Dye-Penetrant Examination Physical Principles

Figure 700-6 illustrates the physical principles of PT. A liquid (referred to as a penetrant) is applied to the surface of the part being examined, and is drawn into cavities in the object by capillary action. The surface tension of the liquid draws it from the wet surface into the dry cavities. A dye (commonly red) is added to the liquid to make it clearly visible.

The surface is then wiped clean of the liquid, which causes a small quantity of the liquid that was drawn into the cavity to reemerge on the surface. Reemergence of the liquid onto the surface at this point is due to reverse capillary action, resulting from the now dry surface and wet cavity. Sufficient liquid may reemerge to be visible when the surface is wiped clean, but a developer is usually applied to the surface that greatly enhances the ability to detect the cavity.

The developer, a very fine powder, serves both to draw liquid penetrant out of the cavity, and to form an opaque layer on the surface that masks the potential confusing background of the workpiece surface. The developer is applied to the surface after it is wiped clean of the liquid penetrant. The powder forms a “spongelike” layer that draws more liquid out of the cavity by capillary action, which can be thought of as blotting, to make it more visible. Furthermore, the color of the developer (usually white) is chosen to contrast with the dye in the liquid to further enhance the visibility of the liquid reemerging from the cavity.

Physical Principles of Dye-Penetrant Examination (PT)

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