T.O. 33B-1-14-53Figure 4-42. Decrease in Crack Response with Increasing Lift-Off.4.5.4.4 LiftOffCompensationEffectsOnSensitivity.Lift-off must be minimizedor compensated for in order to maintain a known level of sensitivity during an eddy currentinspection. A meter type of eddy current instrument requires some form of lift-off adjustment. Otherwise, slightvariations in lift-off would provide strong signals which would completely mask the response from cracks. Themagnitude of crack response is considerably reduced by lift-off compensation. The reduction in sensitivity dependsupon the particular eddy current system in use. Each system must be calibrated for the particular application.4.5.4.5 PhaseResponseFromCracks.Difference in phase between lift-off response and crack response is essential for the detection of cracks in mostapplications of eddy current inspection. Depending upon the location of the impedance of the signal on the impedancediagram, the phase angle between lift-off and crack response can be very small. This makes it very difficult to detectthe difference between lift-off and probe motionfrom crack indications. Referring to Figure 4-42, as lift-off increasesand/or the frequency decreases, the impedance of the system approaches the air null point, the phase angle between lift-off and the conductivity line decreases. By maintaining a high fill-factor or low liftoff and operating at a high enoughfrequency, a crack indication (loss of conductivity) can be easily distinguished from lift-off signals because of the largerphase angle. These relationships, as seen on an impedance plane analysis eddy current instrument, are shown in Figure4-43 for aluminum, titanium and steel alloys. As crack depth increases, the phase angle approaches more closely thephase angle for conductivity changes.
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