Basic Processing Steps of a Liquid Penetrant Inspection

Basic Processing Steps of a Liquid Penetrant Inspection

1. Surface Preparation: One of the most critical steps of a liquid penetrant inspection is the surface preparation. The surface must be free of oil, grease, water, or other contaminants that may prevent penetrant from entering flaws. The sample may also require etching if mechanical operations such as machining, sanding, or grit blasting have been performed. These and other mechanical operations can smear metal over the flaw opening and prevent the penetrant from entering.
   
2. Penetrant Application: Once the surface has been thoroughly cleaned and dried, the penetrant material is applied by spraying, brushing, or immersing the part in a penetrant bath.
   
3. Penetrant Dwell: The penetrant is left on the surface for a sufficient time to allow as much penetrant as possible to be drawn from or to seep into a defect. Penetrant dwell time is the total time that the penetrant is in contact with the part surface. Dwell times are usually recommended by the penetrant producers or required by the specification being followed. The times vary depending on the application, penetrant materials used, the material, the form of the material being inspected, and the type of defect being inspected for. Minimum dwell times typically range from five to 60 minutes. Generally, there is no harm in using a longer penetrant dwell time as long as the penetrant is not allowed to dry. The ideal dwell time is often determined by experimentation and may be very specific to a particular application.
   
   
4. Excess Penetrant Removal: This is the most delicate part of the inspection procedure because the excess penetrant must be removed from the surface of the sample while removing as little penetrant as possible from defects.  Depending on the penetrant system used, this step may involve cleaning with a solvent, direct rinsing with water, or first treating the part with an emulsifier and then rinsing with water. 
   
5. Developer Application: A thin layer of developer is then applied to the sample to draw penetrant trapped in flaws back to the surface where it will be visible. Developers come in a variety of forms that may be applied by dusting (dry powdered), dipping, or spraying (wet developers).
   
6. Indication Development: The developer is allowed to stand on the part surface for a period of time sufficient to permit the extraction of the trapped penetrant out of any surface flaws. This development time is usually a minimum of 10 minutes.  Significantly longer times may be necessary for tight cracks.
   
7. Inspection: Inspection is then performed under appropriate lighting to detect indications from any flaws which may be present.
   
8. Clean Surface: The final step in the process is to thoroughly clean the part surface to remove the developer from the parts that were found to be acceptable.

Common Uses of Liquid Penetrant Inspection

Liquid penetrant inspection (LPI) is one of the most widely used nondestructive evaluation (NDE) methods. Its popularity can be attributed to two main factors: its relative ease of use and its flexibility.   LPI can be used to inspect almost any material provided that its surface is not extremely rough or porous. Materials that are commonly inspected using LPI include the following:

• Metals (aluminum, copper, steel, titanium, etc.)
• Glass
• Many ceramic materials
• Rubber
• Plastics

LPI offers flexibility in performing inspections because it can be applied in a large variety of applications ranging from automotive spark plugs to critical aircraft components. Penetrant materials can be applied with a spray can or a cotton swab to inspect for flaws known to occur in a specific area or it can be applied by dipping or spraying to quickly inspect large areas. In the image above, visible dye penetrant is being locally applied to a highly loaded connecting point to check for fatigue cracking.

Penetrant inspection systems have been developed to inspect some very large components. In the image shown right, DC-10 banjo fittings are being moved into a penetrant inspection system at what used to be the Douglas Aircraft Company's Long Beach, California facility. These large machined aluminum forgings are used to support the number two engine in the tail of a DC-10 aircraft.

Liquid penetrant inspection can only be used to inspect for flaws that break the surface of the sample. Some of these flaws are listed below:

• Fatigue cracks
• Quench cracks
• Grinding cracks
• Overload and impact fractures
• Porosity
• Laps
• Seams
• Pin holes in welds
• Lack of fusion or braising along the edge of the bond line

As mentioned above, one of the major limitations of a penetrant inspection is that flaws must be open to the surface. To learn more about the advantages and disadvantages of LPI,