Dye penetrant inspection
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2. Penetrant is applied to the surface.
3. Excess penetrant is removed.
4. Developer is applied, rendering the crack visible.
Dye penetrant inspection (DPI), also known as liquid penetrant examination (LPE), is a type of nondestructive testing used generally in the detection of surface breaking flaws in non-ferrous alloys. It can also be used for the inspection of ferrous materials where magnetic-particle inspection is difficult to apply. In some cases it can be used on non-metallic materials. Variations include the use of fluorescent dyes, where a black (UV) light is used to illuminate the residual penetrant. This technique has even higher sensitivity than normal LPE but can only be used in the absence of other light sources.
[edit] Main Steps
Below are the main steps of Dye - Penetrant Inspection:
1. Pre-cleaning:
The test surface is cleaned to remove any dirt, paint, oil, grease or any loose scale, etc.
2. Application of Penetrant:
The penetrant is then applied to the surface of the item being tested. The dye is allowed time to soak into any flaws (generally 10 to 30 minutes). The soak time mainly depends upon the material being testing and the type of flaws sought.
3. Excess Penetrant Removal:
The excess penetrant is then removed from the surface (a lint - free cloth soaked with solvent remover is normally used for this purpose in case of solvent removable penetrant inspections). it is important to not spray the solvent remover on the test surface directly, because this can the remove the penetrant from the flaws.
4. Application of Developer:
After the removal of excess penetrant, a white developer is sprayed in a thin, even coating on the surface. This is to draw any dye which has entered defects to the surface, a process similar to the action of blotting paper. Any colored stains indicate the positions and types of defects on the surface under inspection. The penetrants are usually red, purple or sometimes orange, to give a good contrast against the white developer used in the process.
5. Inspection:
The inspection of the test surface is carried out immediately after the application of the developer and 15 minutes after the immediate inspection.
6. Post Cleaning:
After the inspection and recording of defects if found, the test surface should be cleaned.
[edit] Features
- The flaws are more visible, because:
- The colour of the penetrant and the developer (e.g. red dye with a white developer, or a fluorescent penetrant and illumination by ultraviolet light) have a high contrast.
- The developer draws the penetrant out of the flaw over a wider area than the real flaw, so it looks wider.
- Limited training is required for the operator — although experience is valuable.
- Low testing costs.
- Proper cleaning is necessary; rusty parts have to be sanded to bare metal.
- Penetrant, developer and cleaner are polluting. Many of the penetrant dyes stain cloth, skin and other porous surfaces brought into contact.
