Detection of micro-flaws on thin copper tubes using SQUID-NDI system based on eddy current technique

We constructed a SQUID-NDI system for inspection of micro-flaws on heat exchanger copper tubes employing HTS-SQUID gradiometer and Helmholtz-coil-type inducer. The HTS-SQUID gradiometer was cooled using a coaxial pulse tube cryocooler. The detection of artificial flaws several tens of μm in depth on copper tubes, 6.35 mm in outer diameter and 0.825 mm in thickness, was demonstrated using the SQUID-NDI system. With excitation field of 1.6 μT at 5 kHz, the system could detect a 30-μm-depth flaw with SN ratio of about 20, where conventional NDI methods should fail to detect. The amplitude of the magnetic signal due to the flaw was in proportion to the square of flaw depth. Taking into account of the system's noise level, the results suggest that the SQUID-NDI system has a potential to detect sub-10-μm-depth flaws on copper tubes