18.104.22.168 Safety Precautions During Radiographic Inspections. Follow safety precautions and instructions contained
this manual and the Nondestructive Inspection Methods manual listed in Table 1-1.
Assure compliance with all applicable precautions set forth in TM 55-1500-335-23 (Nondestructive
Inspection Methods manual) listed in Table 1-1. A hazard associated with exposure to ionizing
radiation is that serious injury can be inflicted without pain, burning, or other sense of discomfort
during the exposure period. Radiation protection shall be utilized in accordance with AR40-
of Radiographic Film Processing Chemicals. Exercise extreme care when working with film processing
chemicals. Fixer solution is highly acidic and developer is highly caustic. Avoid contact with the skin. Flush any skin
contact with water.
1.4.11 Eddy Current (ET) Method. The eddy current method
is used for the detection
conductive materials. The method is effective when inspecting for discontinuities originating: (1) at the radii of mounting
lugs, flanges, or crevices; (2) at pressed-in (interference fit) grease fittings, guide pins, etc.; and (3) from fastener holes
and bushing/bearing bores. Eddy current method will locate surface cracking on any conductive material, but probes and
techniques for inspection of magnetic materials may differ considerably from those used on nonmagnetic materials.
Eddy current has great value for inspecting areas where paint stripping is not desirable and/or impossible. The method
also has wide application in confirming surface indications found by other methods.
The capability and reliability of the eddy current method has been greatly enhanced by the use of modern phase analysis
(impedance plane display) instruments used in conjunction with shielded probes. These instruments display a
representation of the impedance plane which illustrates both the magnitude and direction of impedance changes.
Impedance variables (conductivity, probe lift-off, permeability variations, etc.) can be separated by their characteristic
video response and are readily recognized by the trained operator. The interaction of the probe coils and the part is
represented by a '"flying spot"' (or dot) in the video display.
Equipment is standardized on a test block (reference block) which is constructed of a known material that contains known
good areas, and either simulated or actual defects of known size. The response of the equipment (eddy current machine
and probe) to the good material is set as the starting point by nulling the equipment on the sound area of the block. By
this action, all subsequent readings represent deviations from the null point and have both magnitude and direction.
Careful manipulation of the controls allows the operator to separate the response (deviation from the null point) for lift-off
and flaw (geometric) effects.
Shielded probes have a cylinder of material which encircles the coil of the probe. This serves to constrict the probe's
field and, therefore, limits the spread of eddy currents from much beyond the probe's diameter. This concentrated
electrical field is most useful for scanning around fasteners, near edges, and into specific small areas. Other types of
probes are used for wide area scans, alloy sorting, conductivity comparisons, coating thickness comparisons, skin
thickness comparisons, etc.