A comparison of the performance of planar and conventional second-order gradiometers coupled to a SQUID for the NDT of steel plates

The authors have developed a DC magnetic NDT (nondestructive testing) technique using a SQUID (superconducting quantum interference device) coupled to a planar gradiometer for the inspection of structures such as flat steel plates and steel pipes. Design criteria for the gradiometer are presented and used to demonstrate the superior performance of the planar approach compared to that of a second-order axial gradiometer of conventional design. The authors show an example of the use of system for mapping the magnetic field above a steel plate containing ideal defects. The proposed SQUID system is shown to operate normally in an unscreened laboratory close to a steel specimen in the presence of a DC magnetizing field in excess of 20 mT