Impedance of a curved circular spiral coil around a conductive cylinder

•Flexible spiral coils have advantages in eddy-current NDE because they conform to the surface.•Expressions for the impedance of curved circular spiral coil around a long cylinder are derived.•Good agreement was obtained between the theoretical predictions and experiments.•A curved circular spiral coil has almost identical response to a corresponding square spiral coil.•This work increases the quantitative understanding of a canonical geometry in eddy-current NDE.

Expressions are presented for the inductance of a curved circular spiral coil in free space and the change in impedance when the coil is wrapped around the surface of a magnetic, conductive cylinder. Inductance and impedance measurements of a thin conformable coil on a cylindrical Al-alloy rod were used to test the validity of these expressions. The theoretical predictions are in very good agreement with the experimental measurements over the frequency range 1 kHz–10 MHz. The findings are compared with previous work using rectangular spiral coils and it was found that the change in normalized impedance at high frequencies for a circular coil was almost identical to that obtained for a similarly-sized square coil. The implications for use in eddy-current nondestructive testing are discussed.