Subscribe to our newsletter to receive the latest news and events from TWI:

Subscribe >
Skip to content

What is Liquid Penetrant Testing? (Definition, Advantages and Examples)


Liquid or dye penetrant testing is a non-destructive testing method that uses capillary forces to locate surface-breaking flaws such as cracks, laps and porosity.

The flaw must reach the surface in order to be tested, but liquid penetrant testing offers an inexpensive solution for testing large areas quickly.

The process works through a liquid penetrant being drawn into the surface-breaking flaw through capillary action before excess surface penetrant is removed. A developer, typically a dry powder, is then applied to the surface to draw the penetrant from within the flaw and produce a surface indication. The process can locate cracks as narrow as 150 nanometres and is often used to locate deficiencies in welds, pipes and bars, among other materials.

Liquid penetrant testing is also known as dye penetrant testing, a dye penetration test, dye penetrant inspection, liquid penetrant examination, or simply penetrant testing.


Click the below to skip to the section in the guide:


TWI provides support to our Industrial Members for a range of engineering challenges, including material and product testinginspection and testing services, and failure investigations, as well as research and technology and engineering consultancy. As well as offering training services and professional development and support for engineers with The Welding Institute.

TWI is an Industrial Membership based organisation. TWI's experts can provide your company with an extension to your own resources. Our experts are dedicated to helping industry improve safety, quality, efficiency and profitability in all aspects of materials joining technology. Industrial Membership of TWI currently extends to over 600 companies worldwide, embracing all industrial sectors.

You can find out more by contacting us, below:

How Does Liquid Penetrant Testing Work?

Liquid penetrant testing works through the infiltration of a liquid dye into open surface discontinuities. Used to locate surface defects such as cracks, pores, poor fusion or inter-granular corrosion, this testing method is performed on both parent materials and welds. However, the nature of this technique means it is not suitable for highly porous materials.

The testing is usually performed using a red dye penetrant, which can be seen in daylight, whereas fluorescent dyes are easier to evaluate in darkness or with an ultraviolet light. Liquid penetrant can be applied to non-porous, clean materials – whether metallic or non-metallic. However, it is unsuitable for dirty, rough or particularly porous surfaces.

The process is typically broken down into six stages, as follows:

1. Surface pre-cleaning

This can be performed manually, semi-automatically or automatically. It can be achieved by grinding, wire brushing or simply by wiping the test piece with a cloth moistened with the cleaner / removing agent  

2. Penetrant liquid application

The liquid penetrant is typically applied as an aerosol or with a brush. Once applied, time must be allowed for the penetrant to permeate into cracks and voids. This typically takes between 5 and 30 minutes, but the penetrant should not be left so long that it dries. Timings can vary and so it is best to follow the manufacturer’s recommendations for this

3. Remove excess penetrant

Excess penetrant should be removed with clean, dry, lint-free cloths. The part should be rubbed until there are no signs of the penetrant on the cloths. Once this has been achieved, a cleaner / remover should be sprayed on another cloth and the part rubbed again until there is bo visible penetrant on the cloth

4. Application of developer

A thin coating of developer is then sprayed on the part or material. This should ten be left for a dwell time so that the dye can exit the flaws to create an indication in the developer. This typically takes between 10 and 60 minutes according to the manufacturer’s recommendations

5. Inspection and evaluation

The part should be examined in line with the written procedure for the test. The length of the indication is measured for the evaluation rather than the size of the flaw

6. Post-inspection cleaning

The final step in the process is to clean the part and remove all of the developer following evaluation

The process can, in turn, be split into three groups according to how the excess penetrant is removed:

A.   Water-soluble

B.   Post-emulsifiable (with water rinsing)

C.   Solvent removable

The solution used for the penetrant can contain a dye that makes it visible under white light or a fluorescent that shows under ultraviolet light. 

Continuous-operation production lines with penetrant inspection, whereby specimens are cleaned, dipped, washed and dried on a specific time cycle, are fairly common. The visual inspection has also seen levels of automation with a camera pre-programmed to identify and recognise flaws as well as computerised image enhancement being used. Radioactive tracers have also been used for very high sensitivity penetrant processes, but this requires stringent safety procedures.

The exact choice of technique can be determined by:

a) The specimen’s surface finish

b) Compatibility of the materials with the specimen

c) Sensitivity requirements

d) Size, shape and accessibility of the inspection area

e) The component’s final use


Liquid penetrant testing offers a range of advantages:

  • Globally-accepted method of non-destructive testing
  • Easy to learn with minimal training
  • Capable of identifying surface cracks, pores, inter-granular corrosion and lack of fusion with a high sensitivity to small surface discontinuities
  • Able to test a wide range of materials, including ferritic and composite items
  • Fast, efficient, portable and inexpensive method for testing large areas and large volumes of parts
  • Able to inspect parts with complex shapes
  • Indicates the relative shape, size and depth of a flaw


Despite the many advantages, there are still some drawbacks associated with liquid penetrant testing:

  • Only able to detect flaws that are open to the surface
  • Cannot examine porous surfaces
  • Only able to examine clean and smooth surfaces (dirt, grease, paint, metal smearing or rust need to be removed)
  • Requires direct access to the surfaces to be examined
  • Requires multiple process steps
  • Requires post-cleaning of parts (especially where welding is to follow)
  • Requires proper handling and disposal of chemicals
  • Potential release of hazardous or flammable fumes with inadequate ventilation

Examples / Applications

Due to the ease of use, liquid penetrant testing is a commonly-used non-destructive testing technique.

It is used to inspect a range of bars, castings, forgings, pipes and welds, with the first recorded use being in the railway industry, where it was used to assess cast railroad wheels.

Liquid penetrant testing can be used on most non-porous materials, including aluminium, brass, bronze, cast iron, glass, plastics, rubber, stainless steel and steel.


Liquid penetrant testing, also known as dye penetrant testing, is a valuable tool for assessing new constructions, parts and in-service. While this non-destructive testing technique is capable of being used on a wide range of materials and is inexpensive and easy to learn, there are drawbacks with the process.

The process uses capillary forces to locate even very small surface cracks or pores so that they can be seen.

The nature of the process means that it is best used on clean and smooth materials, and not suitable for porous, rough or dirty material parts.


What is Liquid Penetrant Testing used for?

Liquid penetrant testing is used in a wide range of industries as it is simple to use, easy to learn, efficient and cost-effective. Capable of detecting surface-breaking flaws such as cracks, laps, and porosity, this process is used to locate defects in welds, bars, pipes and other components in a range of different materials.

Where is Liquid Penetrant Testing used?

Liquid penetrant testing can be used on any solid, non-porous and clean material, including ceramics, metals or plastics. This includes both magnetic and non-magnetic metals as well as those that are both conductive or non-conductive. It is frequently used to assess for flaws in castings, forgings and welded parts – including corrosion tests.

What is the Difference between a Dye Penetrant Test and a Liquid Penetrant Test?

Dye penetrant testing (DPT) and liquid penetrant testing (LPT) are the same thing. The process is also known by other names, including liquid penetrant inspection (LPI) liquid penetrant examination (LPE), or simply penetrant testing (PT).

For more information please email: