A non – destructive type of penetrant testing is the liquid dye penetrant test. It is used to detect breaks or flaws on open surfaces such as cracks, seams, laps or cold shuts. Liquid dye penetrant test is used in inspecting ferrous metals and non – ferrous metal products respectively. It is also used on non – porous and non – metallic items such glasses, plastics or ceramics.
The value of the liquid penetrant test is for the liquid used to enter the flaws by capillary action. The effectiveness of this action is dependent on the surface tension, adhesion, cohesion and viscosity. Factors like condition of the surface of material and its discontinuity inside can also influence the liquid dye penetrant test. The surface material should be cleaned thoroughly from all unwanted materials which can hinder the penetration of the liquid into the flaw. This will make the liquid penetrant to work effectively.
The dye penetrant is then applied on the surface after cleaning and it should be given time to penetrate the possible discontinuities. After some time you can completely remove the liquid dye penetrant on the surface followed by the application of a developer. A developer can either be wet or dry depending on the penetrant used. The flaws which have been penetrated by the dye penetrant will bleed out on the surface. The developer will then help in identifying the defects.
If there is any existing defect, the developer will be able to identify their location, nature and degree. For quicker actions in forcing the liquid out of the flaw, this can be done by striking it sharply to make vibrations. Cracks which are too small to be seen in a visual inspection can be detected by using the oil – whiting test. This penetrant test used is older and cruder. The item should be covered with penetrating oil, rubbed dry and then covered with dry whiting.
The whiting will then absorb the oil which has seeped into the flaws showing discolored streaks to identify the cracks.
The dye penetrant test (DPT) is not expensive and does not need any special instruments but with simple applications and very reliable. The red colored penetrant fluid (dye) has properties to seep into surface flaws when applied on impenetrable surface.
Steps involved in doing a dye penetrant test are:
- Clean the surface part from dirt and dust with a piece of cloth.
- Brush the surface part to take away paint, rust or scale with a soft wire brush.
- Spray on the cleaner to take away the oil or grease.
- Spray the dye penetrant on the area to be tested and give it 3 to 5 minutes maybe more for the penetrant to enter into the cracks.
- Wipe the excess dye penetrant with a rag.
- Spray the surface again with a cleaner to take off the remains of the red dye penetrant.
- Apply the developer on the surface to form a thin layer which will absorb the dye penetrant from the cracks thus, producing red spots or lines on the surface of affected flaws.
- The red dye penetrant which is absorbed by the white absorbent will identify any existing cracks.