Sensitivity characteristics of single-coil dual-mode sensors for multi-type defects detection in hybrid structures

Insulator–conductor hybrid structures are increasingly used, yet detecting defects across layers remains challenging. This work extends single-coil sensors by explicitly embedding specimen-induced capacitive coupling into the sensor’s equivalent impedance, enabling sensitivity to defects in insulated layer as well as conductive layer. Closed-form expressions for coil coupling capacitance are derived and validated against measurements with a relative error of ≈13.38%. A detail-preserving finite-element model of PCB coil characterises sensitivity to permittivity and conductivity over a broad band around resonance. Guided by these analyses, five representative excitation frequencies are selected to retain discriminative spectral features while reducing data volume and test time. Finally, experiments verify detection of multiple defect types in both layers and show that integrating multi-frequency features provides a clear advantage for identifying complex composite defects.