Liquid Penetrant Inspection (FPI) or Penetrant Testing

Fluorescent Penetrant Inspection (FPI) or Penetrant Testing is a widely applied and low-cost inspection method used to check surface-breaking defects in all non-porous materials (metals, plastics, or ceramics). The penetrant may be applied to all non-ferrous materials and ferrous materials, unlike Magnetic-Particle Inspection which may only be applied to parts made with ferrous materials. FPI is used to detect casting, forging, and welding defects such as hairline cracks, surface porosity, leaks in new products, and fatigue cracks on in-service components.

FPI 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. Inspection is performed under ultraviolet UVA light.

Wikipedia contributors. (2019, April 18). Dye penetrant inspection. In Wikipedia, The Free Encyclopedia. Retrieved 14:36, June 13, 2019, from [Ref]


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