Published on 14-Apr-2023

What Is Liquid Penetrant Testing?

What Is Liquid Penetrant Testing?

Liquid Penetrant Testing, also known as Dye Penetrant Testing is a non-destructive testing technique that makes use of capillary forces to uncover the cracks or pores present on the surface of the material. It can inspect surface-breaking drawbacks like cracks, laps, porosity cracks, quench cracks, fatigue cracks, grinding cracks, overload cracks, and impact fractures.

Liquid Penetrant Testing is one step ahead of visual inspection and it has so many advantages, such as speed, large-area coverage and it's a low cost method.

It is generally a process of six stages: 

1) Cleaning of the surface.

2) Application of a penetrant liquid on the surface of the material. 

3) After this, excess penetrant is removed.

4) The developer is applied on the surface.

5) Examination of the test surface. 

6) Cleaning is done after the inspection is completed. 

The three main categories of the penetrant procedure are:

a) Water-soluble

b) Post-emulsifiable with water rinsing

c) Removal of solvent, the disparity is the technique utilized to eliminate the additional penetrant.

In all of these three categories, the penetrant solution includes a dye to create the indication noticeable under white light, or a fluorescent substance which fluoresces under acceptable ultraviolet (UV-A) light.

The choice of technique for any specific application is based on:

  • The surface finish of the sample material.
  • The compatibility of the substances with the sample.
  • The requirement of sensitivity.
  • The shape, size and accessibility of the region are to be examined.
  • The utmost utilization of the component.

Fluorescent penetrants are normally utilized when the maximum sensitivity is expected.

Principles Of Liquid Penetrant Testing

The liquid penetrant testing principle is that the liquid penetrant is brought out into the surface-breaking crack by capillary effort and then the additional penetrant present on the surface is eliminated. A developer is applied to the surface of the material, to draw out the penetrant into the crack that will generate an indication. The indications generated are much wider than the real defect and are thus more easily noticeable.

Liquid penetrant testing principle can be applied to any non-porous clear substance, metallic or non-metallic, but is problematic for dirty or very rough surfaces. Cleaning of the surface is an essential aspect of the penetrant testing method. The technique can be physical, semi-automatic or completely automated. Penetrant examination, continuous-operation production lines in which the samples are cleared, dipped, rinsed, dried, etc on a time cycle are ordinary.

Liquid Penetrant Testing Application 


This test is used to inspect the crack that can take place at the time of the solidification process of deposited weld metal, and it may occur in weld metal or in weld heat-affected region. The lack of fusion on the surface can also be detected by this particular test.

The surface porosity is a very general defect present on the surface of the material which can be identified visually & more precisely through a dye penetration test. The acceptance criteria for the liquid penetrant examination for welding have been talked about in the ASME Code Section VIII Div 1 Mandatory appendix 8.

This specific section encompasses the criteria for both pressure vessels and rotating equipment like pumps and compressors. Always consider the API codes such as API 610  and API 617, and do not deal with the acceptance criteria for Penetrant test directly and generally are referred to ASME.


The casting surface porosity, hot tear, cold shout or surface shrinkage, can be identified by liquid penetrant testing. The acceptance criteria have been mentioned in ASME Section VIII Div 1. Mandatory Appendix 7.

This acceptance criterion includes both fixed equipment, for example, heat exchangers, rotating equipment such as compressors & pumps, pressure vessels etc. The ASME Section VIII is a pressure vessel code, but API codes such as API 610 (for Pumps) or API 617 direct the users to the ASME code for acceptance criteria.


The forging surface helps in detecting bursts and laps, these two can easily be spotted by conducting a liquid penetrant test.


This examination is utilized to identify lamination. This is a normal flaw that occurs in plate material. This can be inspected only if the lamination is open to the surface area.

Have a read: Acceptance Criteria for Liquid Penetrant Testing -LPT


Hoping that this blog was an immensely helpful article for you. Want to explore more about Liquid Penetrant Testing? Then do read our articles that cover everything related to non-destructive testing. 

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Application Notes


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