Dye penetrant inspection (DPI) or also known as penetrant testing (PT), is a commonly applied and low-cost inspection method used to check surface-breaking defects in all non-porous materials such as metals, plastics, or ceramics. Penetrant can be applied to all non-ferrous materials and ferrous materials, although magnetic-particle inspection (MPI) is often used instead for ferrous components due to its subsurface detection capability.
DPI is used to detect casting, forging and welding surface defects such as hairline cracks, surface porosity, leaks in new products, and fatigue cracks on in-service components.
How Does It Work?
Based upon capillary action, low surface tension fluid can penetrate into clean and dry surface-breaking discontinuities. Penetrant which has entered a tight discontinuity will remain even when the excess is removed.
Penetrant can be applied to the test component by dipping, spraying, or brushing. After reasonable penetration time has been allowed, the excess penetrant is removed and a developer is applied.
The developer helps to draw penetrant out (or bleed out) of the discontinuity so that an invisible indication becomes visible to the inspector. Inspection is performed under ultraviolet for a fluorescent dye and white light for nonfluorescent/visible dye.
Fluorescent Dyes
The use of fluorescent dyes increases the sensitivity of the technique significantly. However, the technique is not applicable in extreme temperature because the penetrant vehicle, which is normally oil, will become excessively viscous below 5°C, causing an increase in the penetration time with a consequent decrease in sensitivity. Meanwhile, in temperature above 60°C, the penetrant will dry out and the technique will not work.
How to Perform Liquid Penetrant Inspection
1. Pre-cleaning
Clean the test surface using solvents, alkaline cleaning steps or vapor degreasing, to remove any dirt, paint, oil, grease or any loose scale so that the penetrant won’t cause irrelevant or false indications. The cleaning process will open the surface of any defects, and also dry and remove it from contamination.
2. Application of Penetrant
The penetrant is then applied to the surface of the item being tested. The penetrant is allowed "dwell time" to soak into any flaws (generally 5 to 30 minutes). The dwell time mainly depends upon the penetrant being used, the material being tested and the size of flaws sought. Smaller flaws require a longer penetration time. Due to their incompatible nature, one must be careful not to apply solvent-based penetrant to a surface which is to be inspected with a water-washable penetrant.
3. Excess Penetrant Removal
The excess penetrant is then removed from the surface. The removal method is controlled by the type of penetrant used. Water-washable, solvent-removable, lipophilic post-emulsifiable, or hydrophilic post-emulsifiable are the common choices.
Emulsifiers represent the highest sensitivity level and chemically interact with the oily penetrant to make it removable with a water spray.
When using solvent remover and a lint-free cloth, it is important not to spray the solvent on the test surface directly, because this can remove the penetrant from the flaws.
If excess penetrant is not properly removed, once the developer is applied, it may leave a background in the developed area that can mask indications or defects.
In addition, this may also produce false indications severely deterring the ability to do a proper inspection. Also, the removal of excessive penetrant is done towards one direction either vertically or horizontally as the case may be.
4. Application of Developer
After excess penetrant has been removed, a white developer is applied to the sample. There are several developer types available, which include the non-aqueous wet developer, dry powder, water-suspendable, and water-soluble.
The selection of a developer is governed by penetrant compatibility and by inspection conditions. For example, water-soluble or -a suspendable developer cannot be used with water-washable penetrant.
When using non-aqueous wet developer (NAWD) or dry powder, the sample must be dried prior to application, while soluble and suspendable developers are applied with the part still wet from the previous step.
NAWD is commercially available in aerosol spray cans and may employ acetone, isopropyl alcohol, or propellant that is a combination of the two. The developer should form a semi-transparent, even coating on the surface.
The developer draws penetrant from defects out onto the surface to form a visible indication, commonly known as bleed-out. Any areas that bleed out can indicate the location, orientation and possible types of defects on the surface.
Interpreting the results and characterizing defects from the indications found may require some training and experience.
5. Inspection
The inspector will use visible light with adequate intensity (100 foot-candles or 1100 lux is typical) for visible dye penetrant. Ultraviolet (UV-A) radiation of adequate intensity (1,000 micro-watts per centimeter squared is common), along with low ambient light levels (less than 2 foot-candles) for fluorescent penetrant examinations.
Inspection of the test surface should take place after 10 to 30 minute development time, and is dependent on the penetrant and developer used. This time delay allows the blotting action to occur.
The inspector may observe the sample for indication formation when using visible dye. It is also good practice to observe indications as they form because the characteristics of the bleed out are a significant part of interpretation characterization of flaws.
6. Post Cleaning
The test surface is often cleaned after inspection and recording of defects, especially if post-inspection coating processes are scheduled.
Bleeding indicate some defect which is most probably surface prorsity.
Advantages and Limitation
The advantages of Dye penetrant inspection are listed below;
- All nonporous materials
- Portable
- Applicable to small parts with complex geometry
- Simple
- Inexpensive
- Sensitive
- Relatively low skill level (easy to interpret)
The limitations are;
- Will only detect defects open to the surface
- Requires careful space preparation
- Not applicable to porous surfaces
- Temperature dependent
- Cannot retest indefinitely
- Potentially hazardous chemicals
- No permanent record
- Time lapse between application and results
- Messy
Watch This Video on The Basic Principle of DPI;
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