Liquid penetrant testing is probably the most widely used NDT method. The test object or material is coated with a visible or fluorescent dye solution. The excess dye is removed from the surface, and then a developer is applied. The developer acts like a blotter and draws penetrant out of imperfections which open to the surface. With visible dyes, the vivid color contrast between the penetrant and the developer makes the "bleedout' easy to see. With fluorescent dyes, and ultraviolet lamp is used to make the "bleedout" fluoresce brightly, thus allowing the imperfection to be seen readily.

Liquid penetration inspection is a method that is used to reveal surface breaking flaws by bleedout of a coloured or fluorescent dye from the flaw. The technique is based on the ability of a liquid to be drawn into a "clean" surface breaking flaw by capillary action. After a period of time called the "dwell", excess surface penetrant is removed and a developer applied. This acts as a "blotter". It draws the penetrant from the flaw to reveal it's presence. Coloured (contrast) penetrants require good white light while fluorescent penetrants need to be used in darkened conditions with an ultraviolet "black light".

Penetrant inspection can be used on any material. It is essential that the material is carefully cleaned first, otherwise the penetrant will not be able to get into the defect. If surface penetrant is not fully removed, misleading indications will result.

Liquid penetrant testing, cross section of steel forging.

Liquid penetrant testing, cross section of steel forging.