Friday, March 19, 2010

Liquid Penetrent Test

Sometimes, smallest of the defects can be hardly visible for human eyes. That is when engineers use the Liquid Penetrant (PT) or Dye Penetrant test. Liquid Penetrant Examination is a Non-Destructive Testing method used to detect surface breaking defects, such as cracks, laps, porosity etc., which are unperceivable with the bare eye.
The method is rest upon the ability of a fluid to be drawn into a "clean" surface-breaking flaw by smooth action. After a certain time period, unnecessary surface penetrant is displaced and a developer is applied that functions as a "blotter". Color contrast penetrants demand adequate white light whereas fluorescent penetrants are suggested to be used in dark environment and require ultraviolet "black light" where penetrant glows with a luminous color which can be easily seen from our eyes.

Figure: Penetrant testing process

  1. Cleaning the surface (1)
  2. Adding the first chemical liquid (penetrant) (3) where it goes through to the crack (2)
  3. Cleaning the surface once more. Though, some of the first chemical remains inside of the crack
  4. Adding the second chemical liquid (developer) (4), where the first liquid (5) penetrate to the second, so that the area of the crack will be shown to our eyes in a bigger scale

Dye Penetrant Examination is mainly used for clad layer, welds and heat affected zones. Penetrant inspection can be used on any solid material. It is essential that the material is carefully cleaned first; otherwise the penetrant will not be able to get into the flaw. If surface penetrant is not fully removed, misleading indications will result.
Surface defects are also dangerous. They also can grow up and lead in to a major structural failure. So it is very important to do frequent visual inspections for the whole structure and LP test for the identified parts with constant load changes.
The liquid comes in different forms. In the lab, penetrant, developer and the washing liquieds are kept inside of different tanks. the process of the test usually done as a line. Tanks are also kept in the way which makes the testing easier with less time waste. Additionally the liquids from tanks are reusable for quite a long time. The other way of inspection is, using them as sprays. It's easier to use them in the field. On an aircraft, or on a bridge for example. Very powerful black lights are used in outside conditions.

An examples I found in the youtube... Check it out!