Crack test. Researcher's hand holding a metal component under ultraviolet (UV) light to check for cracks. The metal was immersed in a fluorescent dye, which permeated the small cracks in the metal. UV light makes the dye fluoresce green, allowing the cracks to be seen. Cracks in a component may be due to stress or metal fatigue, and can cause the failure of the entire machine of which it is a part.