Liquid Penetrant also known as dye penetrant testing is a nondestructive test method using a visible or fluorescent die that causes bleed out from the discontinuity.
The process is based upon capillary action, where low surface tension fluid penetrates into clean and dry surface-breaking discontinuities. Penetrant may be applied to the test component by dipping, spraying, or brushing. After adequate penetration time has been allowed, the excess penetrant is removed and a developer is applied. The developer helps to draw penetrant out of the flaw so that an invisible indication becomes visible to the inspector.
This method is used to detect surface-breaking defects.
Liquid Penetrant can be used on metals, plastics, or ceramics and can be applied to non-ferrous and ferrous materials.