Introduction to Dye Penetrant Testing
- This method is frequently used for the detection of surface breaking flaws in non-ferromagnetic materials.
- The subject to be examined is first of all chemically cleaned, usually by vapour phase, to remove all traces of foreign material, grease, dirt, etc. from the surface generally, and also from within the cracks.
- Next the penetrant (which is a very fine thin oil usually dyed bright red or ultra-violet fluorescent) is applied and allowed to remain in contact with the surface for approximately fifteen minutes. Capillary action draws the penetrant into the crack during this period.
- The surplus penetrant on the surface is then removed completely and thin coating of powdered chalk is applied. After a further period (development time) the chalk draws the dye out of the crack, rather like blotting paper, to form a visual, magnified in width, indication in good contrast to the background.
- The process is purely a mechanical/chemical one and the various substances used may be applied in a large variety of ways, from aerosol spray cans at the most simple end to dipping in large tanks on an automatic basis at the other end. The latter system requires sophisticated tanks, spraying and drying equipment but the principle remains the same.
Advantages of Dye Penetrant Testing
=> Simplicity of operation.
=> Best method for surface breaking cracks in non-ferrous metals.
=> Suitable for automatic testing, with reservation concerning viewing. (See automatic defect recognition in Magnetic Particle Inspection)
Disadvantages of Dye Penetrant Testing
=> Restricted to surface breaking defects only.
=> Decreased sensitivity.
=> Uses a considerable amount of consumables.