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9/8/98 AC 43.13-1B (7) The intensity of the longitudinal shots is kept just below the level at which leakage fields develop across sharp changes of section, such as radii under bolt heads, threads, and other sharp angles in parts. This does not apply when checking chrome-plated parts for grinding cracks. (a) For example, longitudinal mag- netization of a round steel bar would be pro- duced by placing the DC coil around the bar. After application of the magnetic particles, ei- ther during or subsequent to magnetization, discontinuities perpendicular to the longitudi- nal axis of the bar would be disclosed. (b) When a yoke is used, the portion of the part between the ends of the yoke com- pletes the path of the magnetic lines of force. This results in a magnetic field between the points of contact. c. Permanent Magnets and Electro- magnetic Yoke. The stability of the magnetic field generated by permanent magnets requires some agitation of the oxide particles within the field. The wet method is considered most sat- isfactory. Use a well-agitated plastic squirt bottle for the most effective application of the magnetic particle suspension. When the di- rection of possible cracks in a suspect area is not known, or would not necessarily be normal to the lines of force between the poles of the magnet, reposition the magnet to the best ad- vantage and recheck. Usually, two shots, 90 degrees apart are required. The part must be demagnetized between each magnetization when the field direction is changed unless the next shot is at least 10 percent stronger than the previous shot, if this is the case demagneti- zation is not necessary. 5-49. DETERMINATION OF FIELD STRENGTH. Factors such as part size, shape, magnetic properties of the material, and the method of magnetization will affect the field strength induced within a part by a given applied magnetizing force. The factors vary considerably, making it difficult to establish rules for magnetizing during examination. Technique requirements are best determined on actual parts having known defects. a. A magnetization indicator, such as a Quantitative Quality Indicator (QQI), should be used to verify that adequate magnetic flux strength is being used. It effectively indicates the internally-induced field, the field direction, and the quality of particle suspension during magnetization. b. The level of magnetization required for detection of service-related defects in most cases can be lower than that required for mate- rial and manufacturing control. Contact the manufacturer for correct specifications. NOTE: If the examination must be performed with less current than is desired because of part size or equip- ment limitations, the lower field strength can be partially accommo- dated by reducing the area of exami- nation for each magnetization, or the examination can be supplemented by using electromagnetic yokes. Examine only 4 inches on either side of a coil instead of 6, or apply additional mag- netization around the periphery of a hollow cylinder when using an inter- nal conductor. 5-50. SPECIAL EXAMINATION TECH- NIQUES. a. Magnetic Rubber. Magnetic rubber formulations using finely divided magnetic particles in a silicone rubber base are used for the inspection of screw, bolt, or other bore holes, which are not easily accessible. The liquid silicone rubber mixture is poured into holes in magnetic parts to be inspected. Par 5-48 Page 5-25PDF Image | AFS-640
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