A Model-Based Study of the Effect of Semi-Elliptical Surface Notch Geometry on the Signal of a Split-D Eddy Current Probe

Eddy current testing (ECT) of tiny fatigue surface cracks are mostly carried out by means of differential probes. Among their various designs, Split-D reflection differential probes, named after their D-shaped internal cores and receiver coils, are particularly interesting because of their small footprint that makes them suitable for inspecting short surface cracks. In many of ECT theories and modelling trials that have been published on the interaction of fatigue cracks with the magnetic field of ECT probes, fatigue cracks are simplified and replaced by semi-elliptical notches. Therefore, in model-based ECT studies, electrical discharge machined (EDM) notches are frequently used since they are advantageous in terms of their low manufacturing cost as well as their usefulness in calibration and model verification procedures. Additionally, ECT signals obtained from EDM notches can roughly estimate those obtained from real fatigue cracks having the same size. Accordingly, in the present study a commercially available split-D surface probe is modeled based on its actual dimensions and material properties. The dimensions of the probe are extracted from a CT-scan reconstruction and inserted in the 3-D model. The probe scanning over 3 semi-elliptical notches having different sizes is simulated using the AC/DC module of COMSOL Multiphysics. A test frequency of 500 kHz is considered in the simulations, and the test block containing the notches is assigned with material properties of aluminum. The effect of simulation parameters, such as mesh size and distribution, is investigated, and hence the parameters are finely tuned to achieve consistent results. Afterwards, the reliability of the simulation outputs is assessed by comparing them to impedance measurements of semi-elliptical surface notches in an aluminum block. This validation study shows acceptable matching of the probe’s impedance obtained from both simulations and measurements. After validation of the model, the sensitivity of eddy current signals to variations of the notch geometry (e.g., notch opening, depth and length) is studied using the 3-D simulations. The importance of studying the notch opening originates from the fact that by decreasing this opening in the simulations, a better approximation of a fatigue crack shall be achieved. On the other hand, the ECT signals obtained from different notch lengths and depths establish a size dependent signal archive at the selected test frequency. This archive can be used as a basis for inversion purposes using artificial intelligence algorithms to be pursued in future studies.

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