Fluorescent penetrant inspection
Encyclopedia
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, simple process, FPI is used widely in a variety of industries.
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.
1. Initial Cleaning:
Before the dye 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. Methods such as sand blasting or chemical etching can be used to rid the surface of undesired contaminates and ensure good penetration when the dye is applied. Even if the part has already been through a previous DPI operation it is imperative that it is cleaned again. Most dyes are not compatible and therefore will thwart any attempt to identify defects that are already penetrated by any other dye. This process of cleaning is critical because if the surface of the part is not properly prepared to receive the dye, 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.
2. Penetrant Application:
The fluorescent penetrant is applied to the surface and allowed time to seep into flaws or defects in the material. Time varies by material and the size of the flaws that are intended to be identified but is generally around 30 minutes. It requires much less time to penetrate larger defects because the dye is able to soak in much faster. The opposite is true for smaller flaws.
3. Excess Dye Removal:
Penetrant on the outer surface of the material is next removed. This highly controlled process is necessary in order to ensure that the dye is removed only from the surface of the material and not from 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 cloth that is used to carefully clean the surface.
4. Developer Application:
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. Time is then allowed for the developer to achieve desired results before inspection.
5. Inspection:
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 defects 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.
6. Final Cleaning:
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 never be cleaned if it is considered not to be cost effective.
Penetrant dyes stain cloth, skin and other porous surfaces brought into contact. One should verify compatibility on the test material, especially when considering the testing of plastic components.
Further information on inspection steps may be found in industry standards (e.g. the American Welding Society, American Society for Testing and Materials, the British Standards Institute, and the Society for Automotive Engineers).
Dye penetrant inspection
Dye penetrant inspection , also called liquid penetrant inspection or penetrant testing , is a widely applied and low-cost inspection method used to locate surface-breaking defects in all non-porous materials...
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, simple process, FPI is used widely in a variety of industries.
Materials
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
See the following main steps in a Fluorescent Penetrant Inspection:1. Initial Cleaning:
Before the dye 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. Methods such as sand blasting or chemical etching can be used to rid the surface of undesired contaminates and ensure good penetration when the dye is applied. Even if the part has already been through a previous DPI operation it is imperative that it is cleaned again. Most dyes are not compatible and therefore will thwart any attempt to identify defects that are already penetrated by any other dye. This process of cleaning is critical because if the surface of the part is not properly prepared to receive the dye, 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.
2. Penetrant Application:
The fluorescent penetrant is applied to the surface and allowed time to seep into flaws or defects in the material. Time varies by material and the size of the flaws that are intended to be identified but is generally around 30 minutes. It requires much less time to penetrate larger defects because the dye is able to soak in much faster. The opposite is true for smaller flaws.
3. Excess Dye Removal:
Penetrant on the outer surface of the material is next removed. This highly controlled process is necessary in order to ensure that the dye is removed only from the surface of the material and not from 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 cloth that is used to carefully clean the surface.
4. Developer Application:
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. Time is then allowed for the developer to achieve desired results before inspection.
5. Inspection:
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 defects 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.
6. Final Cleaning:
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 never be cleaned if it is considered not to be cost effective.
Advantages
- Highly sensitive fluorescent penetrant is ideal for even the smallest imperfections
- Little training is needed for the operator/ inspector
- Low cost and potentially high volume
Potential Disadvantages
- Requires adequate cleaning (neglect of this step can have costly repercussions)
- Test materials can be damaged if compatibility is not ensured
- Dyes stain clothe and skin and must be treated with care
Penetrant dyes stain cloth, skin and other porous surfaces brought into contact. One should verify compatibility on the test material, especially when considering the testing of plastic components.
Further information on inspection steps may be found in industry standards (e.g. the American Welding Society, American Society for Testing and Materials, the British Standards Institute, and the Society for Automotive Engineers).