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9/8/98 AC 43.13-1B SECTION 3. EDDY CURRENT INSPECTION 5-25. EDDY CURRENT INSPECTION. Eddy current is used to detect surface cracks, pits, subsurface cracks, corrosion on inner sur- faces, and to determine alloy and heat-treat condition. a. Eddy Current Instruments. A wide variety of eddy current test instruments are available. The eddy current test instrument performs three basic functions: generating, re- ceiving, and displaying. The generating por- tion of the unit provides an alternating current to the test coil. The receiving section proc- esses the signal from the test coil to the re- quired form and amplitude for display. In- strument outputs or displays consist of a vari- ety of visual, audible, storage, or transfer tech- niques utilizing meters, video displays, chart recorders, alarms, magnetic tape, computers, and electrical or electronic relays. b. Principles of Operations. Eddy cur- rents are induced in a test article when an al- ternating current is applied to a test coil (probe). The alternating current in the coil in- duces an alternating magnetic field in the arti- cle which causes eddy currents to flow in the article. (See figure 5-3.) (1) Flaws in or thickness changes of the test-piece influence the flow of eddy currents and change the impedance of the coil accord- ingly. (See figure 5-4.) Instruments display the impedance changes either by impedance plane plots or by needle deflection. (2) Figure 5-5 shows typical impedance plane display and meter display instrument re- sponses for aluminum surface cracks, subsur- face cracks, and thickness. FIGURE 5-3. Generating an eddy current. FIGURE 5-4. Detecting an eddy current. 5-26. EDDY CURRENT COILS AND PROBES. A wide variety of eddy current coils and probes is available. Coils and probes are not always interchangeable between vari- ous types of instruments and, for best results, should be matched to a specific instrument and frequency range. Special probe holders can be fabricated to facilitate eddy current inspection of contoured or shaped parts including part edges. 5-27. FIELD APPLICATION OF EDDY CURRENT INSPECTION. Eddy current techniques are particularly well-suited for de- tection of service-induced cracks in the field. Par 5-25 Page 5-15PDF Image | AFS-640
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