An ECT probe featuring array TMR sensors and a coaxial excitation for the inspection of defects beneath rivets

Detecting defects beneath rivets in multilayer metallic structures remains a challenge in eddy current testing (ECT). This paper presents a rotational eddy current probe that integrates a coaxial excitation coil with an array of high-sensitivity tunnelling magnetoresistance (TMR) sensors for the inspection of defects around non-countersunk rivets. The coaxial excitation induces predominantly circumferential eddy currents around the rivet, leading to a near-zero tangential magnetic-field response in defect-free regions and thereby suppressing rivet-induced background signals. When a defect is present, the local disturbance of the eddy-current path produces a measurable magnetic-field perturbation that can be captured by the sensors. By rotating the probe around the rivet axis, a high-resolution spatial profile of the tangential magnetic field is recorded for defect visualisation. Experiments on double-layer aluminium specimens demonstrate that the proposed probe can detect both surface and subsurface defects, including flaws buried 3 mm beneath the upper layer. Defects as small as 2.0 mm × 0.2 mm × 3.0 mm were successfully identified. The results indicate that the proposed coaxial-excitation TMR-array probe provides an effective solution for rivet inspection and offers a promising approach for high-resolution eddy-current imaging of multilayer aerospace structures.