Liquid Dye Penetrant

Liquid Penetrant inspection (often abbreviated PT, FPI or LPI) is a nondestructive inspection method that provides detection of flaws open to the surface. It is a surface examination method only.

Basic Principle

This method of inspection works on the principle of capillary action. Liquid Penetrant is applied by immersion, spraying or brushing to the part to be inspected. The Penetrant works its way into surface openings via capillary action. Excess surface Penetrant is removed while allowing the Penetrant in flaws to remain. A developer is applied which acts as a blotter to draw the Penetrant from the flaws creating an indication on the surface of the part. This indication is either visible or fluorescent depending on the type of Penetrant used.

LPI process steps
Process Steps

  1. Preclean and dry test specimen surface
  2. Penetrant application
  3. Removal of excess surface penetrant
  4. Drying
  5. Developer application
  6. Inspection
  7. Postclean


Liquid Penetrant inspection started in the 1890’s as the Oil and Whiting method. The Oil was actually kerosene and was used as the Penetrant. The whiting was chalk dust suspended in alcohol and was used as the developer. This method was utilized by the railroad industry to detect large cracks in their components.


This method of inspection can be used on virtually any material and the results appear directly on the surface of the part. This method of inspection is used predominantly on nonferrous materials (aluminum, titanium, magnesium, etc.).


The single largest limitation of this method is that it will only reveal flaws that are physically open to the surface. Therefore surface preparation is critical to the effectiveness of this inspection method. In certain instances it is necessary to perform a pre-penetrant etch to remove smeared metal or excessive oxides that may have formed blocking the opening to flaws.

Other Details

The Penetrant material is an oil based chemical with either visible or fluorescent dye. There are various types, methods and sensitivity levels within the Penetrant inspection method.

Type I – FluorescentMethod A – Water WashableSensitivity Levels 1-4
Type II – Visible dyeMethod B – Post Emulsified (lipophilic)
Method C – Solvent Removable
Method D – Post Emulsified (hydrophilic)

The most commonly used Penetrant systems in aerospace are Type I, Methods A, B and D, Sensitivity Levels 3 and 4.


Liquid Fluorescent Penetrant Inspection is used for testing aerospace aluminum and other non-ferrous metals


Parts are solvent washed and immersed in a fluorescent dye


Removal of excess surface penetrant


Examination is performed under UV light. Cracks and other surface irregularities become visible.