Liquid Penetrant Testing

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Liquid penetrant testing

• This is a method which can be employed for the
  detection of open-to-surface discontinuities in
  any industrial product which is made from a
  non-porous material. This method is widely used
  for testing of non-magnetic materials. In this
  method a liquid penetrant is applied to the
  surface of the product for a certain
  predetermined time, after which the excess
  penetrant is removed from the surface. The
  surface is then dried and a developer is applied
  to it. The penetrant which remains in the
  discontinuity is absorbed by the developer to
  indicate the presence as well as the location,
  size and nature of the discontinuity.
• Penetrants used are either visible dye penetrant
  or fluorescent dye penetrant. The inspection for
  the presence of visible dye indications is made
  under white light while inspection of presence of
  indications by fluorescent dye penetrant is made
  under ultraviolet (or black) light under darkened
  conditions. The liquid penetrant processes are
  further sub-divided according to the method of
  washing of the specimen. The penetrants can be.
• (i) water-washable
  (ii) postemulsifiable

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i.e. an emulsifier is added to the excess
penetrant on surface of the specimen to make it
water-washable, and (iii) solvent removable, i.e.
the excess penetrant is needed to be dissolved in
a solvent to remove it from the test specimen
surface. In order of decreasing sensitivity and
decreasing cost, the liquid penetrant processes
canbelistedas

• Post emulsifiable fluorescent dye penetrant.
• Solvent removable fluorescent dye penetrant.
• Water washable fluorescent dye penetrant.
• Post emulsifiable visible dye penetrant.
• Solvent removable visible dye penetrant.
• Water washable visible dye penetrant.

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Some of the limitations of liquid penetrant
testing are as follows

• Test surface must be free of all contaminants
  (dirt, oil, grease, paint, rust, etc.
• Detects surface discontinuities only.
• Cannot be used on porous specimens and is
  difficult to use on very rough surfaces.
• Removal of all penetrant materials, following the
  test, is often required.
• There is no easy method to produce permanent
  record.