Published on 13-Dec-2022

Liquid Penetrant Testing: An Overview

Liquid Penetrant Testing: An Overview

There are various non-destructive testing techniques, but very few are favourable for all types of applications. For example, for most composite substances, applying electromagnetic testing methods will not work at all. Still, if you go on with computed tomography & X-rays, they will be a perfect choice!

  • Porosity
  • Seams
  • Laps
  • Fatigue Cracks
  • Pin holes in welds
  • Grinding cracks
  • Quench cracks
  • Absence of fusion/braising along the edge of the bond border
  • Overload & consequence fractures


The Process Of Liquid Penetrant Testing — Explained In 8 Simple Steps

1) Preparation Of The Surface

The first step in liquid penetrant testing is surface preparation. The material's surface must be free of dirt, grease, oil, water, or additional contaminants that might restrict the penetrant from seeping into the flawed material.

The given testing material might also need etching if mechanical operations like sanding, machining, or grit blasting have been conducted previously. These and many other mechanical operations can actually coat or mark material over the opening of the flaw and prevent the liquid from penetrating. The fluid is applied to the surface of the material and penetrates into the crack of the surface.


2) Application of the penetrant 

After cleaning and drying the material's surface, the penetrant must be applied by brushing, spraying, or immersing the part completely into the penetrant.


3) Letting The Penetrant Enter Into The Material

After the second step, the penetrant is left on the material's surface for a specific time to allow it to penetrate into the material. The penetrant dwell time is the complete time given to the penetrant to seep into the surface of the material.

The penetrant manufacturers or the required specification decides the dwell time. Here, the time might vary depending on the application, penetrant used, the sample material, the form of the substance being examined, and the type of substance being assessed for!

But the maximum dwell time generally ranges from 30-60 minutes. Typically, there is no disadvantage in utilizing a penetrant with a longer dwell time as long as the penetrant is not permitted to dry up. However, the perfect & ideal dwell time is actually specified by experiment and might be very specific to a respective application. The surplus penetrant that is on the surface of the piece must be rubbed out.

4) Removal Of Excess Penetrant

This is considered to be the most delicate & important part of the entire examination process. This Is so because the extra penetrant must be eliminated from the material's surface. Depending on the penetrant utilized, this particular step might include cleaning it by using a solvent, direct water rinsing, or treating the material with an emulsifier & then rinsing it with water. The developer helps draw out the penetrant trapped in the cracks so that it can be clearly visible to an examiner.


5) Application Of The Developer 

A layer of developer is applied to the sample material. Various forms of developer might be applied by spraying, dusting, or dipping.

6) Developer Time

In the sixth step, the developer is made to sit on the material's surface for a specific time to allow the extraction of penetrant out of any surface defects. The development time is generally a minimum of 10 to 15 minutes.

7) Assessment

Next, a thorough examination is conducted under proper lighting to identify the indications coming from any defects which might be present in the substance.

8) Surface Cleaning

So the last step in this whole process is to precisely clean the material's surface to eliminate the developer from the parts of the material that were found to be admissible.


Advantages Of Liquid Penetrant Testing 

There are a lot of advantages to using liquid penetrant testing. Some of them are mentioned below:

  • Delivers clear, real-world outcomes.
  • Operates on complex geometric structures.
  • The liquid penetrant testing materials are solely very cost-effective in nature.
  • It is quite sensitive to minor surface interruptions.
  • The materials are compact.

Only a few material limitations are there, such as works on metallic/non-metallic, magnetic/non-magnetic, and conductive/non-conductive materials. 

Disadvantages Of Liquid Penetrant Testing

 With so many benefits, liquid penetrant testing has a few disadvantages also! Let's check them out:

  • The user is reliable.
  • It is sensitive to surface-breaking deformities.
  • The testing procedure is multi-purpose.
  • Zero depth sizing.
  • It's a time taking procedure where post-cleaning of the material is very important.
  • A direct connection to the surface of the material under examination is significant.
  • For progress in the monitoring process, no recordable data can be attained.
  • It only works on fairly non-porous surface substances.

Therefore, in absoluteness, despite the quick captivation of this cost-effective testing method, it holds many downsides that must be looked into even before dismissing more advanced and additional costly examination solutions, whether you accord assessments or conduct them on yourself!

Key takeaways


Liquid penetrant testing is one of the most commonly utilized non-destructive testing techniques. The material & procedures that are utilized in the liquid penetrant testing are developed to create the outcomes of this capillary activity and the ability of interpretation. Liquid penetrant testing is an adequate way of uncovering and specifying the stringency of surface flaws in the materials, including those that are not visible to the eye.

Well, we hope that this article has been useful to you, and for more similar articles, visit our website now. 

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Application Notes


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