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Introduction & Basic Principle

LPI is based upon capillary action, where low surface tension fluid penetrates into clean and dry surface-breaking discontinuities. Penetrant may be applied to the test component by dipping, spraying, or brushing. After adequate penetration time has been allowed, the excess penetrant is removed, and a developer is applied. The developer helps to draw penetrant out of the flaw where a visible indication becomes visible to the inspector. Inspection is performed under ultraviolet or white light, depending upon the type of dye used - fluorescent or nonfluorescent (visible).

Liquid penetrant inspection is a widely applied and low-cost inspection method used to locate surface-breaking defects in all non-porous materials (metals, plastics, or ceramics). Penetrant may be applied to all non-ferrous materials, but for inspection of ferrous components magnetic particle inspection is preferred for its subsurface detection capability. LPI is used to detect casting and forging defects, cracks, and leaks in new products, and fatigue cracks on in-service components.

Process:

Application

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:

  1. Metals (aluminum, copper, steel, titanium, etc.)

  2. Glass

  3. Many ceramic materials

  4. Rubber

  5. 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 material 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. Above, visible dye penetrant 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 is used to inspect for flaws that break the surface of the sample. Some of these flaws are listed below:

  1. Fatigue cracks

  2. Quench cracks

  3. Grinding cracks

  4. Overload and impact fractures

  5. Porosity

  6. Laps

  7. Seams

  8. Pin holes in welds

  9. 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.

Primary Advantages

  1. The method has high sensitivity to small surface discontinuities.

  2. The method has few material limitations, i.e. metallic and nonmetallic, magnetic and nonmagnetic, and conductive and nonconductive materials may be inspected.

  3. Large areas and large volumes of parts/materials can be inspected rapidly and at low cost.

  4. Parts with complex geometric shapes are routinely inspected.

  5. Indications are produced directly on the surface of the part and constitute a visual representation of the flaw.

  6. Aerosol spray cans make penetrant materials very portable.

  7. Penetrant materials and associated equipment are relatively inexpensive.

Primary Disadvantages

  1. Only surface breaking defects can be detected.

  2. Only materials with a relatively nonporous surface can be inspected.

  3. Precleaning is critical since contaminants can mask defects.

  4. Metal smearing from machining, grinding, and grit or vapor blasting must be removed prior to LPI.

  5. The inspector must have direct access to the surface being inspected.

  6. Surface finish and roughness can affect inspection sensitivity.

  7. Multiple process operations must be performed and controlled.

  8. Post cleaning of acceptable parts or materials is required.

  9. Chemical handling and proper disposal is required.

 

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