Simulation and design of a superconducting eddy current probe for nondestructive testing

Abstract

Numerical modeling studies were carried out for the design and performance evaluation of an eddy current probe for superconducting quantum interference device (SQUID) based noncontacting electromagnetic nondestructive testing. The probe utilizes high-Tc SQUID from electromagnetic interference and to focus magnetic flux due to the eddy current excitation coil into the object under test (OUT). This is very important in order to achieve “self-nulling,” where the probe responds only to the defects in the OUT. Simulations are carried out with the eddy current probe located a finite distance above a defect free conducting half-space of infinite thickness. Eddy current densities in the OUT are then calculated for various probe designs. These eddy current densities give an indication of the flux focusing, and hence increased spatial resolution for combinations of HTS and μ-metal shielding. Shielding factors are also calculated for the screening of the primary excitation field and the “defect-free” eddy current field. Agreement with the analytical solution of a simple solenoid above a conducting half-space verifies the numerical method.

Simulation of the magnetic field due to defects and verification using high-Tc SQUID

“…The similarity in the depth dependence between the defect ®eld and the eddy current was owing to the fact that the defect ®eld is caused by the dierence in the eddy-current distribution between the ¯awed and the un¯awed sample. The defect ®eld is generated by the current anti-dipole [4] at the position of the ¯aw. Hence, the depth dependence of the defect ®eld must be related to the depth dependence of the eddy-current density.…”

“…This dierence suggests that the conducting material surrounding the ¯aw has induced additional eddy currents due to the disturbed eddy current near the ¯aw, which has a non-negligible contribution to the defect ®eld. Therefore, for various excitation frequencies, it is inaccurate to calculate the phase of the defect ®eld with the current anti-dipole [4] without the contribution from the conducting material surrounding the ¯aw.…”

“…In addition, the sample was assumed to be non-magnetic with permeability l l 0 . For the eddy-current problem with a cylindrical symmetry, the analytic solution for the eddy-current distribution in the ¯awless sample was found to be [4]…”

“…The theoretical analysis for the eddy-current problem is important for the quantitative nondestructive evaluation (NDE) with SQUIDs. The analytic formulas for the eddy-current distributions were studied for the un¯awed samples excited by the sheet inducer [1,2] and by the circular coil exciter [3,4]. However, for the sample with a buried ¯aw, it is dicult to obtain the analytic formula for the eddy-current distribution because of complex boundary conditions imposed by the ¯aw.…”

Low-frequency nondestructive analysis of cracks in multilayer structures using a scanning magnetic microscope

“…The frequency of measurements and the configurations of the excitation coil, which are the essential parameters, have been widely described by several authors [9][10][11]. In particular, Kreutzbruck et al [12] reported on measurements using an HTS rf-SQUID and a circular excitation coil to measure artificial crack signals close to rivets in multilayer samples, maintaining a high dynamic range.…”