Liquid Dye Penetrant Test
A liquid penetrant test is non-destructive type. It detect flaws that are open to the surface e.g., cracks, seams, laps, lack of bond, porosity, cold shuts, etc. It can be effectively used not only in the inspection of ferrous metals but is especially useful for non-ferrous metal products and on non-porous, non-metallic materials such as ceramics, plastics and glass.
They are also influenced by factors such as the condition of the surface of material and the interior of the discontinuity. For the liquid to penetrate effectively, the surface of the material must be thoroughly cleaned of all foreign matter that would obstruct the entrance of the liquid into the defect.
After cleaning, the liquid penetrant is applied evenly over the surface and allowed to remain long enough to permit penetration onto possible discontinuities. The liquid is then completely from the surface of the component and either a wet or a dry developer is applied. The liquid that has penetrated the defects will then bleed out onto the surface, and the developer will help delineate them.
This will show the location and general nature and magnitude of any defect present. To hasten this action, the part may be struck sharply to produce vibrations to force the liquid out of the defect. The oil-whiting test is one of the older and cruder penetrant test used for the detection of cracks too small to be noticed in a visual inspection. In this method, the piece is covered with penetrating oil, such as kerosene, than rubbed dry and coated with dry whiting.
in a short time the oil that has seeped into any cracks will be partially absorbed by the whiting, producing plainly visible discolored streaks delineating the cracks.
the Dye Penetrant Test (DPT) based on liquid penetrant is a sensitive extremely versalite and a very reliable method of test. It is quite inexpensive, does not require any special apparatus and is quite simple on application. Only a moderate skill is required. In this test, the strongly coloured red penetrant fluid (or dye) has a property of seeping into surface flaws when applied on a impervious surface.
The steps involved in dye penetrant test are :
- Clean the surface of the component free of dust and drt with a piece of cloth.
- Brush the surface of the component to remove scale, rust, paint, etc, by a soft wire brush.
- Spray the cleaner to remove oil, grease, etc.
- Apply the dye penetrant (by spraying) adequately to cover the area to be tested. Allow 3 to 5 minutes or more for dye to penetrate onto the cracks.
- Wipe off the excess penetrant on the surface with a rag.
- Again spray the surface with the cleaner to remove the remnants of the red dye.
- The developer evenly on the surface to give a thin even layer. This layer absorbs the penetrant from the cracks and red spots or lines appear on the surface to give a visible indication of the flaws.
- The crack if any will be indicated with the red dye absorbed by the white absorbent.