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Fluorescent penetrant inspection (FPI) is a non-destructive testing method that detects surface flaws in various materials, such as metals, plastics, ceramics, and composites. FPI is widely used in industries where the integrity of components is critical for safety and performance.

Why FPI?

The basic principle of FPI is to apply a liquid penetrant that contains fluorescent dye to the surface of the test piece. The penetrant seeps into any cracks or pores on the surface due to capillary action.

After a sufficient dwell time, the excess penetrant is removed from the surface, usually by wiping or rinsing. A white powder is then applied to the surface to draw out the trapped penetrant from the flaws. The test piece is then inspected under ultraviolet light, causing the dye to glow, revealing defects.

Pre-Cleaning

The test piece must be thoroughly cleaned before applying the penetrant to remove any dirt, grease, oil, paint, or other contaminants that could interfere with the penetration or detection of flaws. The cleaning method depends on the type and condition of the material. 

Penetrant Application

The penetrant can be applied by spraying, dipping, brushing, or pouring. The application method depends on the size and shape of the test piece, the type of penetrant, and the accessibility of the surface.

The penetrant should cover the entire surface to be inspected. It should be allowed to dwell sufficiently to ensure complete penetration into any flaws. The dwell time depends on the type and size of flaws, the material properties, the temperature, and the type of penetrant. 

Excess Penetrant Removal

The excess penetrant on the surface must be removed before applying the developer to avoid masking or reducing the contrast of flaws. The removal method depends on the type of penetrant used.

There are two main types of penetrants: water-washable and solvent-removable. Water-washable penetrants can be rinsed off with water or a water-based emulsifier. Solvent-removable penetrants require wiping with a solvent-soaked cloth or spraying with a solvent. 

Developer Application

The developer acts as a blotter that draws out the penetrant from the flaws and spreads it on the surface, indicating defects. The developer can be applied by spraying, dipping, or dusting. 

Inspection

The inspection is performed under ultraviolet light in a darkened area to enhance the visibility of fluorescent indications. The inspector should wear protective eyewear and clothing to avoid exposure to ultraviolet radiation. The inspector should examine the test piece from different angles and distances to detect any flaws that may be present on the surface. 

Post-Cleaning

The test piece must be cleaned after inspection to remove any remaining penetrant, developer, or contaminants from the surface. The cleaning method depends on the type and condition of the material. It can include wiping, rinsing, solvent cleaning, alkaline cleaning, or vapor degreasing.

FPI is an effective and reliable technique for detecting surface flaws in various materials and applications. As long as this technique is used correctly, you will be able to detect flaws in a non-destructive way.

For more information on non-destructive testing, contact a professional near you.