This inspection method applies to many kinds of compact and nonporous materials (metallic materials, plastic, composites…) of any geometry and size and allows the test of the whole testpiece surface.

The stages of liquid penetrant inspection are as follows:

• Firstly, the surface to be inspected is cleaned thoroughly to remove all traces of dirt and grease.
  A brightly coloured (contrast) or fluorescent liquid (penetrant) is then applied liberally to the component surface and allowed to penetrate any surface-breaking cracks or cavities (porosity, pinholes).
  The time the liquid is allowed to soak into the material's surface is normally about 20 minutes.
• After soaking, the excess liquid penetrant is wiped from the surface and a developer applied.
  The developer is usually a dry white powder, which draws penetrant out of any cracks by reverse capillary action to produce indications on the surface. These (coloured) indications are broader than the actual flaw and are therefore more easily visible.

There are number of different liquid penetrant systems used in industry:

• colour contrast dye
  These are the most commonly used systems. Solvent removable or water washable red dye systems. Thay typically comprise of three aerosol cans - cleaning fluid, penetrant and developer. These systems are often used to check weld quality during fabrication.
• Fluorescent dye
  Fluorescent penetrants are normally used when the maximum flaw sensitivity is required. However, these penetrants must be viewed under darkened conditions with a UV lamp (UV-A Radiation also known as black light), which may not be practical in some situations.

Important considerations

Despite being one of the popular NDT methods, liquid penetrant testing is often misused. Test surfaces are not cleaned adequately, the contact time between the penetrant and the test surface is too short, or the excess penetrant is removed carelessly (i.e. from flaws as well as from the test surface). For these reasons, it is important that personnel carrying out liquid penetrant inspection are properly trained and qualified (for example, in accordance with the British Institute of Non Destructive Testing's PCN certification scheme or equivalent schemes such as those operated by CSWIP and ASNT).

Where is it used?

If applied correctly, liquid penetrant testing offers a fast, cheap and relatively simple means of surface inspection, making it attractive to a number of industries. At the 'high-tech' end of the market, the aerospace industry use automated fluorescent penetrant testing to look for fatigue cracking in turbine blades. At the other end of the scale, the construction industry uses dye penetrant testing as a quick and simple method for checking that welds and other susceptible areas are free from surface-breaking flaws.