T.O. 33B-1-13-223.2.6.2 Considerations.If the critical area of an assembly is completely accessible for inspection without any disassembly, and if the inspectionmedium (magnetic powder or paste) can be removed after inspection, then it is acceptable to inspect those area or partsin place without disassembly. For example, steel propeller blades may be inspected in the blade area when they are inplace on the aircraft, but to inspect the shank area, which is concealed by the hub, it is necessary to disassemble.SECTION IIIMAGNETIC PARTICLE INSPECTION TECHNIQUES3.3 MAGNETICPARTICLEINSPECTIONTECHNIQUES.3.3.1 General.a. Magnetic particle inspection is a method of nondestructive testing that uses very small ferromagneticparticles to reveal discontinuities in parts capable of being magnetized. Magnetizable parts are thoseparts made of metals classed as ferromagnetic. To find flaws in metals that are not ferromagnetic, suchas aluminum, titanium, brass, etc., other inspection methods must be used.b. This section includes a description of the types of electric currents used; the various techniques,procedures and equipment used to magnetize parts; and the kinds of magnetic particles available foreither wet or dry application. Special magnetization techniques are also included.3.3.2 FactorsdeterminingthechoiceOfTechnique.The choice of technique for a particular magnetic particle inspection depends upon:a. The type of discontinuity or defect being sought.b. The part's material, shape, and size.c. The magnetic particle inspection equipment available.3.3.3 TechniqueVariations.The following points must be considered and the appropriate alternatives selected in order to achieve a particularinspection result. A discussion of each alternative and the effect that it has upon the results of the inspection willfollow.a. Type and amount of electric current to be used for magnetization.b. The direction of the resultant magnetic fields obtained using these currents.c. The kind of magnetic particles to be used, i.e., wet or dry.d. How the inspection media is to be applied.3.3.4 TypesOfElectricCurrent.Commonly, three types of electric current are used in magnetic particle inspection: alternating current (AC), directcurrent (DC), and half-wave direct current (HWDC). Alternating current is used for the detection of surfacediscontinuities only, whereas direct or half-wave direct currents are used for both surface and subsurfacediscontinuities.
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