Automated Real-Time Eddy Current Array Inspection of Nuclear Assets

Abstract

Inspection of components with surface discontinuities is an area that volumetric Non-Destructive Testing (NDT) methods, such as ultrasonic and radiographic, struggle in detection and characterisation. This coupled with the industrial desire to detect surface-breaking defects of components at the point of manufacture and/or maintenance, to increase design lifetime and further embed sustainability in their business models, is driving the increased adoption of Eddy Current Testing (ECT). Moreover, as businesses move toward Industry 4.0, demand for robotic delivery of NDT has grown. In this work, the authors present the novel implementation and use of a flexible robotic cell to deliver an eddy current array to inspect stress corrosion cracking on a nuclear canister made from 1.4404 stainless steel. Three 180-degree scans at different heights on one side of the canister were performed, and the acquired impedance data were vertically stitched together to show the full extent of the cracking. Axial and transversal datasets, corresponding to the transmit/receive coil configurations of the array elements, were simultaneously acquired at transmission frequencies 250, 300, 400, and 450 kHz and allowed for the generation of several impedance C-scan images. The variation in the lift-off of the eddy current array was innovatively minimised through the use of a force-torque sensor, a padded flexible ECT array and a PI control system. Through the use of bespoke software, the impedance data were logged in real-time (≤7 ms), displayed to the user, saved to a binary file, and flexibly post-processed via phase-rotation and mixing of the impedance data of different frequency and coil configuration channels. Phase rotation alone demonstrated an average increase in Signal to Noise Ratio (SNR) of 4.53 decibels across all datasets acquired, while a selective sum and average mixing technique was shown to increase the SNR by an average of 1.19 decibels. The results show how robotic delivery of eddy current arrays, and innovative post-processing, can allow for repeatable and flexible surface inspection, suitable for the challenges faced in many quality-focused industries.

Keywords: automated eddy current testing; eddy current arrays; non-destructive evaluation; robotic NDE.

Figure 1

Eddy current inspection hardware.

Figure 2

Canisters with a matrix of 16 stress corrosion cracks. Depositions of 5 µL droplets of sea water, 3.03 g/L of MgCl2, 15.2 g/L of MgCl2 and 30.03 g/L of MgCl2 were used to induce the cracks in the top row, left, central and right columns, respectively.

Figure 3

Eddy current array transmit and receive configurations. (a) Generic Eddy current array layout with two vertical columns of coils. (b) Axial transmit and receive configuration where x (in blue) corresponds to the transmit/receive pair centres of the excited eddy current channels in the test part. (c) Transversal transmit and receive configuration where x (in green) corresponds to the transmit/receive pair centres of the excited eddy currents in the test part resulting from the first/odd column of coils, and where x (in orange) corresponds to the transmit/receive pair centres of the excited eddy currents in the test part resulting from the second/even column of coils.

Figure 4

Illustration of complex impedance data positional compensation performed between axial and transversal configurations. (a) Axial complex array positional compensation. (b) Transversal complex array positional compensation.

Figure 5

A flow chart showing the data transfer between different software and hardware elements.

Figure 6

Illustration of the multi-threaded C and LabVIEW programs.

Figure 7

Illustration of mixing datasets $Z_1$ and $Z_2$ impedance data to make $Z_m$ mixed data.

Figure 8

Axial vertical impedance component C−scan images at 250, 300, 400, and 450 kHz on a dB scale alongside impedance plane plots of the response from the highlighted defect.

Figure 9

SNR vs. Angle of phase rotation for the axial dataset acquired at 250 kHz.

Figure 10

Phase-rotated axial vertical impedance component C−scan images at 250,300,400 and 450 kHz on a dB scale alongside impedance plane plots of the response from the highlighted defect.

Figure 11

Mixed vertical impedance component C−scan.

Figure 12

Photo of crack matrix and micrograph (a) Photo of crack matrix with the defect of interest highlighted in a red circle. (b) Micrograph of the defect of interest at 96× zoom with desaturated background.