Imaging of Small Defects in Nonmagnetic Tubing Using a SQUID Magnetometer

Abstract

Using' a recently developed superconducting quantum interference device (SQUID) magnetometer system which offers higher resolution and greater sensitivity than previous designs, we have created high-resolution spatial maps of the magnetic anomalies created by small defects in thin-walled, nonmagnetic tubes. Images obtained using three different sensing methods (injected current, induced current, and ferromagnetic decoration) are presented, and compared in terms of sensitivity and signal-to-noise ratio (SNR). All three methods possess the sensitivity to detect holes on a tube's outer diameter as small as 0.37 mm diameter X 0.54 mm deep, with varying SNR, as well as holes on the tube's inner diameter as small as 1.0 m m diameter x 0.3 mm deep. The ferromagnetic decoration method produces the strongest signals and excellent SNRs. Using this approach, we have detected inner-and outer-diameter slots with dimensions 0.76 X 0.15 X 0.08 mm3. In addition, we examine results of numerical inversion calculations applied to the experimental images. With inversion techniques it is possible to determine the original current density distribution given the measured magnetic field, hence revealing the approximate shape and location of defects.

A Comparison of Squid Imaging Techniques for Small Defects in Nonmagnetic Tubes

“…Although the technique does not appear suitable for quantitative defect characterization owing to the somewhat random nature of the application, it produces very strong signals with -excellent SNRs. With this approach, we have detected defects as small as 0.76 x 0.15 x 0.08 mm 3 and achieved SNRs at least five times larger than with other methods. In addition, we have demonstrated the technique's potential to simultaneously detect flaws on the tube's ID and OD surfaces.…”

“…Often, the amplitude of these background signals was approximately the same as that of the defect signals. Since our experiments were concerned with detecting artificial tube defects, image processing techniques were used to minimize the background signals and enhance the defect signals [3]. In most cases, a three-step subtraction method was used; for the smallest defects, digital filtering techniques produced better results.…”

SQUIDs for nondestructive evaluation

“…With this arrangement it was shown that the magnetic field due to the edges of the sample plate are essentially cancelled and the interfering signal due to the current cables is remarkably reduced. The cancellation principle was also studied [39] for transverse and longitudinal currents flowing in tubes and rods and later applied [40] to such samples. One of the key elements in the research at Vanderbilt has been the development of theoretical modelling in parallel with experimental work.…”

“…The effectiveness of SQUID susceptometry was further demonstrated in a novel NDE technique called surface decoration susceptometry. In this technique, the surface of a nonmagnetic sample is decorated with a ferromagnetic [40,101] or a superparamagnetic [102] tracer, and a high-resolution magnetometer or susceptometer is used to image the remanent magnetization or susceptibility distribution respectively of the decorated sample. An analysis of the resulting image is then utilized for detection of very fine surface-breaking cracks.…”

“…In other experiments, they utilized the FEM to compute the current distributions in a superconducting and normal state YBa 2 Cu 3 O 7−x , thin film [154]. To investigate the current distribution in flawed conducting tubes, Wikswo's group adopted lead field analysis [101] for inverting the measured magnetic field data. In this experiment, current flowed axially along a 12.1 mm metal tube with a 4 mm diameter hole in the wall of the tube.…”