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