T.O. 33B-1-15-79Figure 5-79. Mechanical Impedance Analysis Display.5.4.6.5 Eddy-SonicMethod.Since this method is based on the generation of eddy currents in the test part, it will work only on metal structures. Theinstrument sends electrical pulses, with frequencies in the low kilohertz range, to a coil in the probe. The resultantpulsating magnetic field produces eddy currents in the part; the eddy currents cause the part to vibrate, and amicrophone on the axis of the coil detects the sonic vibrations. Unbonds cause changes in the vibrational response ofthe part. The detected changes produce an indication on a meter or an LED array. The probe usually has a mechanicallift-off adjustment that sets the air gap between the coil and the test surface to minimize the noise produced by probescanning. This method works best on metallic honeycomb structures with thin skins (0.062 inch or less). Othermethods do as well on such configurations, and because the eddy-sonic is rather limited in its application, it is notcommonly used.NOTEFor a reliable bond inspection, the inspection surfaces of the test part must be free ofloose paint and foreign matter.CAUTIONGradual changes in indications on an instrument display should be evaluated to seeif the part thickness is changing. If the part thickness has changed, recalibration isrequired. When possible, scanning should be performed in directions of constantthickness.Figure 5-80. Typical Multiple Mode Bond Tester.
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