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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 nanometers can be detected. The indications produced are much broader than the actual flaw and are therefore more easily visible.

The image shows the steps for dye penetrant testing. It starts with a contaminated workpiece, which is then cleaned. Flaw detection ink is applied, followed by wiping the workpiece. A developer is then applied, and finally, the workpiece is inspected for flaws indicated by red dye.

Liquid penetrant testing can be applied to any non-porous clean material, metallic or non-metallic, but unsuitable for dirty or rough surfaces. Surface cleaning is a vital part of the penetrant testing technique. The method can be manual, semi-automatic, or fully automated. Penetrant inspection, continuous-operation production lines in which the specimens are cleaned, dipped, washed, dried, etc., on a time cycle are standard.