Penetrant method of non-destructive testing is dedicated to detecting surface defects invisible to the naked eye. It is based on use of capillary properties of liquids namely, on capillary penetration of special indicator liquids inside defects. This method reveals defects by forming an indication patterns on the surface of tested items. These patterns have high optical luminance and color contrast and width exceeding width of the defect opening.

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Liquid penetrant inspection is used to detect any surface-connected discontinuities such as cracks from fatigue, quenching, and grinding, as well as fractures, porosity, incomplete fusion, and flaws in joints. DPI is based upon capillary action, where low surface tension fluid penetrates into clean and dry surface-breaking discontinuities. Penetrant may be applied to the test component by dipping, spraying, or brushing. After adequate penetration time has been allowed, the excess penetrant is removed, a developer is applied. The developer helps to draw penetrant out of the flaw where an invisible indication becomes visible to the inspector.