Commonly referred to as PT (Penetrant Testing), DPI (Dye Penetrant Testing) or FPI (Fluorescent Penetrant Testing).
This method is used to find surface breaking defects. Penetrant Testing can be done on a wide variety of materials, but it is mainly used on non-ferrous metals. Penetrant Testing is based upon the capillary action, where low surface tension fluid penetrates into clean and dry surface-breaking discontinuities. After adequate penetration time has been allowed, the excess penetrant is removed and a developer is applied. The developer helps to withdraw the penetrant out of the flaw and makes an invisible indication visible to the inspector by it’s contrast to the developer. Inspection is either performed under ultraviolet or white light, depending on the type of dye used.