Dye penetrant testing (PT) is a nondestructive method for crack detection in metals. PT is a process by which a liquid dye is applied to the surface to be inspected; this liquid dye seeps into cracks and other voids or depressions in the surface. The surface is then wiped clean, removing all of the dye from the surface. Dye that has penetrated into a crack is not removed by the cleaning and emerges from the crack due to capillary action. This results in a visible indication of the crack on the surface.(1,2)
To ensure that the indication is easy to observe visually, the color contrast between the liquid dye and the surface undergoing inspection may be enhanced. For typical field inspections of bridges, this is accomplished by applying a developer to the surface after it has been cleaned. Developers are typically white in color; the liquid dye is typically red. The color contrast between the white developer and the red dye ensures that crack indications will be easily observed by an inspector.
Another method of improving the contrast between an indication and the surface being inspected is to use fluorescent dye. A fluorescent dye contains particles that, when illuminated with ultraviolet light, emit intense visible spectrum light. This improves the light intensity contrast between the fluorescing dye crack indication, and the surface under inspection, which is not fluorescing. However, generally dark ambient light conditions (= 2 fc) are required to observe the light contrasts between the dye and the surface being inspected.(1,2) Consequently, fluorescent PT is not commonly used for bridges in the field.