Liquid penetrant inspection is a sensitive, nondestructive method for detecting minute discontinuities (cracks, porosity, holes, or surface seams) in non-magnetic materials where magnetic particle inspection cannot be used. A liquid with high penetrating qualities is applied to the part surface and is drawn into extremely small surface openings by capillary action. The excess liquid is removed, and a developer such as talcum powder is applied to the surface. The penetrant trapped in discontinuities flows back up to color the developer and provide an indication.
In dye penetrant testing the liquid penetrant is colored with a visible dye, and the surface is inspected under normal white light. In fluorescent penetrant testing the liquid penetrant contains a fluorescent dye and the surface is inspected under fluorescent light. Information on liquid penetrant testing is contained in the following specifications:
Source: Investment casting Institute Handbook 1997 Edition