Effect of a double-pancake coil configuration on reliable crack detection in curved thin-walled structures

The coil configuration of an eddy current probe is a key factor in determining the magnitude and distribution of induced currents in a test sample, thereby influencing the crack detection capability. This study investigates the effect of a double-pancake coil configuration on reliable crack detection in curved thin-walled structures. Three configurations, each with identical excitation and detection coil dimensions, are evaluated for detecting surface and back-surface cracks in an aluminium tube. The mutual inductance characteristics of two conventional configurations are first analysed using electromagnetic field theory, leading to the development of a zero-mutual-inductance design. Numerical simulations are conducted to compare electromagnetic field distributions and detection signal responses, enabling an assessment of detection sensitivity and suppression of lift-off variations. Experiments are then performed on an aluminium cylinder using an industrial six-axis robot scanning system. The results show that coil configuration entails a trade-off between lift-off noise suppression and crack detection sensitivity in curved thin-walled structures, with back-surface crack detection being more strongly affected by coil configuration than surface crack detection.