Liquid Penetrant Inspection (LPI)

Also known as Dye Penetrant Inspection (DPI), Penetrant Testing (PT) or Fluorescent Penetrant Inspection (FPI). It 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 penetration time, excess surface penetrant is removed and a developer applied. This ‘draws’ the penetrant from the flaw to reveal its presence. Colour contrast (Or “visible”) penetrants require good white light while fluorescent penetrants need to be used in darkened conditions with an ultraviolet light (Also known as black light).

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