Dye Penetrant inspection or liquid penetrant inspection LPI is used on non porous metal and non metal components to find material discontinuities that are open to the surface and may not be evident to normal visual inspection. The part must be clean before inspection. The basic purpose of dye penetrant inspection is to increase the visible contrast between a discontinuity and its background. This is accomplished by applying a liquid of high penetrating power that enters the surface opening of a discontinuity. Excess penetrant is removed and a developer material is then applied that draws the liquid from the suspected defect to reveal the discontinuity. The visual evidence of the suspected defect can then be seen either by a color contrast in normal visible white light or by fluorescence under black ultraviolet light.
The penetrant method does not depend upon ferro-magnetism like magnetic particle inspection, and the arrangement of the discontinuities is not a factor. The penetrant method is effective for detecting surface defects in non magnetic metals and in a variety of non metallic materials. The method is also used to inspect items made from ferromagnetic steels and its sensitivity is generally greater than that of magnetic particle inspection. The method is superior to visual inspection but not as sensitive as other advanced forms of tests for detection of in-service surface cracks.
The major limitations of dye penetrant inspection is that it can detect only those discontinuities that are open to the surface; some other method must be used for detecting subsurface defects. Surface roughness or porosity can limit the use of liquid penetrants. Such surfaces can produce excessive background indications and interfere with the inspection. The method can be used on most airframe parts and assemblies accessible to its application.
Dye penetrant inspection equipment.
Equipment varies from simple aerosol cans used in portable systems to fully automated computer-controlled systems. Whether fluorescent or visible penetrants are used, different penetrant bases are available but may require different cleaning methods. Water-washable penetrants can often be removed by a simple water washing process, whereas oil-base penetrants may require special solvents for removal. Some oil-base penetrants have emulsifiers, either added to the penetrant before it is applied or added afterwards, that allow water washing to be used. Developers used, can be applied either by a wet or dry bath. Therefore, each penetrant inspection process may require different cleaning facilities and procedures.
Nadcap approved LPI
BASIC STEPS TO PERFORM PENETRATION INSPECTION. It is important to ensure that parts are thoroughly cleaned and dried before doing dye penetrant inspection. All surfaces to be inspected should be free of contaminants, paint, and other coatings that could prevent penetrant from entering discontinuities.
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