Calibration of Commercial Eddy Current Instruments for Quantitative NDE

Eddy current testing is typically used in the field to detect defects in a specimen that exceed a set threshold. In most conventional techniques this is done by calibrating the test setup with a reference standard in a way that will correlate the output signal to a known and standard flaw size, such as 10% metal loss due to corrosion thinning in a aircraft skin. However, these standard methods cannot completely characterize a defect in a quantitative manner. In general, reliable quantitative NDE requires quantitative measurements and a theory to interpret them. Theoretical eddy current NDE usually models the test coil’s impedance change (in ohms) as the quantitative measure that varies with specimen and flaw parameters. Specific laboratory instruments such as impedance analyzers are capable of making quantitative measurements that allow researchers to compare experimental data directly to theory. But typical commercial eddy current instruments are not designed to measure probe impedances quantitatively. Rather, they measure relative changes in the coil impedance