black" light to become visible.
When performing penetrant testing, it is imperative that the surface being tested is clean and free of any foreign materials or liquids that might block the penetrant from entering voids or fissures open to the surface of the part.
