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Liquid Penetrant Examination

Last updated: January 14, 2021

What Does Liquid Penetrant Examination Mean?

Liquid penetrant examination is among the most accepted nondestructive examination (NDE) techniques in engineering and other industries. It is versatile, cost effective and easier to facilitate compared to other NDE procedures.

This test looks for flaws in materials that are exposed on the surface. This is accomplished by allowing a thin liquid to flow to the surface and drawing it out through a developer with the consistency of chalk. The penetrant can be applied to ferrous and non-ferrous materials.

Among the most frequently inspected products are bars, butt plates, castings, pipes and forgings.

This method is also known as dye penetration inspection (DPI) and liquid penetration testing (PT).


Corrosionpedia Explains Liquid Penetrant Examination

Liquid penetration examination is based on the principle of capillary action, which is when a fluid with a low surface tension penetrates a dry and clean surface, breaking discontinuities. Penetrants are applied in a variety of ways, such as brushing, spraying and dipping. Once ample penetration has occurred, any excess penetrant is removed and followed by applying a developer. This aids in pushing the penetrant out to produce a visible indicator. The test is performed beneath a white or ultraviolet light according to the type of dye used in the test.

The major steps of a liquid penetration examination:

  • Pre-cleaning: The surface to be tested is cleaned to eliminate paint, dirt and loose scale that may cause false indications.
  • Application: The penetrant is applied to the surface and allowed to soak into any flaws for 5 to 30 minutes.
  • Removal: Excess penetrant is removed from the testing surface. The type of removal depends on the type of penetrant. A developer is applied afterward.
  • Inspection: The examiner will use adequate visible or ultraviolet light to detect visible penetrant.
  • Post-cleaning: The surface is usually cleaned after the inspection.


dye penetration inspection (DPI), penetration testing (PT)

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