T.O. 33B-1-14-93/(4-94 Blank)inspection technique. MOI may also reduce inspection time and associated costs, and may have unique capabilities tosolve special inspection problems. Magneto-optic imaging depends on the ability of certain materials to rotate theplane of polarization of light in the presence of a magnetic field. This (Faraday) effect is used to detect disturbances inthe magnetic field produced by passing an alternating current in a thin planar foil of doped yttrium iron garnet. Whenthe foil is placed near the surface of a metallic test object, eddy currents are produced which modify the magnetic fieldin the foil. When defects or other material discontinuities, such as rivets or holes, divert the otherwise uniform flow ofelectric current near the surface of the test piece, magnetic fields perpendicular to the surface of the test piece areproduced which can be imaged in real time by an appropriately designed optical system. Since the system providesoptical information, the results can be videotaped for analysis and permanent documentation.4.9.3.5.1The MOI NDI technique is based on the Faraday magneto-optic effect. When light passes through a polarizer, thewave motion of the light is altered to produce motion in one plane perpendicular to the propagating light wave. Whenthe polarized light then passes through a magneto-optic material in a direction parallel to an applied magnetic field, theplane of polarization is rotated an angle theta (q) as shown by the arrows. This design uses a reflector, which causesthe light to pass through the sensor twice, thereby doubling the rotation angle of the polarized light caused by themagnetic field.4.9.3.5.2A 1992 study sponsored by the United States Federal Aviation Agency (FAA) evaluated the performance of acommercially available MOI system. The purpose of the study was to compare the effectiveness of the MOI systemwith conventional eddy current methods of detecting corrosion in aircraft test panels, previously identified by eddycurrent scanning. The study indicated that MOI might not be able to detect gradual differences in thinning that are lessthan ten percent of base metal thickness. Also, with MOI it appeared to be more difficult to provide quantitativeestimates of remaining thickness than is the case with eddy current scanning. On the other hand, MOI visualization ofthe extent of corrosion was simple and free of the labor intensive point-by-point mapping required by eddy currentscanning. Other recent studies conducted by various aircraft manufacturers have demonstrated the potential for MOI todetect cracks in both prepared samples and samples extracted from aging aircraft. MOI also appears to have anadvantage over conventional eddy current NDI for the detection of linear cracks under thick (greater than 0.005 inches)paint and coatings. Development and application studies using MOI are continuing, and this technique may findspecific applications to NDI of aircraft structures in the near future.4.9.4 ApplicationofAdvancedTechniques.Several of the advanced techniques and processes discussed above do not have fully developed and recognized testprocedures, process controls, and qualification procedures. Specific application of these processes and techniques shallbe in accordance with approved procedures and engineering approval.
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