Dye Penetrant Inspection

Dye Penetrant inspection (DPI), is also called Liquid Penetrant Inspection (LPI), and it is a widely applied, low-cost inspection method used to locate surface-breaking defects by use of bleed out of a colored or fluorescent dye in the defect. This technique can only be on all non-porous  materials, such as metals, plastics, or ceramics.  LPI is used to detect casting and forging defects, cracks, and leaks in new products, and fatigue cracks on in-service components.

This technique uses the capillary action to draw the penetrant in the flaw to the clean surface, thereby giving the inspector a visual indication of the defect.

The following the basic steps for Dye Penetrant Inspection:

  1. Surface Preparation: probably the most important step. The surface must be free of any oil, grease, water or any other contaminants that may prevent the penetrant from entering the surface breaking flaws
  2. Penetrant Application: Once the surface has been cleaned and dried, the dye penetrant material is applied by spraying, brushing or immersing the part in a penetrant bath. The penetrant is left on the surface for a sufficient time to allow as much penetrant as possible to be drawn from or to seep into a defect. The penetrant is then left to dwell. This is the total time that the penetrant is in contact with the part surface. Dwell times are usually recommended by the penetrant producers or required by the specification criteria being followed. The times may vary depending on the application, penetrant materials used, material, shape of the material being inspected, and the type of defect being inspected for. Dwell times usually range from 5 to 60 minutes. The general rule of thumb is that longer dwell time is fine, as long as the penetrant is not allowed to dry. The ideal dwell time is usually determined by experimentation or experience or may be very specific to a particular application.
  3. Excess Penetrant Removal: This is the most sensitive part of the PT inspection steps because the excess penetrant must be removed from the surface of the test piece while removing as little penetrant as possible from defects.  This step may involve cleaning with a solvent, rinsing directly with water, or first treating the part with an emulsifier and then rinsing the penetrant off with water.
  4. Developer Application: A thin layer of developer is then applied to the test piece to draw the penetrant trapped in flaws back to the surface where it is then visible. Developers may be applied by dusting (dry powdered), dipping, or spraying (wet developers).
  5. Indication Development: The developer is then allowed to stand on the test piece surface for a period of time, that time must be sufficient to permit the extraction of the trapped penetrant out of any surface breaking flaws. This development time is usually a minimum of 10 minutes.  Longer times may be necessary for tight cracks.
  6. Inspection: The technician is then able to perform the inspection, under appropriate lighting, to detect indications from any flaws which may be present in the test piece.
  7. Cleaning: After inspection is complete, all developer is then cleaned off the test piece.

Some of the Advantages of of Penetrant Inspection are:

  • High sensitivity to be able to detect small surface discontinuities.
  • PT has few material limitations able to inspect metallic and nonmetallic, magnetic and nonmagnetic,  conductive and nonconductive materials.
  • Large areas, test pieces and large volumes of parts/materials can be inspected quickly with relatively low cost.
  • Parts with complex geometric shapes are often, easily inspected.
  • Indications are surface breaking, giving the technician a visual representation of the flaw.
  • It is very portable, due to the use of aerosol spray cans.

Some of the Disadvantages of Penetrant Inspection are:

  • Only defects that break the surface can be detected.
  • Only relatively nonporous materials can be inspected.
  • If pre-cleaning is insufficient, contaminants can easily mask defects.
  • Metal smearing from machining, grinding, needle gunning, grit or vapor blasting has to be removed prior to LPI.
  • The inspector must have direct access to the surface being inspected.
  • Surface finish and roughness can very easily affect the inspection sensitivity and results.
  • Temperature of materials can affect test results
  • Post cleaning if generally required of acceptable parts after inspection is complete.
  • Chemical handling and proper disposal is required.