Liquid penetrant
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Liquid penetrant inspection, also known as dye penetrant inspection, is a type of nondestructive testing used generally in the detection of 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, liqued penetrant inspection can also be used on non-metallic materials.
There are two processes available: one is the use where the penetrant contains an emulsifying agent, and in the other a dye solvent has to be applied separately after the penetration time has elapsed. These processes can be sub-divided even further where the use of dry developer may produce a better result or by the use of a wet developer.
All the processes consist of applying a penetrant dye (generally red) to the surface of the item being tested, waiting a certain time to allow the dye to soak into any flaws, then removing any extra dye remaining on the surface. A white Developer is then applied, the purpose of which is to draw to the surface any dye which has penetrated into defects in the surface, a process similar to the action of blotting paper. Any red stains indicate the position and type of defects in the surface under inspection.