The Ultimate Guide to Fluorescent Penetrant Testing [Infographic]

Posted By: Steve Published: 03/05/2020 Times Read: 2087

The Importance of Liquid Penetrant Inspection

Liquid penetrant inspection is a nondestructive test method which does not harm the parts being inspected. The testing methods detect cracks, fractures, laps, seams and other flaws that are open to the surface that may have been caused by fatigue, impact, quenching, machining, grinding, forging or overload. The process is used on machined parts, castings, forgings and other items that will be placed into service or already in service requiring maintenance.

The process involves applying a penetrant chemical, visible or fluorescent, directly to the part that’s to be inspected. The excess penetrant is removed and a developer is applied to draw the penetrant deep from the cracks to the surface of the part. The penetrant contrasts with the surface of the part so the crack is identified more easily. Fluorescent penetrants are used under an ultraviolet light that makes penetrant fluoresce. The chemical, equipment and NDT accessory choices have grown but the fundamentals and benefits of liquid penetrant inspection have endured the test of time.

Some major benefits to this method are:

  • Versatile material uses and applications
    • Liquid penetrant inspection can be performed on most materials that are not extremely rough or porous, including material composition that is metallic or nonmetallic, magnetic or nonmagnetic, and conductive or non-conductive
  • Rapidly inspect large areas and high volumes of parts
    • Liquid penetrant inspection is a fast, easy and efficient means of surface inspection. Large quantities of parts or materials can be inspected quickly. Parts of almost any shape, size and geometry can be inspected
  • Identify small surface discontinuities with high sensitivity
    • Sensitivity levels are a classification system specifically for fluorescent liquid penetrants that are not applied to visible penetrants. Higher sensitivity penetrants have the capability to detect smaller cracks and defects

Selecting the Correct Penetrant Method Per AMS 2644

Penetrant Types

  • Type 01: Fluorescent dye
  • Type 02: Visible dye

Penetrant Methods

  • Method A: Water washable
  • Method B: Post emulsifiable, lipophilic
  • Method C: Solvent removable
  • Method D: Post emulsifiable, hydrophilic

Developer Forms

  • Form a: Dry powder
  • Form b: Water soluble
  • Form c: Water suspendible
  • Form d: Nonaqueous Type 1 (fluorescent)
  • Form e: Nonaqueous Type 2 (visible)

Click here to learn the three steps to narrow down a penetrant method through governing specifications, part composition, and test parts.

Differences Between Sensitivity

AMS 2644 Penetrant Sensitivity Levels

Sensitivity levels are a classification system for fluorescent liquid penetrants which is outlined in AMS 2644 Inspection Material, Penetrant specification. Sensitivity levels are not applied to visible penetrants.

  • Level ½: Ultra low sensitivity
  • Level 1: Low sensitivity
  • Level 2: Medium sensitivity
  • Level 3: High sensitivity
  • Level 4: Ultra high sensitivity

How to Determine Which Sensitivity Level to Use

The primary consideration when deciding on which sensitivity level penetrant to use is the guidance provided by governing specifications. The penetrant sensitivity level is stated in the working specifications for the process. This is particularly true when penetrant inspection involves safety critical parts. The specifications and standard operating procedures will dictate the penetrant sensitivity to be used.

If the penetrant sensitivity level is not already established by a specification or procedure, evaluation on actual parts with a few penetrants is strongly recommended. A lower sensitivity penetrant works well on rough surfaces, while a higher sensitivity penetrant is suitable for highly machined surfaces. Penetrant evaluation on actual parts is important to confirm proper sensitivity with acceptable background fluorescence to detect the type of defects typically found on the parts.

Penetrant evaluation on actual test parts can also confirm proper processing parameters such as dwell time and rinse time. It is possible that more than one penetrant sensitivity level will be used in a facility because of different parts, processes, and customer specifications in use at a given location.

Click here to understand fluorescent liquid penetrant sensitivity levels and how to find the right one for your application

Common Applications

Aerospace processes commonly use Method D Level 4 penetrants for safety-critical rotating parts including turbine bladesAutomotive processes commonly use Method A Level 1 penetrants to inspect non-ferrous components such as steering knuckles and subframesPower Generation commonly uses Method A Level 2 penetrants to inspect non-ferrous parts such as large castings, forgings, impeller blades, and more

Common Terms

  • Background – the surface of the test part against which the indication is viewed. It may be the natural surface of the test part or the developer coating on the surface
  • Bleedout – the action of an entrapped liquid penetrant in surfacing from discontinuities to form indications
  • Developer – a material that is applied to the test surface to accelerate bleedout and to enhance the contrast of indications
  • Developing Time – the elapsed time between the application of the developer and the examination of the part
  • Dwell Time – the total time that the penetrant or emulsifier is in contact with the test surface, including the time required for application and the drain time
  • Inspection – visual examination of the test part after completion of the liquid penetrant processing steps
  • Liquid Penetrant Testing – a nondestructive test that uses liquid penetrant materials to penetrate and detect various types of discontinuities open to the surface
  • Liquid Penetrant – a solution of dye with the ability to penetrate into fine openings
  • Pre-cleaning – the removal of surface contaminants from the test part so that they will not interfere with the examination process
  • Sensitivity Level – the descriptive term for identifying the capability of a penetrant system to indicate the presence of a surface-connected discontinuity


"The Ultimate Guide to Fluorescent Penetrant Testing [Infographic]". Magnaflux Blog, 28 January 2020,

For more information on the liquid penetrant process, quotes and availability of liquid penetrant materials, equipment, & accessories please reach out to us at:

Tags: non-destructive testing (ndt), magnaflux, liquid penetrant inspection (lpi), ams, 2644, fluorescent, visible, type, 1, 2, method, a, b, c, d, form, e, water washable, post emulsifiable, lipophilic, solvent removable, hydrophilic, sensitivity level,

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