There is a need for innovative methods to not simply detect fatigue cracks at fastener sites in aircraft structures, but to provide accurate sizing information to better guide maintenance actions and support life prediction.&nbsp; This paper presents progress on the characterization of discontinuities in multilayer fastener sites using bolt hole eddy current (BHEC) techniques.&nbsp; The inversion process leverages fast surrogate models built using VIC-3D® simulations [1].&nbsp; A comprehensive approach has been presented to perform model-based inversion of cracks and electrical discharge machining (EDM) notches using eddy current techniques with an automated BHEC system [2].&nbsp; While prior work demonstrated adequate performance for sizing discontinuities, some model discrepancy remained, resulting in greater inversion error for smaller corner discontinuities [3].&nbsp; As well, sensitivity to probe liftoff and the calibration process was also observed during experimental testing, indicating the need to compensate for the varying probe state and the adjacent material conditions [3].Recent progress has focused on addressing the remaining model discrepancy and improving inversion accuracy over a wide range of test conditions.&nbsp; Better representation of the probe geometry and increased mesh resolution provided improvements in model agreement.&nbsp; As well, the presence of high and low pass filters typically used in rotating bolt-hole eddy current NDE were found to greatly distort the circumferential response relative to the base BHEC model.&nbsp; A process was implemented to use calibration data to optimize the model fit through both gain and filter parameter adjustments.&nbsp; As well, by controlling the phase of the data in the model fit to ensure both measurement components are above the background noise level, the model calibration and inversion processes were found to be more consistent.&nbsp; Another key factor that was addressed was sensitivity to tape wear during repeated BHEC inspections. &nbsp;A comprehensive experimental study was performed including many repeated runs with corner EDM notches oriented in varying stack-ups and near- and far- side positions, and repeated calibration panel scans.&nbsp; A proposed liftoff metric was found to be well correlated with the changes in signal response for the calibration notches due to tape wear, and a compensation scheme based on this metric was shown to provide the best inversion performance for outlier cases. Through implementation of these refinements to the inversion process, error in corner EDM notch depth and length estimates was generally reduced.&nbsp; Lastly, progress on the transition and full validation study including the manufacture of cracks of varying shape and orientation will be discussed.References:[1] Sabbagh, H. A., et al., Computational Electromagnetics and Model-Based Inversion - A Modern Paradigm for Eddy-Current Nondestructive Evaluation, Springer, (2013).[2] Aldrin, J. C., Sabbagh, H. A., Zhao, L., Sabbagh., E., Murphy, R. K., Keiser, M., Flores-Lamb, J., Forsyth, D. S., Motes, D., Lindgren, E. A., Mooers, R., “Model-based Inverse Methods for Sizing Cracks of Varying Shape and Location in Bolt-hole Eddy Current (BHEC) Inspections,” 42nd Annual Review of Progress in QNDE, Incorporating the 6th European-American Workshop on Reliability of NDE, Vol. 1706, p. 090020, AIP Publishing, (2016).[3] Aldrin, J. C., Keiser, M., Motes, D., Flores-Lamb, J., Forsyth, D. S., Sabbagh, H. A., Sabbagh., E., Murphy, R. K., Mooers, R., Henry, C., Lindgren, E. A., “Progress and Challenges of Inverse Methods for Sizing Cracks in Multilayer Bolt-holt Eddy Current Inspections,” Electromagnetic Non-Destructive Evaluation (XXII), IOS Press, Ed. Tamburrino, A., Deng, Y., Chakrapani, S., (2020), pp 92-97.Acknowledgements:&nbsp;This work was funded by the U.S. Air Force through the contract FA8650-19-C-5218 to TRI/Austin.

Model-Based Inversion Approach for Crack Sizing Addressing Model Discrepancy and Variability in Bolt-Holt Eddy Current Inspections

Paper Proceedings: https://asmedigitalcollection.asme.org/qnde/QNDE2021/volume/85529

ASME 2021 48th Annual Review of Progress in Quantitative Nondestructive Evaluation (QNDE2021)

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