Liquid Penetrant Testing

Liquid penetrant inspection (LPI), also called dye penetrant inspection (DP), is a widely applied NDT method, which is used to check surface-breaking defects in all non-porous materials (metals, plastics, or ceramics). It is probably one of the most widely used NDT techniques. Test and pre-cleaned objects are coated with visible or fluorescent dye solution. Following a given time, ranging from 5 to 30 minutes depending on the material inspected, excess penetrant is removed from the surface, and a developer in liquid or powder form is applied. The developer acts as blotter, drawing trapped penetrant out of imperfections open to the surface. This developer absorbs penetrant drawn from discontinuities and reveals a vivid color contrast between the penetrant and developer (usually red on white). With fluorescent dyes, ultraviolet light is used to make the bleedout fluoresce brightly, thus allowing imperfections to be readily seen.