Special fluid with low-viscosity
After the surface of the test object has been carefully cleaned, a low-viscosity penetrant is applied. Capillary action leads to the penetrant being pulled into any irregularities in the surface, after which the remaining penetrant is removed from the part – leaving the dye behind which has penetrated into cracks or holes. A developer fluid is then applied to the surface (in some cases, powder may be used instead); a chemical reaction between the penetrant and the developer results in coloured areas anywhere on the part surface where there is a defect of some kind. Certain types of developers contain an agent which produces a fluorescent effect under UV-light, making identification of defects even easier.
Dye Penetrant Investigation is not suitable for identifying deeper cracks; the dye may penetrate so deeply into the part that it does not reach the surface again even after application of the developer.