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.

Contents
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 1Materials
 2Inspection steps
o 2.1Step 1: Initial cleaning
o 2.2Step 2: Penetrant application
o 2.3Step 3: Excess penetrant removal
o 2.4Step 4: Developer application
o 2.5Step 5: Inspection
o 2.6Step 6: Final cleaning
 3Advantages
 4Potential disadvantages
 5References

Materials[edit]
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.

Inspection steps[edit]
See the following main steps in a fluorescent penetrant inspection process:

Step 1: Initial cleaning[edit]

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.

Step 2: Penetrant application[edit]

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.

Step 3: Excess penetrant removal[edit]

After the identified dwell time has passed, penetrant on the outer surface of the material is then removed. This highly
controlled process is necessary in order to ensure that the penetrant is removed only from the surface of the material and
not from inside any identified flaws. Various chemicals can be used for such a process and vary by specific penetrant types.
Typically, the cleaner is applied to a lint-free cloth that is used to carefully clean the surface.

Step 4: Developer application[edit]

Having removed excess penetrant a contrasting developer may be applied to the surface. This serves as a background
against which flaws can more readily be detected. The developer also causes penetrant that is still in any defects to surface
and bleed. These two attributes allow defects to be easily detected upon inspection. Dwell time is then allowed for the
developer to achieve desired results before inspection.

Step 5: Inspection[edit]
In the case of fluorescent inspection, the inspector will use ultraviolet radiation with an intensity appropriate to the intent of
the inspection operation. This must take place in a dark room to ensure good contrast between the glow emitted by the
penetrant in the defected areas and the unlit surface of the material. The inspector carefully examines all surfaces in
question and records any concerns. Areas in question may be marked so that location of indications can be identified easily
without the use of the UV lighting. The inspection should occur at a given point in time after the application of the developer.
Too short a time and the flaws may not be fully blotted, too long and the blotting may make proper interpretation difficult.

Step 6: Final cleaning[edit]

Upon successful inspection of the product, it is returned for a final cleaning before it is either shipped, moved on to another
process, or deemed defective and reworked or scrapped. Note that a flawed part may not go through the final cleaning
process if it is considered not to be cost effective.

Advantages[edit]

 Highly sensitive fluorescent penetrant is ideal for even the smallest imperfections
 Low cost and potentially high volume
 Suitable for inspection of non-magnetic materials and electrical insulators.

Potential disadvantages[edit]

 The method requires thorough cleaning of the inspected items. Inadequate cleaning
may prevent detection of discontinuities.
 Test materials can be damaged if compatibility is not ensured. The operator or
his/her supervisor should verify compatibility on the tested material, especially when
considering the testing of plastic components and ceramics. The method is
unsuitable for testing porous ceramics.
 Penetrant stains clothes and skin and must be treated with care
 The method is limited to surface defects
 Training is required for the inspector

References[edit]

 Manufacturing Processes Reference Guide, Industrial Press Inc. 1994

 http://www.cnde.iastate.edu/faa-casr/fpi/TechResults.htm
 ASTM E 1417 Standard practice for liquid penetrant examination