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 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. In this paper, 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.