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Transient Eddy-Current Nondestructive Evaluation Benchmark Data for Backface Slots in a Plate.

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Abstract

The results of a benchmark experiment for transient eddy-current non-destructive evaluation are reported. The benchmark configuration corresponds to the canonical geometry of an air-cored probe coil positioned above a conductive plate containing a long back-face slot. The coil is excited by an exponentially-damped step function current and measurements are made of the change in the transient magnetic field due to the slot. The aim of the work is to provide experimental data for validation of theoretical models under development within the wider NDE community and to provide a common geometry against which the performance of a range of such models can be compared. A further aim of the work is to stimulate the ongoing development of quantitative methods in transient eddy-current research.

Executive Summary

Eddy-current testing (ET) is a well-established and reliable non-destructive evaluation (NDE) technique that is used extensively to ensure the structural integrity of military platforms. It relies on a time-varying magnetic field, produced by a probe coil, to induce eddy currents in a metallic component under test. The presence of defects such as fatigue cracks or corrosion perturb the local eddy-current distribution, leading to a change in the induced coil voltage, signalling the presence of a flaw.; The two principal variants of ET are distinguished by the time variation of the applied field. Conventional ET utilises an alternating current to excite a probe coil, and measurements are performed in the frequency domain. The emerging technique of transient eddy-current testing consists of time-domain measurements where the probe coil is excited by a pulsed current. Significant interest has been generated in the transient technique because of its potential advantages over conventional ET in circumstances such as the detection of buried defects in multi-layer metallic structures.; A range of mature numerical modelling packages can be used to predict the probe response and optimise inspection parameters for conventional ET. However, there is a distinct lack of such tools for transient eddy-current testing. Further development of the transient technique requires similar validated quantitative models to underpin probe design and inspection philosophy.; With this in mind, the aim of the present work is to provide high-quality experimental transient eddy-current data for a well-characterised benchmark configuration. These data will then be employed to validate numerical models, currently under development within the wider NDE community. The benchmark experiment also provides a common geometry against which the performance of a range of numerical models can be compared, and is expected to stimulate ongoing development of quantitative transient eddy-current methods.; The benchmark configuration is typical of the case where transient eddy-current NDE could be considered in preference to conventional eddy-current NDE. The experimental configuration and parameters for the benchmark problem are presented within the body of the report and, for ease of access, the transient magnetic field data are provided in electronic form in a series of files accompanying this report.; This work was initiated by the TTCP Technical Panel MAT-TP-5 Nondestructive Evaluation for Asset Life Extension and Integrity through the operating assignment TTCP MAT-TP-5 O33 ‘Transient Eddy-current Systems.’

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