Penetrant inspection is a non-destructive test method for revealing fine surface discontinuities such as cracks, pores or seams in weld metal or base metal. It is useful for detecting discontinuities in magnetic and nonmagnetic materials where magnetic particle
inspection cannot be used.
Penetrant inspection is accomplished by applying a liquid with high wetting capabilities to the surface which can be drawn into surface cavities or openings. The excess penetrant is then removed from the surface, and a liquid-propelled or dry powder developer is applied. If there is a significant discontinuity, the penetrant will be held in the cavity. Blotter action draws the penetrant from the discontinuity to provide a
contrasting indication on the surface. This is a relatively reliable and inexpensive method for obtaining information on questionable welds.
The following sequence is normally used in the application of a typical penetrant test. When the order is changed or short cuts are taken, the validity of the test is suspect.
(1) Clean the test surface.
(2) Apply the penetrant.
(3) Wait for the prescribed dwell time.
(4) Remove the excess penetrant.
(5) Apply the developer.
(6) Examine the surface for indications, and record the results.
(7) Clean to remove the residue.
Dye Penetrant
The dye penetrant method uses a bright red dye with high wetting capabilities. To begin this method, the part is cleaned with a cleaning solution to prepare the surface. The dye penetrant is applied by brush, spray, or dipping, and allowed to remain for at least
five minutes. (Detection of very small cracks may require two or three such applications). After applying the dye penetrant, excess penetrant is removed with a cleaning solution, and the developer is sprayed on or applied with a brush. As the developer dries, the penetrant is drawn to the surface, and the discontinuities are revealed.
Fluorescent Penetrant
In fluorescent penetrant examination, a highly fluorescent liquid with good wetting or penetrating properties is applied to the surface of the part to be inspected.
The liquid is drawn into very small surface openings by capillary action. Excess liquid is removed from the surface and a developer in the form of a fine powder, or water suspension of a fine powder, is applied to the surface. The developer draws the penetrant from the pores and cracks and makes them more visible under ultraviolet light.
Before the fluorescent penetrant test is started, the part must be thoroughly clean, because any dirt, grease, or paint could close the discontinuities to the penetrant. The penetrant may be applied by spraying, dipping or brushing. The time the penetrant must
remain on the surface will vary from a few minutes to several hours, depending on the thickness of the work-piece, but 5 to 15 minutes is usually required.
After the penetrating time, the excess penetrant must be removed carefully to avoid removing more than the surface penetrant. Water-wash penetrants can be washed with a low-pressure water spray. Some commercial penetrants require a solvent wash or an emulsifier. The emulsifier is applied to the surface and allowed to remain for one to four minutes before washing with the water spray. The parts are then dried by wiping, air blower, or hot air oven.
The dry developer is applied to the dried parts with a powder gun, spray bulb, or by dipping the part into the developer powder. The penetrant is drawn from the discontinuities, making the discontinuities visible in ultraviolet light. If a wet developer is used, drying after washing is not necessary. The wet developer in the form of a colloidal suspension is applied by spraying or dipping. The developer should remain on the surface for at least half the penetrating time. After this, the part is dried by hot air.
When viewed under ultraviolet light, the indications of discontinuities are brilliantly fluorescent, revealing the depth and length of discontinuities by the amount
of penetrant which bleeds out. Contrast is enhanced when viewed in a darkened location, which allows the finer indications to be observed.
Historical Background. The old “oil and whiting” method might be considered the forerunner of penetrant inspection. A light oil was applied to a surface, wiped off, then the surface was coated with chalk. The oil showing through the chalk pointed up the location of cracks.