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Penetrant testing is widely used in the non-destructive testing of pressure vessels, and it is mainly divided into two types: fluorescent penetrant testing and colored penetrant testing. Penetrant testing uses green fluorescent penetrants or red colored penetrants to reveal the traces of enlarged defect images, thereby enabling the detection of cracking defects on the surface of the test piece with the naked eye. The method that uses fluorescent penetrant is called fluorescent penetrant inspection, while the method that uses red-colored penetrant is called colored penetrant inspection. The basic procedure for using penetrant testing is as follows. 01 Penetration. First, the test piece is immersed in the penetrant, or the penetrant is applied to its surface using a sprayer or brush; if the test piece has cracks, pores, and other defects, the penetrant will seep into them. This process is called osmosis. The penetrant used for fluorescence detection is a fluorescent liquid, and ultraviolet light is applied during the inspection. 02 Cleaning. After the penetrant has fully penetrated into the defect, wash away the penetrant from the surface of the test piece using a cleaning agent or water. This process is called cleaning. Since the cleaning agent cannot penetrate into the cracks and pores containing penetrant, the penetrant in those cracks and pores remains. 03 Imaging. By spraying the developer on the surface of the test specimen, the penetrant remaining in cracks and pores is drawn out to the surface by the developer, resulting in amplified yellow-green fluorescent or red visible traces; this process is called development. 04 Observation. The traces left by fluorescent penetrants emit strong fluorescence under ultraviolet light, while those left by colored penetrants appear red in natural light, making them easy to identify; even very minor defects can be detected with the naked eye. There are 3 types of marks: continuous lines represent cracks, dotted lines represent narrow cracks, and scattered dot marks represent pores. Note: In fact, in addition to the basic processes mentioned above, preprocessing is sometimes required to facilitate penetration. Additionally, drying treatment is sometimes required for imaging purposes ； Or, to make the penetrating fluid easy to wash off, some of these fluids are sometimes subjected to emulsification. The minimum size that can be detected by penetrant testing is determined by factors such as the properties of the detecting agent, the testing method, the quality of the testing procedure, and the surface roughness of the test piece; it is generally around 0.02 mm in depth and 0.001 mm in width. Furthermore, in fluorescent penetrant testing, if a penetrant with high fluorescence intensity is used and alternating stress is applied to the test piece during testing, the detection sensitivity can be further improved.