Also known as: Dye Penetrant Inspection (DPI), Liquid Penetrant Inspection (LPI), Penetrant Testing (PT), and Fluorescent Testing. Liquid penetrant testing is one of the oldest methods of nondestructive testing still in use today because of its simplicity and remarkable sensitivity to locating very fine surface discontinuities. Used on ferrous, nonferrous, and even nonmetallic materials, dye penetrant inspection is the most common and cost effective of all NDT inspection methods.


  1. Fast
  2. Portable
  3. Cost-Effective
  4. High Sensitivity
  5. Flaws are visually indicated on surface
  6. Appropriate for large & complex shapes


As the name implies, Liquid Penetrant testing involves a colored penetrating liquid that is applied to the surface of the material being tested. The liquid is drawn into defective areas in the material’s surface through capillary action and then drawn out to reveal itself to detection using UV lights or other methods, depending upon the type of penetrant and dye coloring used.

Frontline Testing Laboratories provide a full range of economical liquid penetrant NDT services. Whether it’s high-volume production or testing individual, large, and complex objects, we offer quick turn-around and a high degree of reliability


  • Step 1: Preparation: The surface is prepared by removing any contaminants that could interfere with the process.
  • Step 2: Application of Penetrant: The penetrant is applied by spraying, brushing or immersing in a bath. It is then left to site for an amount of time to allow as much dye penetration as possible.
  • Step 3: Removal: In this step, the inspector carefully removes excess penetrant without removing penetrant from the defects. Water or emulsifiers are sometimes used.
  • Step 4: Application of Developer: A layer of developer is applied to the surface, drawing penetrant out of flaws and to the surface, making it visible.
  • Step 5: Inspection: With the aid of proper lighting, visual inspection is performed to detect any indications that may be present.
  • Step 6: Cleaning: The last step is to remove the developer material from the areas of the surface lacking in defects.

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