Liquid Penetrant Inspection

Steps of Liquid Penetrant Inspection.

In liquid penetrant inspection (LPI), a fluid with high capillary potential is applied to a sample and drawn into surface breaking flaws. The excess is removed after a period of time and an appropriate developer is applied to draw the penetrant back out. As it bleeds out laterally on the part surface, it is visually noted due to contrast with the developer. A penetrant that fluoresces under UV light may also be used to generate dramatic visualization of the flaws.

It may be used on any non-porous material and is used to find surface flaws, sometimes to detect leaks. Equipment for LPI can be as simple as a collection of aerosol spray cans, or may be a fully automated system. There are considerable choices of penetrant, remover, and developer materials. Selection is based on the required sensitivity, portability requirements, and the nature of the material to be tested with respect to compatibility issues.

Liquid Penetrant Inspection is performed in six steps:

  1. Pre-clean and dry the specimen
  2. Apply penetrant to the area of interest
  3. Remove excess penetrant so dye is left only in defects
  4. Apply developer to draw penetrant out and form indication
  5. View specimen
  6. Post-clean (if necessary)

Advantages of Liquid Penetrant Inspection

  • Large areas or volumes of parts/materials can be inspected rapidly and at low cost
  • Parts with complex geometries are routinely inspected
  • Indications are produced directly on surface of the part, providing a visual image of the discontinuity
  • Equipment investment can be quite low
  • Aerosol spray cans can make testing very portable

Limitations of Liquid Penetrant Inspection

  • Only detects surface breaking defects
  • Test material must be nonporous
  • Precleaning is critical - contaminants can mask defects
  • Post cleaning is sometimes necessary to remove chemicals
  • Requires multiple operations under controlled conditions
  • Chemical handling precautions may be necessary
  • Metal smearing from machining, grinding and other operations inhibits detection
  • Some materials may need to be etched prior to inspection

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Last updated: 2/9/2016