Fluorescent penetrant inspection

Fluorescent penetrant inspection (FPI) is a type of dye penetrant inspection in which a fluorescent dye is applied to the surface of a non-porous material in order to detect defects that may compromise the integrity or quality of the part in question. Noted for its low cost and simple process, FPI is used widely in a variety of industries.

There are many types of dye used in penetrant inspections. FPI operations use a dye much more sensitive to smaller flaws than penetrants used in other DPI procedures. This is because of the nature of the fluorescent penetrant that is applied. With its brilliant yellow glow caused by its reaction with ultraviolet radiation, FPI dye sharply contrasts with the dark background. A vivid reference to even minute flaws is easily observed by a skilled inspector.

Because of its sensitivity to such small defects, FPI is ideal for most metals which tend to have small, tight pores and smooth surfaces. Defects can vary but are typically tiny cracks caused by processes used to shape and form the metal. It is not unusual for a part to be inspected several times before it is finished (an inspection often follows each significant forming operation).

Selection of inspection type is, of course, largely based on the material in question. FPI is a nondestructive inspection process which means that the part is not in any way damaged by the test process. Thus, it is of great importance that a dye and process are selected that ensure the part is not subjected to anything that may cause damage or staining.

See the following main steps in a fluorescent penetrant inspection process:

Before the penetrant can be applied to the surface of the material in question one must ensure that the surface is free of any contamination such as paint, oil, dirt, or scale that may fill a defect or falsely indicate a flaw. Chemical etching can be used to rid the surface of undesired contaminants and ensure good penetration when the penetrant is applied. Sandblasting to remove paint from a surface prior to the FPI process may mask (smear material over) cracks making the penantrant not effective. Even if the part has already been through a previous FPI operation it is imperative that it is cleaned again. Most penetrants are not compatible and therefore will thwart any attempt to identify defects that are already penetrated by any other penetrant. This process of cleaning is critical because if the surface of the part is not properly prepared to receive the penetrant, defective product may be moved on for further processing. This can cause lost time and money in reworking, overprocessing, or even scrapping a finished part at final inspection.

The fluorescent penetrant is applied to the surface and allowed time to seep into flaws or defects in the material. The process of waiting for the penetrant to seep into flaws is called Dwell Time. Dwell time varies by material and the size of the indications that are intended to be identified but is generally less than 30 minutes. It requires much less time to penetrate larger flaws because the penetrant is able to soak in much faster. The opposite is true for smaller flaws/defects.

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