A compact dual - mode EMAT - ECT sensor with lift-off compensation for thickness gauging of coated aluminum plates

Electromagnetic acoustic transducers (EMATs) offer contactless, single-sided inspection for nuclear applications. In CAP1400 baffle plates, nonuniform protective coatings cause spatially varying lift-off distances that result in severe attenuation of signal amplitudes. Conventional EMAT systems cannot sense these lift-off variations, leading to unreliable thickness measurement results. To overcome this limitation, this paper presents a compact dual-mode EMAT-eddy current testing (ECT) sensor. Shear and longitudinal bulk waves are generated using a single M-shaped magnetizer and distributed excitation coils, enabling simultaneous generation and independent acquisition of both wave modes within a single scan. An integrated ECT channel estimates the local lift-off between the sensor and the specimen, and this information is used to compensate for the EMAT signal amplitude. Simulation results show a nonlinear decay of the bias magnetic field with lift-off. A quadratic polynomial is therefore utilized to fit this decay characteristic, and the resulting calibration curve is then used for compensation of the signal amplitude. The ECT channel is calibrated for lift-off estimation over 0−1 mm, with a maximum error of 0.038 mm. When this estimated lift-off is used to compensate for EMAT signals, the amplitude variation falls from 32% to 9%. Experimental validation was performed on a 10-mm-thick 5052-H32 aluminum plate. The longitudinal and shear waves returned thicknesses of 9.93 mm and 9.96 mm, respectively. These values are within 0.1 mm of the nominal thickness and are consistent with one another, offering cross-validation of the measurement. This work provides a practical solution for liftoff-tolerant thickness gauging of coated safety-critical structures in nuclear environments.