Abstract: In the field of electromagnetic non-destructive testing, eddy current technique is used for the inspection of conducting materials, and capacitive imaging technique is used for the inspection of insulating materials. For some insulator-conductor hybrid structures, e.g., structures with glass fibre composite repairs, it is difficult to fullfill the requirements with only one inspection method. By using planar coil probe and high frequency excitation, the capacitive effect of the coil can be enhanced, together with the inductive effect of the coil, capacitive-eddy current dual-modality inspection can be achieved, and the inherent limitations of each single mode method can be overcome. The theory of the dual-modality technique, including the capacitive effect, the equivalent circuit and the influence of the specimen on the impedance measurement, was analysed, dual-modality inspection system with planar coil probe was built, surface and hidden defects in two types of typical specimens, namely glass fibre reinforces plastic (GFRP) and GFRP-metal hybrid structure were inspected. The experimental results indicate that, the dual-modality inspection technique based on the capacitive-inductive effect of planar coil can simultaneously realize the detection of defects in insulating materials and conducting materials, and can discriminate defects based on the probe responses at different excitation frequencies, so as to meet the comprehensive inspection requirements of insulator-conductor hybrid structure, such as structure with composite repair.

Key words: dual modality inspection, capacitive-inductive effect, defect imaging, hybrid structure