What is Dye Penetration Inspection (DPI)?

What is Dye Penetration Inspection?

The Dye Penetrant Test Method is the simplest NDT method. Dye Penetration Inspection (DPI) is generally known as Penetration test also called Liquid Penetration test (LPI). This method is used to find defects in the exposed surface of solid components, both metal and non-metal, such as ceramics and plastic fibers.

Through this method, defects in the material will be seen more clearly. The method is clear to apply the low viscous liquid with good penetration power to the surface of the metallic body. The excess fluid on the sample specimen layer is washed after the liquid has filled in. The defect will be clearly seen if the color difference between the penetrant and the background is quite contrasting.

After the inspection is completed the developer is sprayed to pull out the penetrants. The developer is then sprayed to pull out the penetrant from the defect. The contrasting color difference between the developer and the emerging penetrant fluid represents an indication of a defect.

This test uses the capillary properties of liquid objects used are non-viscous liquids and have a small surface tension, which is usually colored as a penetrant. The test material is dipped or sprayed with this liquid, because of its capillary properties, the liquid enters cracks, fissures or pores on the surface of the test material to the deepest part.

Basic things to do before conducting a DPI test:

  • The object inspected for its surface must be clean of all kinds of dirt, oil, olie, paraffin and so forth. Where the dirt will cover the defects that are examined

  • The object being inspected must be dry and porous

  • If the surface of the object is painted, then remove the paint with rubbing paper.

Materials used for DPI test:

Cleaner: Used to clean the surface of the component before applying the penetrant to make it clean from dirt, scale, grease etc

Penetrant: It penetrates in to crack and draws back when the developer is applied.

Developer: It is applied above the penertrant to remove the excess penetrant from the surface and act as a blotting paper to reveal the crack by drawing back the penetrant from the crack.

General procedure for DPI test:


The item to be inspected is first cleaned with a cleaner fluid from dirt, oil, dust, or other dirt that can cover the crack. If the object you want to check turns out to be coated with paint, the paint must first be cleaned.

Application of penetrant:


Apply the penetrants on the surface of the component, either by dipping the component in the penetrant or spraying. After that lift and let stand a few moments so that the penetrant can enter the crack gap optimally. It take time penetrant to enter the crack and the time according to the penetrant used.

Excess Penetrant Removal:


Clean the penetrant on the surface. Different methods can be used depending on the penetrant used. If the penetrant is washable by apply water, we can use pressurized water. But never use water to hit on the surface perpendicularly which can cause the removal of penetrant in the crack also.

To dry the remaining water after the washing process, the object is wiped and put into a heater with specified temperature and time

Application developer:

Now spary the developer on the surface, so that penetrant liquid in the crack can be pulled out. For penetrant types of fluorescence, can be seen with ultraviolet tools so that penetrants drawn by the developer will be clearly attracted.

To ensure that the penetrant that appears is from a crack, the penetrant that comes out must be cleaned with a cleaner and sprayed on the surface with developer fluid. If you see that there is still a penetrant liquid coming out of the surface, you can be sure that there is a crack on the surface of the object.


For visible dye penetration, the inspector will use visible light with sufficient intensity (100 foot candles are typical). Ultraviolet (UV-A) radiation of sufficient intensity (1,000 micro watts per centimeter square is common) along with low ambient light (less than 2 foot candles) for penetrating fluorescent examinations.

Test surface Inspection must take place after a development time of 10 minutes. This delay in time allows the blotting action to take place. The specimen may be detected by the inspector when using visible dye.


  • Grinding cracks

  • Heat affect zone cracks

  • Poor weld penetration

  • Heat treatment cracks

  • Fatigue cracks

  • Hydrogen cracks

  • Inclusions

  • Laminations

  • Micro shrinkage

  • Gas porosity

  • Hot tears

  • Cold shuts

  • Stress corrosion cracks

  • Intergranular corrosion