Liquid Penetrant Inspection (LPI)
LPI, although the least complex of the methods, is highly sensitive. In manual operation, it uses simple equipment to detect flaws (flaws, with some exceptions, are voids) open to the surface, and works on any material as long as it is not porous. Penetrants are petroleum- or water-based liquids coloured by a dye. Applied to the surface, the liquid seeps into the open voids. It is then removed from the surface and replaced by a white developer. The developer acts as a blotter, drawing out the penetrant trapped in voids. The penetrant stains the developer, indicating the presence and location of flaws.