Non-destructive testing (NDT) methods are used in any application where the structural soundness of a material must be determined without altering the material itself. There are dozens of inspection techniques grouped by method to inspect thousands of materials. Liquid dye penetrant testing and magnetic particle inspections are two of these methods.
Dye Penetrant Testing
The dye penetrant method involves covering a material with a special dye, either fluorescent or non-fluorescent depending on technique. The dye is then removed from the surface of the material. Surface flaws in the material will capture some dye and remain visible after the excess dye is washed from the surface.
Magnetic Particle Inspection
Magnetic particle inspections vary widely by technique but the method principles are consistent. A magnetic force is applied to the subject material, it is dusted with magnetic particles, excess particles are removed, then the material is inspected for groups of residual particles that were caught in the magnetic flux of a crack.
Dyes Vs. Magnets
While some magnetic particle techniques involve a liquid as a carrier of the magnetic particles, the magnetic particles themselves, not the liquid, are the indicating material. With dye penetrant, the liquid penetrant itself is the indicating material. The subject material in magnetic testing must be magnetized, whereas liquid penetrant techniques may be used on a wider variety of materials.