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Magnaflux Inspection Magnetic Particle Inspection Method (NDI)

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Magnetic particle (Magnaflux Inspection) inspection is a method for detecting cracks, laps, seams, voids, pits, subsurface holes, and other surface, or slightly subsurface, discontinuities in ferro-magnetic materials. Magnetic particle inspection can be used only on ferro-magnetic materials (iron and steel). It can be performed on raw material, billets, finished and semi finished materials, welds, and in-service assembled or disassembled parts. Magnetic particles are applied over a surface either dry, as a powder, or wet, as particles in a liquid carrier such as oil or water. Common uses for magnetic particle inspection are; final inspection, receiving inspection, in-process inspection; and quality control, maintenance, and overhaul.

Principle of Operation: Magnetic particle inspection uses the tendency of magnetic lines of force, or flux, of an applied field to pass through the metal rather than through the air. A defect at or near the metals surface distorts the distribution of the magnetic flux and some of the flux is forced to pass out through the surface. (See figure 1.) The field strength is increased in the area of the defect and opposite magnetic poles form on either side of the defect. Fine magnetic particles applied to the part are attracted to these regions and form a pattern around the defect. The pattern of particles provides a visual indication of a defect. (See figure 2.)

Figure 1

Figure 2

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