Recommended NDT Practice and its Model-based Investigation for Eddy Current Inspection of Aerospace Structures

This paper describes a model-based investigation of inspection standards being developed as a recommended practice for eddy current (EC) NDT of aerospace structures for surface cracks. Compared to experimental investigations that can be laborious and expensive, model-based approach provides cost-effective and quantitative verifications of the recommended practice that covers wide ranges of materials, probes, instruments, frequencies, and test conditions. Probe signals for a number of specified standard notches are predicted and the expected amplitudes and phase-angle ranges are determined for various test conditions, in order to examine the influence of material electrical conductivity, frequency, and probe types. For a few selected situations, the model predictions are validated against experimental data. This study verifies the adequacies of the recommended specifications, especially of the reference notch selections and their response amplitudes for various test conditions, as well as confirming the suitability of using vertical amplitude alone for all the test situations. The influence of probe cable and the issue of three-point calibration are also discussed specifically in this investigation.