Quantitative Analysis of Eddy Current NDE Data

A new method for analyzing eddy current inspection data is presented. The key concept behind this method is extraction and isolation of the sample response from the measured signal. The measured signal depends on many factors including, not only the characteristics of the probe itself, the sample material, the operation frequency, and the probe/sample geometry, but also the measurement instrumentation and cabling. To start, complex impedance measurements of the (1) isolated probe and (2) probe with sample as a function of frequency were conducted. The data frequency dependence was found to exhibit resonance behavior that could be fit to a model RLC circuit. The validity of this model circuit was confirmed by the predictable resonance change upon introduction of additional capacitors into the measurement configuration. Without external capacitance, the value for C generally reflects the stray capacitances of the instruments and cables and, hence, is unaffected by presence of a sample. Furthermore, data at a fixed-frequency can be easily related to those of swept frequency measurements via a conformal mapping deduced from analysis of the model circuit. Using this approach, data from fixed-frequency measurements can be effectively mapped to a corrected R and ωL plane. Data for a variety of materials reveals sample responses that can be easily explained in terms of surface impedance variations. In addition, for this corrected R and ωL plane, lift-off behavior scales in a simple predictable fashion and data at different lift-off conditions can be analyzed to further reduce the properties of the sample. Furthermore, defects characteristics, such as crack width and depth, can be quantified with simple physical reasoning and the condition of multiple conductive layers can be evaluated. This improved understanding of the defect signals can be exploited for the design of more efficient probes that are matched to the materials under test. Moreover, this quantitative method allows one to present data taken with different probe and instrument settings in an invariant form representative of the fundamental surface impedance for the specimen.