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.
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.