Dye Penetrant Testing method is used for detecting surface discontinuities in ferrous, non-ferrous and all types of relatively hard metallic as well as non-metallic products. It is a non-destructive testing method, which is simple in principle and simple in application and in the form of portable kits offers the basic facilities for conducting inspection.
The liquid penetrant is applied to the clean and dry test surface of the specimen. In narrow discontinuities such as crack or pin hole it flows in due to capillary action and if the discontinuities are wide such as tear or surface porosity it gets entrapped in the discontinuity.
Sufficient time is allowed for the penetrant to remain on the surface of the specimen during which it penetrates in all the discontinuities including the most narrows ones.
Excess penetrant on the surface of the specimen is removed penetrant, which is entrapped into the discontinuities.
A thin coat of developer, which acts as a blotter, is then applied to the test surface. The blotting action of the developer draws the penetrant from the discontinuity and the penetrant appears on the surface of the specimen as an indication.
The surface of the specimen is then visually examined for the indications which are formed on the developer coating. The indication is always greater than the discontinuity due to diffusion of the penetrant in the developer.
And finally, the surface is cleaned to remove developer coating. Some types of penetrant require additional sub-steps.
The quality of a penetrant inspection is highly dependent on the quality of the penetrant materials used. Only products meeting the requirements of an industry specification, such as AMS 2644, should be used. Deterioration of new penetrants primarily results from aging and contamination. Virtually all organic dyes deteriorate over time, resulting in a loss of colour or fluorescent response, but deterioration can be slowed with proper storage. When possible, keep the materials in a closed container and protect from freezing and exposure to high heat. Freezing can cause separation to occur and exposure to high temperature for a long period of time can affect the brightness of the dyes.
The temperature of the penetrant materials and the part being inspected can have an effect on the results. Temperatures from 27 to 49oC (80 to 120oF) are reported in the literature to produce optimal results. Many specifications allow testing in the range of 4 to 52oC (40 to 125oF). A tip to remember is that surfaces that can be touched for an extended period of time without burning the skin are generally below 52oC (125oF).
Advantages and Disadvantages of Penetrant Testing
Primary Advantages
- The method has high sensitivity to small surface discontinuities.
- The method has few material limitations, i.e. metallic and non-metallic, magnetic and nonmagnetic, and conductive and nonconductive materials may be inspected.
- Large areas and large volumes of parts/materials can be inspected rapidly and at low cost.
- Parts with complex geometric shapes are routinely inspected.
- Indications are produced directly on the surface of the part and constitute a visual representation of the flaw.
- Aerosol spray cans make penetrant materials very portable.
- Penetrant materials and associated equipment are relatively inexpensive.
Primary Disadvantages
- Only surface breaking defects can be detected.
- Only materials with a relatively nonporous surface can be inspected.
- Precleaning is critical since contaminants can mask defects.
- Metal smearing from machining, grinding, and grit or vapor blasting must be removed prior to LPI.
- The inspector must have direct access to the surface being inspected.
- Surface finish and roughness can affect inspection sensitivity.
- Multiple process operations must be performed and controlled.
- Post cleaning of acceptable parts or materials is required.
- Chemical handling and proper disposal is required.