Liquid or dye penetrant testing (PT) is based on the infiltration of liquid penetrant into open surface discontinuities. Penetrant testing does not apply to highly porous materials. The dye penetrant method is used to determine surface defects, including surface cracks, pores, lack of fusion, and intergranular corrosion. Dye penetrant testing is performed on welds and parent material.

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. The method can be manual, semi-automatic, or fully automated. Penetrant inspections on continuous-operation production lines in which the specimens are cleaned dipped, washed, and dried on a time cycle are common.

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