Non-destructive testing

Penetrating liquid

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Inspection by penetrating liquids.

The inspection by penetrating liquids is a type of non-destructive test which is used to detect and identify discontinuities present in the surface of a material under investigation. It is generally employed with non-ferrous alloys, although it may also be used in the inspection of ferrous materials when the inspection by magnetic particles is difficult to implement.

In some cases the technique may be used on non-metallic materials. The procedure involves applying a colored or fluorescent liquid to the service to be examined, and which will seep – by capillarity - into any discontinuities which may exist. After a predetermined amount of time, the excess liquid is removed, and a revealing agent is absorbing the liquid which has penetrated the discontinuities, leaving behind a film on which can be seen the contours of the imperfections.

The applications for this technique are wide ranging, going from the inspection of critical parts – such as aeronautical components to the inspection of ceramics.

Regarding their color
  • Colored penetrating liquids: Inspection is done with the naked eye. All that is required is a good source of white light. They offer a lower sensitivity than other methods.
  • Fluorescent penetrating liquids: Investigation is conducted with the help of an ultraviolet light source (black light). In the absence of this light, the imperfections are invisible. This method offers a higher sensitivity.
Regarding their solubility
  • Colored penetrating liquids: Inspection is done with the naked eye. All that is required is a good source of white light. They offer a lower sensitivity than other methods.
  • Fluorescent penetrating liquids: Investigation is conducted with the help of an ultraviolet light source (black light). In the absence of this light, the imperfections are invisible. This method offers a higher sensitivity.