Non-destructive evaluation of aircraft structures with a multiplexed HTS rf SQUID magnetometer array

Abstract

Eddy current (EC) testing of aircraft components for material flaws hidden deeply in the tested structure is facilitated by using an array of HTS rf superconducting quantum interference device (SQUID) sensors. Recently, the multiplexed operation of three planar HTS rf gradiometers with one electronics and one cable was shown [IEEE Trans. Appl. Supercond. 11 (2001) 1168]. In this paper, a multiplexing setup with three magnetometers is presented. SQUID magnetometers have proven advantageous over short-baseline gradiometers for EC measurements of deep flaws. Using three standard HTS rf washer SQUID magnetometers with step-edge junctions, we implemented a multiplexed SQUID array. In conjunction with EC excitation and lock-in readout, measurements of aluminium aircraft samples were carried out in an unshielded laboratory environment. Newly developed software controls the continuous switching of the SQUIDs during the scan of the samples. The quasi-simultaneously obtained traces of the magnetometers are lock-in demodulated to yield in-phase and quadrature components, respectively. For EC excitation, a double-D excitation coil was selected. We performed measurements to localize an artificial crack of 20 mm length in an aluminium sheet with 0.6 mm thickness uncovered and covered by unflawed aluminium plates. In addition, the angle between crack and scanning direction was varied. Recorded signals of the SQUID array are presented and compared to signals received by scanning with just one and two magnetometers.