T.O. 33B-1-13-133.1.8.3.3Frequency of reversals is an important factor affecting the success of this method. With high frequency of currentreversals, the field generated in the part does not penetrate deeply into the part section since penetration decreases asfrequency increases. At a frequency of perhaps one reversal per second, penetration of even a large section is probablynear 100%. For moderate sized parts, the 50 or 60 hertz commercial frequencies of alternating current give quitesatisfactory results.3.1.8.4 LimitationsofDemagnetization.“Complete” demagnetization is usually not possible, even though it is often specified. All practical demagnetizationmethods leave some residual field in the part. Therefore, demagnetization is either the best effort that existing meanspermit or reduction in magnetism to a residual level considered permissible in the particular part involved. It isextremely difficult to bring the steel back to the original zero point by any magnetic manipulation. In fact, it is sodifficult that for all practical purposes, it may be said that the only way to completely demagnetize a piece of steel is toheat it to its Curie temperature or above, and cool it with its length directed east and west in order to avoidmagnetization by the earth’s field. This method of demagnetization is never used because it is not only impractical butsuch heating will alter the properties of the part.3.1.8.4.1It must be remembered that the earth’s magnetic field can determine the lower limit of practical demagnetization.Long parts, or assemblies of long parts, such as welded tubular structures, are especially likely to remain magnetized, ata level determined by the earth’s field, in spite of the most careful demagnetization technique.3.1.8.4.2Many articles and parts become quite strongly magnetized from the earth’s field alone. Handling of parts, such astransporting from one location to another, may produce this effect. Long bars, demagnetized at the point of testing,have been found magnetized at the point of use. It is not unusual to find that steel aircraft parts are magnetized afterhaving been in service for some time, even though they may never have been near any intentionally produced magneticfield. Parts may also become magnetized by being near electric lines carrying heavy currents, or near some form ofmagnetic equipment.3.1.8.4.3The limits of demagnetization may be considered to be either the maximum extent to which the part can bedemagnetized by available procedures, or the level to which the terrestrial (earth’s) field will permit it to becomedemagnetized. These limits may be further modified by the practical degree or limit of demagnetization that is actuallydesired or necessary.3.1.9 MagnetizingEquipment.3.1.9.1 General.Selection of magnetic particle inspection equipment must consider the type of current to be used and the location andnature of inspection. Magnetic particle inspection equipment must provide a rapid and convenient means formagnetizing each part in a reliable and reproducible manner.3.1.9.2 StationaryEquipment.A variety of stationary, bench-type MPT units are available, with many characteristics that fit different testingrequirements. The smaller size units are used for small parts easily transported and handled on the unit by hand. Thelarger ones are used for heavy parts such as long engine crankshafts, where handling must be by crane. Such units aremade to deliver AC or DC with various types of current control.3.1.9.3 MobileEquipment.Mobile magnetic particle equipment delivering AC or half-wave DC magnetizing currents up to 6000 amperes areavailable. Such equipment is used when it is necessary to conduct inspections in another shop area or in the field. Thistype of equipment is sturdy and well suited for both fabrication and overhaul inspections.
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