Entwicklung eines dreikanaligen SQUID-Sensorarrays für die Flugzeuginspektion nach dem Wirbelstromverfahren

Conventional eddy current (EC) systems employed for non-destructive detection of fatigue cracks hidden deeply in aircraft fuselage are at their sensitivity limit. Due to their wide dynamic range, Superconducting Quantum Interference Devices (SQUIDs) are well suited for EC testing of layered aircraft structure . Offering high sensitivity at low excitation frequencies and high linearity, they allow localization of deep flaws and quantitative evaluation of magnetic field maps from the investigated components. However, the requirement to take maps of the magnetic field, usually by meander-shaped scans, leads to unacceptably long measurement times. Due to their inductive coupling to a tank circuit, several rf-SQUID sensors may be read out sequentially by selectively coupling to their tank circuits reducing the measurement time considerably . Generally, many parameters of multichannel SQUID systems scale approximately linearly with the number of channels: cost, size, power consumption, adjustment effort, number of cables from the warm to the cold end, and coolant evaporation rate, due to thermal conduction into the cryostat. With growing number of SQUID channels, it becomes increasingly important to introduce multiplexing, i.e. sharing some parts of the system among several channels . Using three standard HTS rf washer SQUID magnetometers, a multiplexed SQUID array was implemented and operated with one read-out electronics. The SQUIDs are positioned in line with 7 mm spacing. Air gaps of - 1 mm between neighboring magnetometers and tank circuits have improved the coupling of the system considerably, thus enhancing return losses and ameliorating operation stability of the system. In order to couple the magnetometers inductively to the electronics, three tank circuits and three coupling coils were fabricated. The coupling coils are connected in series. For joining them to the read-out electronics, one coaxial cable was used. Cross-talk between SQUID channels was deterrnined to be negligible. To Show the performance of the SQUID array, EC measurements of aluminium aircraft samples were carried out. A holder with magnetometers, tank circuits, and coupling coils was installed in a bath cryostat. At the lower end of the cryostat, a double-D excitation coil of diameter 30 mm was field. The SQUIDs were arranged exactly along the centerline of the excitation coil where the compensation of the primary field is optimum. Newly developed Software controls the continuous switching of the SQUIDs during the scan and allocates the recorded signal of each magnetometer to the proper signal trace. Thus three EC Signal traces of the sample are obtained simultaneously . These traces are lock-in demodulated to yield in-phase and quadrature component, respectively . To suppress the eddy current frequency fEc in the outgoing signal, the time constant of the lock-in has to be selected sufficiently large