Magnetic Particle Examination Physical Principles
A bar of ferromagnetic material can be magnetized by placing it in contact with the north and south poles of a horseshoe magnet, as depicted in Figure 700-7a. North and south poles are created in the magnetized bar opposite to the poles of the horseshoe magnet, and magnetic lines of force flow through the bar from the south pole to the north pole in a straight line, as shown in Figure 700-7b.
A discontinuity (flaw) in the material will disrupt the magnetic lines of force flowing through the material, as shown in Figure 700-7c. New north and south poles are created at the discontinuity, and some of the magnetic lines of force “leak” from the surface of the material to bridge the gap resulting from the discontinuity. A discontinuity that is open to the surface results in a relatively large leakage of magnetic lines of force, whereas appreciably less leakage is caused by a discontinuity below the surface. Deep discontinuities cause greater leakage than shallow ones, but the leakage produced decreases as the gap that must be bridged by the magnetic lines of force widens. Discontinuities that are perpendicular to the magnetic lines of force cause the greatest leakage, whereas discontinuities parallel to the lines of force may not produce any leakage.
Magnetic particles applied to the surface of the material will be attracted by the magnetic lines of force leaking from the surface at a discontinuity and will adhere to the surface at this location, revealing the presence of the discontinuity.
Ferromagnetic materials can also be magnetized by the passage of an electric current. In these materials, an electric current creates magnetic lines of force that are at right angles to the flow of the current, as illustrated in Figure 700-8.
Since detection of a surface flaw does not depend on a liquid being drawn into the flaw, MT can detect flaws that a liquid penetrant cannot enter because they are too tight or are filled with corrosion scale.
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