Development of eddy current testing system using HTS-SQUID on a hand cart for detection of fatigue cracks of steel plate used in expressways

We have been developing an HTS-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 non-destructive evaluation of a steel plate paved with asphalt. In this work, a new excitation coil intended as a current dipole source is designed and evaluated. The current dipole coil can be expected to generate a unidirectional excitation field and exhibit small field decay by distance, and thus seems suitable for a measurement under a long liftoff condition such as the case for crack detection through an asphalt pavement. It is also found that a strong single peak appears just above a slit hole as a simulated crack by applying the excitation field using a tilted current dipole coil. The developed measurement system was mounted on a hand cart, and successful detection of a 50 mm long slit hole and its stable operation were demonstrated in a field operation test.