Penetrant Testing (PT)

Dye Penetrant Testing (PT) is used to find surface breaking defects. PT inspection is suitable for a wide variety of materials, but it is mainly used on non-ferrous metals. This method makes use of the capillary action principle. A liquid dye with low viscosity is applied to the test piece. Due to its low viscosity, it gets sucked into surface-breaking discontinuities. After allowing adequate penetration time, the excess penetrant liquid is rinsed off. After drying, a chalk-like developer is applied. The developer draws the penetrant dye from defects making a visible indication that can be interpreted by the inspector.

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PT advantages:

  • High sensitivity to small surface discontinuities
  • Suitable for large range of materials: metallic and non-metallic, magnetic and non-magnetic, and conductive and non-conductive
  • Large surface areas and large volumes parts can be inspected rapidly and at low cost
  • Parts with complex geometry can be tested without changing the inspection procedure
  • Indications are produced directly on the surface of the part
  • Aerosol spray cans make penetrant materials very portable
  • Required materials and equipment are relatively inexpensive

PT limitations:

  • Only surface breaking defects can be detected
  • Only materials with relatively nonporous surface can be inspected
  • Precleaning is critical
  • Metal smearing from machining, grinding, and grit or vapor blasting must be removed prior to inspection.
  • Surface finish and roughness can affect inspection sensitivity
  • Multiple process operations must be performed with waiting intervals
  • Post cleaning of acceptable parts or materials is required
  • Handling and proper disposal of chemicals is required
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  • Are all PT indications defects?

    No. Non-relevant indications can appear because of inherent surface roughness or seams. Fingerprints or fibers can also produce non-relevant indications.

  • During which phase of the manufacturing process should PT inspection take place?

    PT testing is useful immediately after any manufacturing process which is known to cause discontinuities. Therefore, it may sometimes be performed more than once during the manufacture of a part. It is important to carry out penetrant inspection before machining operations such as shot blasting, peening or grinding which can prevent penetrant inspections from finding these discontinuities.

  • What are the main differences between magnetic particle and dye penetrant testing?

    MT: fast inspection technique
    PT: relatively slow process

    MT: can only be applied in examination of ferromagnetic materials
    PT: provides optimal results in all metals and alloys

    MT: detects not only surface cracks but also imperfections that are very near to the surface
    PT: only detects discontinuities which are on surface and open for the dye to penetrate

    Surface conditions
    MT: no pre-cleaning and post-cleaning processes are necessary because surface pollutants don’t have adverse effect on inspection results
    PT: Strict pre-cleaning is required 

    MT: is applicable up to extreme temperatures (normally below Curie temperature)
    PT: limited temperature conditions (5-60 degree Celsius)

    MT: power supply is required to operate yoke magnet
    PT: no power supply needed

Henk Schroots

Area Sales Manager NDT & Laboratory

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