These tests reveal discontinuities that are open to the surface and may use dyes or fluorescent materials. They are specially useful for non ferrous metals and non - metallic substances. The simple test involves dipping the component into kerosene, wiping it dry, and then coating it thinly with whiting. cracks will be revealed by a discolored 'line" appearing in the "whiting", this being produced when kerosene trapped in the cracks seeps out slowly. Dyes may be used instead of kerosene , but fluorescent materials are probably the most widely used.
In the fluorescent test, the sample is immersed for sometime in a hot bath of a strongly fluorescent compound such as anthracene, used as a penetrating agent. The solution enters the cracks, if any, and remains there. The metal is then dried and examined under a quartz tube mercury vapor lamp. Any penetrated solution will be detected by the fluorescence caused by the ultraviolet radiation of the light.
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