Conference: Publication Date: 24 October 2016Testing Method:

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.

References

- Huang, H., et al., “Design of an eddy-current array probe for crack sizing in steam generator tubes”.
*NDT & E International*. 36(7): p. 515-522, 2003. - Chen, Z. and K. Miya, “A new approach for optimal design of eddy current testing probes”.
*Journal of Nondestructive Evaluation*. 17(3): p. 105-116, 1998. - Song, S.-J. and Y.-K. Shin, “Eddy current flaw characterization in tubes by neural networks and finite element modeling”.
*NDT & E International*. 33(4): p. 233-243, 2000. - Babbar, V., et al., “Finite element modeling of second layer crack detection in aircraft bolt holes with ferrous fasteners present”.
*NDT & E International*. 65: p. 64-71, 2014. - Rosell, A. and G. Persson, “Finite element modelling of closed cracks in eddy current testing”.
*International Journal of fatigue*. 41: p. 30-38, 2012. - Mooers, R.D. and J.C. Aldrin., “Effects of angular variation on split D differential eddy current probe response”. in
*42nd Annual Review of Progress in Quantitative Nondestructive Evaluation: Incorporating the 6th European-American Workshop on Reliability of NDE*. AIP Publishing, 2016. - Mooers, R., J. Knopp, and M. Blodgett., “Model based studies of the split D differential eddy current probe”. in
*Review of Progress in Quantitative Nondestructive Evaluation*. Volume 31, AIP Publishing, 2012. - Mooers, R.D., J.C. Aldrin, and J.S. Knopp., “Model the effects of core/coil size and defect length on eddy current response”. in
*41st Annual Review of Progress in Quantitative Nondestructive Evaluation*. Volume 34, AIP Publishing, 2015. - Mooers, R.D., et al., “Split D differential probe model validation using an impedance analyzer”. in
*40th Annual Review of Progress in Quantitative Nondestructive Evaluation: Incorporating the 10th International Conference on Barkhausen Noise and Micromagnetic Testing*. AIP Publishing, 2014. - Boller, C., “Ways and options for aircraft structural health management”.
*Smart materials and structures*. 10(3): p. 432, 2001. - Nakagawa, N., T. Khan, and J. Gray., “Eddy current probe characterization for model input and validation”. in
*Review of Progress in Quantitative Nondestructive Evaluation*. Volume 19, AIP Publishing, 2000. - Yusa, N., et al., “Numerical modeling of general cracks from the viewpoint of eddy current simulations”.
*NDT & E International*. 40(8): p. 577-583, 2007. - Xu, B., et al., “Intelligent Eddy Current Crack Detection System Design Based on Neuro-Fuzzy Logic”. in
*International Workshop on Smart Material and Structures/ NDT in Canada conf./ NDT for the Energy Industry*. Calgary, Alberta, October 7-10, 2013. - Santandréa, L. and Y. Le Bihan., “Using COMSOL-Multiphysics in an Eddy Current Non-Destructive Testing Context”. in
*Proceedings of the Comsol Conference*. Paris, 2010.

Metrics

Usage | Shares |
---|---|

Total Views 37 Page Views |
Total Shares 0 Tweets |

37 0 PDF Downloads |
0 0 Facebook Shares |

Total Usage | |

37 |