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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 three 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, proceed to
the next page.
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