Pulsed eddy current transient technique with HTS SQUID magnetometer for non-destructive evaluation

• Published in: Physica. C, Superconductivity Vol. 372; pp. 278 - 281
• Main Authors: Panaitov, G, Krause, H.-J, Zhang, Y
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.08.2002
• Subjects: Non-destructive testing; Pulsed eddy current; SQUID; SQUID; Pulsed eddy current; Non-destructive testing
• ISSN: 0921-4534; 1873-2143
• DOI: 10.1016/S0921-4534(02)00696-2

Eddy current non-destructive evaluation (NDE) often uses excitation currents at a fixed frequency, which determines the depth of investigation due to the skin effect. We developed a novel pulsed eddy current NDE technique which potentially may cover a broader depth range, by analyzing the decay of the recorded transient. Eddy currents in the inspected aluminium structures were induced by turn-off of the primary magnetic field pulse in a transmitter coil. The secondary magnetic field created by the induced currents was recorded by an HTS superconducting quantum interference device magnetometer. The quickly decaying early time data of the transient signal corresponds to the information from upper layers of the plate, while the late time data delivers information from deep layers. In order to suppress environmental noise and increase the signal-to-noise ratio, repetitive square-wave pulses were used. The transient response of the plate was recorded in off-time between the transmitter pulses and averaged using a commercial LAPTEM data acquisition unit. Measurements of a crack at different depths in a stacked aluminium sample are presented. The data was analyzed by calculating the apparent conductivity of the sample as a function of depth, a technique often used in Geophysical data interpretation. Possible advantages and drawbacks of this new transient NDE technique are discussed.