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 Basic Knowledge of Penetrant Testing (PT)
Surface cracks which can occur during the manufacturing of technical components or pores in castings which are opened by machining or cutting can be made visible by means of the penetrant testing method. It is a relatively simple method and enables the detection of surface defects like pores, cracks, lacks of fusion and forging laps which are open at the surface.

 Main Applications
  • Inspection of non-ferromagnetic materials
  • On-site inspection of pipelines, vessels and chemical plants (e.g. welds)
  • Re-examinations of aircrafts
  • Testing of aluminum and titanium components in the aerospace and automotive industry
 Principle
The dye penetrant method is always applied when the material is not ferromagnetic and therefore the magnetic particle testing method cannot be used.

Phases of penetrant testing  In a first step the object to be inspected must be cleaned thoroughly (1 and 2).
After thorough cleaning of the surface a (most often) red liquid (dye penetrant) is dispersed on the inspection area. Due to capillary action the liquid penetrates into existing cracks (3).
After a penetration time of approximately 10 minutes (the time depends on the material type and defect size to be detected) the surface is carefully cleaned again. The penetrant must remain in the cracks (4).
Then a white developer is sprayed as a thin film on the surface (5) which “sucks” the penetrant out of the crack (6). This usually takes approx.10 minutes again.
Due to the high contrast between the red penetrant and the white developer, cracks are clearly indicated. The penetrant can also contain fluorescent pigments which generate bright indications with a high contrast under ultraviolet light (UV light).

The three chemicals (penetrant, cleaner and developer) form one test system. According to the standards, only approved tested systems are allowed and should undergo the approval on an external laboratory. An exchange of one chemical within one test system by another chemical of another test system is not accepted.
The method also works if the objects are immersed in a tank for every inspection step. This enables an automated test procedure. However, the final evaluation is usually done visually by the operator.
Under certain circumstances the method can also be applied for leak testing (common abbreviation LT). For this purpose the penetrant is sprayed on one side and the developer on the other side of the object. Leakages then are indicated where the penetrant appears on the developer side.

 Advantages of Dye Penetrant Testing
  • On-site inspection or serial testing in stationary setups
  • Highly sensitve method (if carried out properly)
  • No particular radiation protection required
  • Applicable on almost all solid materials (except foams or porous / sintered materials). Only approved method for complex-shaped aluminium or light metal parts for the aerospace (e.g. turbine blades) and automotive industry (steering knuckles, pistons etc.)
  • Detection of cracks and pores at the same time
  • Almost independent from complex object geometries
  • Low expenses for the test agents, therefore relatively cheap
  • No special requirements concerning the operator’s expertise and skill. However, carefulness and permanent attention is needed.

 Further Information

Knowledge Floater "Dye Penetrant Testing"

You can find further extensive information from the following knowledge floater video "Dye Penetrant Testing":



Note: This video frame links to Youtube. Please ensure that no internal browser setting prevents external loading or click here for direct access to our NDTChannel on Youtube.
Alternative: The Knowledge Floater can be downloaded as zipped Powerpoint ® file here (size 37 MB approx.). The zip-file also contains the necessary powerpoint viewer. Unpack the archive into a separate directory and start the presentation by opening the file "play.bat".


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