Liquid Penetrant

LPI or liquid penetrant inspection or dye penetrant inspection, penetrant testing is a simple, low-cost method of detecting surface-breaking flaws such as cracks, laps, porosity, etc. To be detected, the flaw must reach the surface to be tested.

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There are three major groups of penetrant process: water-soluble, post-emulsifiable with water rinsing and solvent removable, the difference being in the method used to remove the excess penetrant. In each of these three groups the penetrant solution can contain a dye to make the indication visible under white light, or a fluorescent material which fluoresces under suitable ultraviolet (UV-A) light. Fluorescent penetrants are usually used when the maximum flaw sensitivity is required.

The principle of liquid penetrant testing is that the liquid penetrant is drawn into the surface-breaking crack by capillary action and excess surface penetrant is then removed; a developer (typically a dry powder) is then applied to the surface, to draw out the penetrant in the crack and produce a surface indication. Cracks as narrow as 150 nanometres can be detected. The indications produced are much broader than the actual flaw and are therefore more easily visible.

Liquid penetrant testing can be applied to any non-porous clean material, metallic or non-metallic, but is unsuitable for dirty or very rough surfaces. Surface cleaning is a vital part of the penetrant testing technique.

At Testa all of our NDT technicians are qualified to industry recognised PCN (personnel certification in non-destructive testing) in accordance with BS EN ISO 9712.

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Typically we use the technique to test stainless steel and other non-ferromagnetic materials. Carbon steel would be better suited to magnetic particle inspection. Benefits include: instant results, cost effective, on-site inspection.