Liquid Penetrant Testing

Liquid penetrant testing (LPT) is a nondestructive testing technique for detecting surface defects on both ferromagnetic and non-ferromagnetic materials. The technique works based on capillary action, which is the tendency of a liquid to rise, or saturate. The types of defects that can be found using liquid penetrant testing include, but are but not limited to: cracks, porosity, undercut, overlap fatigue cracks, and hairline cracks.

The two most commonly used penetrant testing techniques are the visible method and the fluorescent method. The visible method relies on a penetrant dye that is usually bright red in color, and can be seen with the naked eye. The fluorescent method uses a penetrant dye that cannot be seen by the naked eye and requires the use of ultraviolet light to enhance the indications. That being said, the fluorescent method tends to be more sensitive and can reveal smaller defects.


There are several specific steps that need to be completed for a liquid penetrant exam to take place.

  • Surface preparation: Prior to testing, the surface of a part being tested needs to be cleaned and free of oil, grease, water, or other contaminants. This is done because these contaminants can skew the results of the examination.
  • Application of penetrant: The penetrant can be applied by either spraying it onto the surface, brushing it onto the surface, or immersing the part in penetrant. The penetrant is then left to dwell for some time so that the penetrant has time to seep into any defects.
  • Excessive penetrant removal: After the dwell period, any excess penetrant is removed from the surface of the part. This needs to be done carefully so as little as penetrant as possible is removed from the defects.
  • Application of developer: The developer draws the penetrant out of the flaws they’ve seeped into. This allows them to be more easily seen by inspectors.
  • Evaluation: Because any cracks or defects are now highlighted by penetrant, they become much easier to see, including those that might not have even been visible to the naked eye before at all.
  • Post cleaning: The penetrant and developer is cleaned off of the part.


Liquid penetrant testing has several advantages over other forms of inspection:

  • It is economical. Both penetrant and developer are fairly cheap, and the process doesn’t require any other expensive equipment, except for sometimes an ultraviolet light.
  • LPT allows for quick inspection of large areas.
  • It doesn't rely on outside power sources for the visible method, although the fluorescent method does require an ultraviolet light.
  • Because LPT doesn’t require any bulk equipment, it is portable.


The formal reports consist of a detailed description of the equipment, inspection technique, inspection findings, summary of results, and sketch or photo of the test specimen.


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