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Guides for Nondestructive Testing
- Magnetic Particle Inspection
- Fluorescent Penetrant Inspection
- Visible Red Dye Penetrant Inspection
For the penetrant process to highlight small cracks and porosity the penetrant must enter all discontinuities, this is why every specification states that the part or inspection surface must be clean. Improper cleaning is often the cause for failure of the penetrant inspection process and if indications are not open to the surface, the penetrant inspection process will not work.
Cleaning materials must dissolve, emulsify, oxidize, wet, displace or act in some other way to assist in removing
the undesired contaminant and at the same time not attack the base component. Cleaning and base component attack are often directly related to the concentration of the cleaning material, contact time with the cleaning material, the temperature at which the cleaning takes place, and the form of agitation employed.
There are two types of materials generally used for magnetic particle inspection, wet method and dry method. Dry method materials are primarily used in weld inspection. Production and overhaul situations require high sensitivity, broad area detection capability exhibited by the wet method. Wet method particles are generally smaller than dry method particles and are more easily attracted to weaker leakage fields. The particles are suspended in a liquid carrier fluid which facilitates the mobility of the particles on the part surface. The particles may be visibly colored relying on contrast with the base material or contrast coating for detectability or they may be fluorescent and produce brilliant indications under UV-A illumination. Fluorescent inspection requires the inspection area be darkened to ensure detection of the fluorescent indications.
Fluorescent penetrants are used in inspection procedures where the greatest level of crack detection is required. The sensitivity of the inspection is increased as a result of UV-A light (315–400 nm) that causes fluorescent dyes to glow, thereby enhancing any penetrant in an indication open to the surface.
Visible inspection penetrants are used primarily for field applications were darkening the inspection area and the use of a black light (UV-A) is impractical, in maintenance repair and safety checks, and in manufacturing processes where a visible indication helps in locating the area for subsequent repair work, such as in large rough castings. They are also used for through leak testing of heat exchangers and tanks, where penetrant is applied to one side and developer to the other.
Guide to Visible Penetrant Inspection – English & Spanish