Liquid Penetrant Inspection (PT)

Liquid Penetrant Inspection (PT)Surface cracks and pinholes that are not visible to the naked eye can be located by the liquid penetrant inspection. It is widely used to locate leaks in welds and can be applied with austentic steels and nonferrous materials where magnetic particle inspection would be useless.
Liquid penetrant inspection is often referred to as an extension of the visual inspection method. Many standards, such as the AWS D.1. Code, say that "welds subject to liquid penetrant testing�shall be evaluated on the basis of the requirements for visual inspection."
Two types of penetrating liquids are used - fluorescent and visible dye. With fluorescent penetrant inspection, a highly fluorescent liquid with good penetrating qualities is applied to the surface of the part to be examined. Capillary action draws the liquid into the surface openings, and the excess is then removed. A "developer" is used to draw the penetrant to the surface, and the resulting indication is viewed by ultraviolet (black) light. The high contrast between the fluorescent material and the object makes it possible to detect minute traces of penetrant that indicate surface defects.
Dye penetrant inspection is similar, except that vividly colored dyes visible under ordinary light are used - Fig. 4. Normally, a white developer is used with the dye penetrants that creates a sharply contrasting background to the vivid dye color. This allows greater portability by eliminating the need for ultraviolet light.
The part to be inspected must be clean and dry, because any foreign matter could close the cracks or pinholes and exclude the penetrant. Penetrants can be applied by dipping, spraying or brushing, but sufficient time must be allowed for the liquid to be fully absorbed into the discontinuities. This may take an hour or more in very exacting work.
Liquid penetrant inspection is widely used for leak detection. A common procedure is to apply fluorescent material to one side of a joint, wait an adequate time for capillary action to take place, and then view the other side with ultraviolet light. In thin-walled vessels, this technique will identify leaks that ordinarily would not be located by the usual air test with pressures of 5-20 lb/in.2 When wall thickness exceeds � in., however, sensitivity of the leak test decreases.