Visible Liquid Penetrant Testing
Dye Penetrant Inspection is based upon capillary action, where fluid with low surface tension penetrates into clean and dry surface-breaking discontinuities. The penetrant may be applied to the test component by dipping, spraying, or brushing. After adequate penetration time has been allowed, the excess penetrant is removed and a developer is applied. The developer helps to draw penetrant out of the flaw so that an invisible indication becomes visible to the inspector. Inspection is performed under white light since we are using non-fluorescent (visible) penetrant.
Steps for Liquid Penetrant Testing:
- Section of material with a surface-breaking crack that is not visible to the naked eye
- Penetrant is applied to the surface
- Excess penetrant is removed
- Developer is applied, rendering the crack visible
Fluorescent Liquid Penetrant Testing
Fluorescent Liquid Penetrant Testing (FLPT) is a type of dye penetrant inspection in which a fluorescent dye is applied to the surface of a non-porous material in order to detect defects that may compromise the integrity or quality of the part in question. Noted for its low cost and simple process, FLPT is used widely in a variety of industries.
There are many types of dye used in penetrant inspections. FLPT operations use a dye much more sensitive to smaller flaws than penetrants used in other DPI procedures. This is because of the nature of the fluorescent penetrant that is applied. With its brilliant yellow glow caused by its reaction with ultraviolet radiation, FLPT dye sharply contrasts with the dark background. A vivid reference to even minute flaws is easily observed by a skilled inspector.
Because of its sensitivity to such small defects, FLPT is ideal for most metals which tend to have small, tight pores and smooth surfaces. Defects can vary but are typically tiny cracks caused by processes used to shape and form the metal. It is not unusual for a part to be inspected several times before it is finished (an inspection often follows each significant forming operation).
Selection of inspection type is, of course, largely based on the material in question. FLPT is a non-destructive inspection process which means that the part is not in any way damaged by the test process. Thus, it is of great importance that a dye and process are selected that ensure the part is not subjected to anything that may cause damage or staining.
Water-Washable Dye Penetrant Testing
Water-washable dye penetrants work best on rough surface parts, threaded or grooved parts and those with holes and orifices, which may be difficult to remove with the post emulsification method. It is especially suitable for automation, larger parts, leak testing and use on parts that are incompatible with oil-based systems. Water-washable penetrants are removed by manual or automated water spray, manual wipe or air agitated immersion wash. If a manual spray is used, the water pressure should not exceed 40 psi; the temperature should be between 10ºC – 38ºC using a coarse spray at a minimum distance of 300 millimeters. Although water-washable processing has its advantages, certain specifications will restrict its use because over-washing can occur in shallow discontinuities, which makes rinsing time critical to the process.