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Liquid Penetrant Testing

Liquid Penetrant Testing (LPT) is one of the most widely used nondestructive evaluation (NDE) methods. Its popularity can be attributed to two main factors: its relative ease of use and its flexibility. LPT can be used to inspect almost any material provided that its surface is not extremely rough or porous.

Materials that are commonly inspected using LPT include the following:

  • Metals (aluminum, copper, steel, titanium, etc.)
  • Glass
  • Many ceramic materials
  • Rubber
  • Plastics

LPT offers flexibility in performing inspections because it can be applied in a large variety of applications ranging from automotive spark plugs to critical aircraft components. Penetrant materials can be applied with a spray can or a cotton swab to inspect for flaws known to occur in a specific area or it can be applied by dipping or spraying to quickly inspect large areas. In the image above, visible dye penetrant is being locally applied to a highly loaded connecting point to check for fatigue cracking.

Liquid penetrant inspection can only be used to inspect for flaws that break the surface of the sample. Some of these flaws are listed below:

  • Fatigue cracks
  • Quench cracks
  • Grinding cracks
  • Overload and impact fractures
  • Porosity
  • Laps
  • Seams
  • Pin holes in welds
  • Lack of fusion or braising along the edge of the bond line

As mentioned above, one of the major limitations of a penetrant inspection is that flaws must be open to the surface.

Advantages
  • The method has high sensitivity to small surface discontinuities.
  • The method has few material limitations, i.e. metallic and nonmetallic, magnetic and nonmagnetic, and conductive and nonconductive materials may be inspected.
  • Large areas and large volumes of parts/materials can be inspected rapidly and at low cost.
  • Parts with complex geometric shapes are routinely inspected.
  • Indications are produced directly on the surface of the part and constitute a visual representation of the flaw.
  • Aerosol spray cans make penetrant materials very portable.
  • Penetrant materials and associated equipment are relatively inexpensive.