T.O. 33B-1-13-2b. When a surface or near-surface discontinuity interrupts the magnetic field in a ferromagnetic material,some of the field is forced out into the air above the discontinuity resulting in a leakage field. The sizeand strength of the leakage field depend on the size and proximity to the surface of the discontinuity.The discontinuity is detected by the use of finely divided ferromagnetic particles that are applied to apart’s surface and are attracted to the leakage field. This collection of particles indicates the presenceand location of the discontinuity.3.1.5 BasicTerminology.The following terms and definitions are basic to an understanding of the MPT method.a. CoerciveForce. The negative or reverse applied magnetizing force (H) necessary to reduce the residualmagnetizing force (B) to zero in a ferromagnetic material, after magnetic saturation has been achieved.The magnitude and direction of this force are represented by the line OG in Figure 3-16.b. DirectContactMagnetization. Use of current passed through the part via contact heads or prods toproduce a magnetic field.c. Ferromagnetic. A term that describes a material that exhibits both magnetic hysteresis and saturation,and whose magnetic permeability is dependent on the magnetizing force present. In magnetic particletesting, we are concerned only with ferromagnetic materials.d. Field,CircularMagnetic. The magnetic field surrounding the flow of electric current. For magneticparticle testing, this refers to current flow in a central conductor or the part itself. It is bestcharacterized as having no magnetic poles present.e. Field,LongitudinalMagnetic. A magnetic field that results in magnetic poles. An example would bethe field that exists in a bar magnet.f. Field,Magnetic. The term used to describe the volume within and surrounding either a magnetizedpart or a current-carrying conductor wherein a magnetic force is exerted.g. Field,MagneticLeakage. The magnetic field outside of a part resulting from the presence of adiscontinuity, a change in magnetic permeability, or a change in the part’s cross-section.h. FluxDensity,Magnetic(B). The strength of a magnetic field, expressed in flux lines per unit cross-sectional area.i. FluxLinesorLinesofForce. A conceptual representation of magnetic flux that can be illustrated bythe line pattern produced when iron filings are sprinkled on paper laid over a permanent magnetj. Hysteresis,Magnetic. The behavior of ferromagnetic materials that results from both the retentivity andthe coercive force being greater than zero.k. InducedCurrentMagnetization. Use of current induced in a part to produce a magnetic field.l. MagnetizingCurrent(I). The flow of either alternating or direct current used to induce magnetism intothe part being inspected.m. MagnetizingForce(H). The magnetizing field applied to a ferromagnetic material to inducemagnetization.n. MagneticPermeability(m). The ease with which a ferromagnetic part can be magnetized. It is equal tothe ratio of the flux density (B) produced to the magnetizing force (H) inducing the magnetic field. It
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