A Combined Inductive and Capacitive Non-Destructive Evaluation Technique Using a Single Spiral Coil Sensor

“Insulator-conductor” hybrid structures, such as insulated pipe/vessel and metallic structures with composite repairs, have been widely used in industry in recent years. Due to the two distinct types of materials, the comprehensive Non-destructive Evaluation (NDE) of such hybrid structure is often achieved via complementary NDE techniques, which may cause efficiency and cost issues, i.e., the conducting layer is inspected by magnetic induction related technique and the insulation layer is inspected by capacitive technique. This paper proposes a combined inductive and capacitive non-destructive evaluation technique using sensor with a single spiral coil etched on a Printed Circuit Board (PCB). The proposed technique uses both the inductance and stray capacitance of the planar coil. By measuring the coil impedance at frequencies below and above the resonant frequency, the inductive and capacitive responses can be separated to some extent, offering a simple and efficient way to image and discriminate the defects in both layers of the hybrid. Equivalent circuit analysis, Finite Element simulations and experimental research are applied. A system to implement this technique has been constructed, and its feasibility in imaging defects in insulators, conductors and a hybrid structure has been verified. The developed system successfully detected surface and hidden slots in Perspex, a through hole in aluminium, and a surface crack in steel at various lift-offs (up to 7 mm). In addition, line scan results on a steel plate with a surface crack covered by a Glass-fibre Reinforce Plastic (GFRP) plate with a hidden flat-bottomed circular hole are presented and discussed to further demonstrate the feasibility of the proposed technique on the defect detection and discrimination for “insulator- conductor” hybrid structures.