For parts with smooth and clean surfaces, use a colored (usually red) or fluorescent, highly permeable liquid to coat the surface of the parts to be tested.
If there are microcracks on the surface that cannot be directly detected by the naked eye, due to the strong permeability of the liquid, it will penetrate along the cracks to its roots. Then wash away the penetrating liquid on the surface, and then apply a display liquid with a higher contrast (usually white).
After leaving it for a while, because the crack is very narrow and the capillary phenomenon is significant, the penetrant that originally penetrated into the crack will rise to the surface and spread, showing a thick red line on the white substrate, thus showing that the crack is exposed on the surface. Shape, therefore, is often called tinted flaw detection.
If the penetrant is a fluorescent liquid, the liquid that rises to the surface due to capillarity will emit fluorescence under the irradiation of an ultraviolet lamp, which can better show the shape of the crack exposed on the surface. Therefore, the penetrating liquid at this time is often regarded as Flaw detection is directly called fluorescence flaw detection. This flaw detection method can also be used for flaw detection on metal and non-metallic surfaces. The flaw detection liquid used has a strong smell and is often toxic.
