Liquid Dye Penetrant Testing

Liquid Dye Penetrant is mainly used to detect fatigue and stress cracks in ferrous or non-ferrous materials. Penetrant testing will also reveal any cracks, pinholes, laps, seams, leaks as well as grinding cracks. This works on the capillary effect that small voids will draw in the dye. After the dye has been administered, the surface is cleaned. Then a developing agent is applied to expose the residual dye, and the void is revealed.

Fluorescent penetrants contain dyes that are visible only under black light. Black light is a special light with wavelengths between visible and ultraviolet light. Almost invisible, black light causes many materials such as the fluorescent dye to glow in the dark. This fluorescence is normally a brilliant yellow-green that catches the eye and magnifies the indication.

Visible dye penetrants contain dyes that can be seen in visible or white light. They are also known as color contrast or nonfluorescent penetrants. The dye is usually bright red, but different colors can be used for special applications.

Of the two dye penetrants, the visible or color contrast is the simplest to use. No darkened area or black light is needed. However the primary limitation is getting enough contrast or dye into a discontinuity to form an indication. The best sensitivity can be obtained using the fluorescent penetrants.

When using color contrast the surface is covered with penetrating liquid that seeks surface connected cracks. Liquid in cracks bleeds out to stain powder-coating applied to the surface after the removal of excessive liquid film from the surface of the test object.

Penetrant testing can be used in a variety of applications on nonporous and nonabsorbent materials. It can be used on surfaces, entire objects and complex shapes. It can also be used as a control step in metal processing and joining operations. Cracks can be found that are formed during the testing or operation of equipment. Some applications are weldments, joints, tubing, castings and billets. Fuel and liquid-oxygen tanks and vessels, aluminum parts, gas turbine disks and blades and a variety of other components lend themselves well to inspection by the liquid penetrant method.

There are a few limitations such as access being required for surface cleaning and the discontinuity must be surface-connected and open. There may be false indications form shallow scratches and smearing and surface porosity may mask some indications.