T.O. 33B-1-1SECTION IVPORTABLE MPI TECHNIQUES3.4PORTABLEMAGNETICPARTICLEFIELDINSPECTIONTECHNIQUES.3.4.1General.a.Portable induced field inspection equipment is generally referred to as either a probe or a yoke. Theseterms are synonymous and differ only due to manufacturer’s nomenclature. This category ofinspection equipment is described here in conjunction with the techniques for their use andapplication.b.Magnetic yokes are small and easily portable. They are easy to use and are adequate when testingsmall castings or machine parts for surface cracks and for weld inspection. They induce a strongmagnetic field into that portion of a part that lies between the poles or legs of the yoke. The inducedfield f lows from one leg of the yoke to the other regardless of the style or leg configuration. Yokes orprobes are available with either fixed or articulated legs as shown in Figure 3-29.Figure 3-29.Portable Induced Field Inspection Equipmentc.Either dry powder or wet magnetic particles may be used in conjunction with a yoke for the detectionof discontinuities. Yokes are available for operation from a 115 volt, 60-hertz AC outlet, or from a 12volt DC battery. A permanent magnet yoke is also available, permitting inspections to be performedwithout the use of electric current.d.Deleted.3.4.2CapabilitiesandLimitations.a.The units are designed for simplicity, ease of handling, and operation by one person. They may beused on machine-finished surfaces, as well as castings and weldments fabricated in a variety ofconfigurations. The units induce a strong magnetic field at the surface of the part being inspected.Since no current is f lowing through the part being subjected to inspection is impossible to overheat orburn the part. The f lexibility of a yoke with articulating legs is greatly increased permittinginspections to be performed on parts of varied configurations.b.Yokes or probes are limited to the detection of surface and near surface discontinuities only. Theyshould not be used for deep-seated, subsurface discontinuities due to the limited penetration of theinduced magnetic field. Because of their size they cannot be used with a 100% duty cycle. Rather,they are limited essentially to spot-checking and occasional sample testing rather than continuousproduction testing. Under optimum operating conditions, the fixed leg yoke has a limited inspectionarea governed by the distance between and immediately surrounding the legs. The moveable orarticulated leg yoke can inspect either a larger area (legs apart) or detect finer discontinuities byconcentrating the magnetic field in a smaller area (legs closer together).Change 23-49
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