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 AR40-14/DLAR 1000.28.
18.104.22.168 Mixing 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.
Eddy current inspections shall be performed in accordance with the general application and techniques in
TM 55-1500-335-23 (Nondestructive Inspection Methods manual) and the specific requirements of this
The eddy current method is used for the detection of discontinuities in electrically 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 nonferromagnetic 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.