Dye Penetrant Inspection

Showing an object's true colours.

Dye Penetrant Inspection (DPI) or 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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  • 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 volume components 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

  • This method is typically ‘colour-contrast’ with a red dye, however, for high sensitivity, a fluorescent dye and blacklight can be used. This requires a dark testing environment.


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