The calibration of eddy-current measurement systems is a long-standing problem in nondestructive evaluation. Eddy-current probe calibration is needed for several reasons: to compensate for different probe sensitivities, to set detection thresholds, to validate instrument setup and operation, and to perform quantitative flaw sizing.1,2 The most frequently used calibration method is to scan the probe being calibrated over simulated defects such as electrical-discharge-machined (EDM) slots, saw cuts, or laboratory-produced fatigue cracks. This method has the virtue of calibrating probe and instrument at the same time and it can be performed on the same material as that to be inspected. But it has a number of disadvantages as well. First, a large number of artifact standards must be generated, certified, and maintained, resulting in considerable expense. Second, the signals from EDM slots and saw cuts are not equivalent to the signals from actual defects.3 Third, it is questionable whether quantitative flaw sizing can be performed using such a calibration method. Even if laboratory-produced cracks were to be used for routine calibration (a prohibitively expensive option), the accuracy of calibration or quantitative sizing could be compromised by the occurrence of crack closure effects.4
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