Eddy Current Testing System Using HTS-SQUID With External Pickup Coil Made of HTS Wire

We have developed an high-temperature superconductor (HTS)-superconducting quantum interference device (SQUID) eddy current testing (ECT) system using an HTS-SQUID module and external pickup coil made of Bi-2223 tape to investigate the feasibility of nondestructive evaluation of a steel plate paved with asphalt. Steel plates with simulated cracks were measured by using the system with a liftoff of 80 mm, which corresponds to a typical thickness of the asphaltic pavement. A signal from a backside crack in the steel plate can be detected by reducing the frequency of the excitation field to less than 40 Hz. In addition, it was found that a fluctuation of the background signal caused by an inhomogeneous magnetization decreases with decreasing the frequency while the signal from the crack does not depend on the frequency, resulting in clearer detection of the signal from the crack at lower frequencies. Since the signal from the eddy current decreases with decreasing the frequency, the observed signal from the crack probably originated from the flux leakage of the excitation field while the fluctuation of the background signal originated from the eddy current. The successful detection of the backside crack in the steel plate is expected to lead to diagnosis of an early-stage fatigue damage of social infrastructures.