HTS-SQUID Gradiometer Research Articles

Characteristics of an HTS-SQUID gradiometer with ramp-edge Josephson junctions and its application on robot-based 3D-mobile compact SQUID NDE system

In this paper, we investigated detailed behavior of novel HTS-dc-SQUID gradiometers with ramp-edge Josephson junctions (JJs) in both an ac magnetic field and a dc magnetic field. In the both fields, the novel gradiometers shows the superior performance to the conventional YBa2Cu3O7−x (YBCO) HTS-dc-SQUID gradiometer and a bare HTS-dc-SQUID ring with bicrystal JJs concerning durability against entry and hopping of flux vortices, probably due to their differential pickup coils without a grain boundary and multilayer structure of the ramp-edge JJs. A robot-based compact HTS-SQUID NDE system utilizing the novel gradiometer was reviewed, and detectability of the system to detect non-through cracks in a carbon fiber reinforced plastic (CFRP)/Al double-layer structure was demonstrated. A new excitation coil in which the supplied currents flowed in the orthogonal directions was applied to detect cracks that oriented vertical and parallel to the baseline of the gradiometer.

Nondestructive evaluation of braided carbon fiber composites with artificial defect using HTS-SQUID gradiometer

Braided carbon fiber reinforced plastics (CFRPs) are one of multifunctional materials with superior properties such as mechanical strength to normal CFRPs since the braided CFRPs have continuous fiber bundles. In this paper, we applied the current-injection-based nondestructive evaluation (NDE) method using a HTS-SQUID gradiometer to the braided CFRP for the detection of the breakage of the bundles. We prepared planar braided CFRP samples with and without artificial cracks of 1 and 2mm lengths, and measured the current density distribution above the samples using the NDE method. In the measurement results, not only a few completely-cut bundles but also the additional partially-cut bundles were detected from decrease in the measured current density along the cut bundle around the cracks. From these results, we showed that it is possible to inspect a few partially-cut bundles in the braided CFRPs by the NDE method.

Robot-arm-based NDE of hydrogen fuel tank using a 3D-movable HTS-SQUID gradiometer

In this study, we constructed a robot-arm-based SQUID-NDE system utilizing a novel HTS-SQUID gradiometer with ramp-edge Josephson junctions for hydrogen fuel tank inspection, and developed an automatic 3D-scanning program. A cylindrical hydrogen fuel tank with a double-layer structure, in which a 3mm-thick Al liner was reinforced by a 3mm-thick carbon-fibre reinforced plastic (CFRP) cover, was prepared. The tank had a 10-mm-long through crack in the Al liner made by pressure cycle test. To inspect the tank using the SQUID-NDE system, we adopted a low-frequency eddy current technique that enables to excite deep part in the Al liner. By applying an excitation field of 7.5 μT at 0.4 to 10 kHz to the tank from a double-D coil, the tank was scanned by the system while moving the HTS-SQUID gradiometer along curved surface of the tank in magnetically unshielded environment. Magnetic responses from the deep-lying crack in the tank were successfully detected by the system.

Non-destructive evaluation of multilayer conductor using an HTS SQUID gradiometer

We examined the ability of detecting defects in multilayer aluminum plates by using an HTS SQUID gradiometer in an unshielded environment. The planar HTS SQUID gradiometer with 1mm×1mm pickup loops and 1mm baseline was fabricated by using HTS multilayer and ramp-edge junction technologies, and cooled by thermal conduction from a LN2 cryostat. We tried non-destructive evaluation (NDE) of a structure consisting of 5–10 layers of 2mm thick aluminum plates with a through hole, which has a shape of slit, 30mm in length and 0.5mm in width only in the bottom layer, by changing the frequency of eddy current induced by a double-D type coil. It was found that the observed frequency and depth dependences of the peak gradiometer signal were well fitted by theoretical curves taking account of decay of eddy current and defect-induced magnetic field depending on the distance between the defect and the gradiometer. By employing 200Hz frequency, the slit in the 10-layer structure could be clearly detected, indicating the ability of detecting defects located in depth more than twenty times larger than the gradiometer baseline.

Evaluation of joint interface of friction stir welding between dissimilar metals using HTS-SQUID gradiometer

In this study, we investigated conductive properties of joint interfaces of friction stir welding (FSW) between dissimilar metals, stainless steel SUS304 and aluminum A6063, using a SQUID nondestructive evaluation (NDE) system. With current injection method, the current maps above the FSW specimens jointed under various conditions were measured by a HTS-SQUID gradiometer. The conductivities of the joint interfaces, which were estimated from the current maps, differed between the joint conditions. By destructive tests using optical microscope, large voids were observed on the joint interfaces with low welding speed that generated excess heating. In case of one specimen, which was welded with welding speed of 500 and 200mm/min, the conductivity of the former was higher than that of the latter, although the inside voids in the respective regions were not much different. From these results, it is suggested that the current maps were influenced not only by the conductivity of the joint interface but also by inside voids. By hardness test on the SUS boards near the interfaces, only the SUS jointed with 200mm/min was about half softer than its matrix.

Robot-Based NDE System Using 3D-Mobile HTS-SQUID

A novel HTS-SQUID gradiometer with ramp-edge Josephson junctions (JJs) composed of SmBa <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</sub> base-electrode and La <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> Er <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.95</sub> Ba <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1.95</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</sub> counter-electrode was developed. The robustness of the gradiometer in AC field perpendicular to the device plane was investigated. In an AC field of less than 2.8 mT <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">pp</sub> at 100 Hz, flux-trapping, jumping and increase in noise did not happen in the gradiometer, while a traditional HTS-SQUID gradiometer with YBa <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7-x</sub> thin film and bicrystal JJs with the same dimension as the novel gradiometer could not be operated properly in an AC field of about several tens nT <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">pp</sub> . A robot-based NDE system utilizing the novel HTS-SQUID gradiometer was developed. The system could move the gradiometer three-dimensionally (3D) in an unshielded environment without magnetic shielding or compensation technique. Eddy-current-based detection of a through hole in magnetized stainless steel board was demonstrated by the robot-based HTS-SQUID NDE system.

Eddy-Current-Based SQUID-NDE System for Copper Tubes With Laser Displacement Sensor

An eddy-current-based SQUID-NDE system has been developed to detect shallow surface defects of less than 50 mum in depth on heat-exchanger copper tubes using a HTS-SQUID and excitation coil. In this system, small surface displacement of a tube, variation of the tube thickness and inner ripples cause background magnetic noise besides the surface defects. In this study, main factors of the background magnetic noise were investigated by a HTS-SQUID gradiometer, laser displacement sensor and laser microscope. The magnetic response, surface displacement, outer diameter variation and radial thickness of straight tubes of 9.6 mm in outer diameter and 0.4 mm in thickness were measured. The experimental results suggested that the main factor of the background magnetic noise was not due to local shape displacement near the SQUID, but nonuniformity in tube shape mainly originated inside of the tube. In the case of a tube with inner ripples and a outer-surface defect, large periodical magnetic noise from ripples was measured. Noise reduction method to reduce the periodical magnetic background noise due to the inner ripple was also investigated. The periodical noise due to the ripples was significantly decreased at higher frequency than 300 kHz, where the skin depth into copper was about 0.1 mm, while the magnetic response from the defect of 30 mum in depth on the tube surface was successfully detected.

Detection of wire element breakage in power transmission line using HTS-SQUID

We have proposed the application of nondestructive evaluation (NDE) using HTS-SQUID gradiometer to detect wire breakage in compressive conductor joints in overhead transmission line. In this study, we prepare an aluminum conductor steel reinforced (ACSR) transmission line with a broken wire. The ends of the ACSR cable were compressed with Al sleeves. An AC current was injected toward into the cable axis by applying AC voltage across the both ends of the sleeves. Then the two-dimensional distribution of magnetic field gradient above the cable was measured by a HTS-SQUID gradiometer. In the case of the compressed ACSR with a wire breakage, the field gradient changed periodically along the spiral of the broken wire. In contrast, such changes did not appear above the ACSR cable without defects. These results suggested the possibility to detect the wire breakage in the compressive conductor joints by this method. We also conducted electromagnetic field simulation to verify the experimental results.

NDE of coated-conductor using HTS SQUID array

We developed a non-destructive evaluation (NDE) system using an HTS SQUID array in order to examine rare-earth (RE)-123 HTS coated conductors striated into multi-filamentary lines. The 5-channel HTS SQUID gradiometer array was composed of ramp-edge junctions with LaErBaCuO and SmBaCuO electrode layers, and fabricated by using an HTS multi layer fabrication technique. The planar gradiometers with 1 × 1 mm 2 pickup loops and a baseline of 1 mm detected the vertical element of magnetic field gradient induced around defects by an eddy current. The gradiometer array cooled by thermal conduction from a liquid nitrogen bath was placed above the coated conductor on the main stage with a lift-off of about 1.5 mm. A coated conductor was fed from a reel to reel, and cooled blow its T c by stages connected to Gifford–Mcmahon (GM) coolers. By employing a 3 kHz induction current generating the maximum field of 0.14 mT, we could identify a distribution of defects in a long-length non-striated conductor. Furthermore, we could detect and distinguish three kinds of defects, existence of a spotty normal-state region, electrical short between striated filaments, and delamination of the superconducting layer from the Hastelloy tape for each filamentary superconducting line at a high speed up to 30 m/h.

高温超電導ＳＱＵＩＤを用いたアルミ撚り線の素線断線の検出

A nondestructive inspection method to detect wire breakage in power transmission lines using an HTS SQUID gradiometer was examined. Hard-aluminum transmission lines composed of 19 twisted Al wires, with or without wire breakage, were prepared as specimens. While applying AC voltage to the specimens to induce the equivalent current in each wire, distributions of magnetic field gradient above the transmission lines were scanned by the gradiometer. A periodic pattern in the gradient distribution due to wire breakage was observed in the results of the line with wire breakage, while such pattern was not observed in that of the line without wire breakage. The effect of wire breakage in the transmission line appearing in field gradient generated by the line with current was also investigated by computer simulation, and the result was in approximate agreement with the experimental results.

Study of inspection of wire breakage in aluminum transmission line using SQUID

Taking advantage of the unsurpassed high-magnetic sensitivity of high-temperature superconductor (HTS) superconducting quantum interference device (SQUID), HTS-SQUID-non-destructive evaluation (NDE) technique was applied to detection of single wire breakage in hard-drawn aluminum transmission line (HAL), which was twisted from 19 aluminum wires. While applying AC current of 2.6 mA at 200 Hz to the HAL, distribution of magnetic field gradient above the HAL was two-dimensionally scanned by a HTS-SQUID gradiometer. A periodic pattern was detected along the locus of the broken wire in the distribution from the HAL with wire breakage, while such pattern was not observed in that from normal HAL without wire breakage. These results indicate the possibility of detection of single wire breakage by the magnetic inspection technique.

Mobile HTS-SQUID NDE system with robot arm and active shielding using fluxgate

A robot-arm-based mobile HTS-SQUID NDE system was developed for inspection of advanced structures such as hydrogen fuel cell tanks. In order to realize stable operation of HTS-SQUID exposed in Earth’s field and robot arm’s noise without flux trapping, flux jumping and unlocking during motion, a new active magnetic shielding (AMS) technique using fluxgate was introduced. The high sensitive fluxgate, which could measure magnetic field of up to several 10 μT, was mounted near an HTS-SQUID gradiometer on the robot arm to measure the ambient noise and feed back its output to a compensation coil, which surrounded both SQUID and fluxgate to cancel the ambient noise around them. The AMS technique successfully enabled the HTS-SQUID gradiometer to be moved at 10 mm/s by the robot arm in unshielded environment without flux trapping, jumping and unlocking. Detection of hidden slots in multi-layer composite-metal structures imitating the fuel cell tank was demonstrated.

SQUID-NDE of wire breakage in aluminium transmission line

SQUID-NDE technique was applied to detection of wire breakage in aluminium transmission line, and aluminium clamp composed of compressed conductor joint. Hard aluminium transmission lines twisted from 19 aluminium wires with or without a wire breakage were prepared. While applying ac current of 2.6 mA at 200 Hz to the line, distributions of magnetic field gradient above the transmission lines were measured by a SQUID-NDE system using a HTS SQUID gradiometer. A periodic pattern was detected along the locus of broken wire in the distribution of the field gradient generated from the line with wire breakage, while such pattern was not observed in the normal line without wire breakage. Numerical simulation of distribution of field gradient from the aluminium transmission line was also carried out to compare it with the experimental results.

Robot-arm-based mobile HTS SQUID system for NDE of structures

A robot-arm-based mobile HTS SQUID system was developed for NDE of fixed targets. To realize the system, active magnetic shielding technique using fluxgate as reference sensor for ambient field was applied to a cryocooler-based HTS SQUID gradiometer that was mounted on commercial robot-arm. In this technique, ambient field noise and pulse noise of 550 nT from robot were measured by the fluxgate near the SQUID, and then the fluxgate output was negatively fed back to generate compensation field around the SQUID and fluxgate. The noise from robot was reduced by a factor of about 20 and the shielding technique enabled the HTS SQUID to move in unshielded environment by the robot-arm without flux-trapping or unlocking at 10 mm/s. System noise measurement and inspection of hidden cracks in multi-layer composite-metal structure were demonstrated using the mobile SQUID-NDE system.

Study on nondestructive inspection using HTS-SQUID for friction stir welding between dissimilar metals

We have developed an SQUID-NDI technique for evaluation of friction stir welding (FSW) between aluminum alloy A6063 and stainless steel SUS304 from the electric conductivities in board specimens bonded by FSW. A SQUID-NDI system employing an HTS-SQUID gradiometer was constructed to measure current distribution in the FSW specimens by applying voltage to the specimen. By measuring field gradients dBz/dy and dBz/dx above the FSW specimens made with various FSW conditions and then converting them to current vector Jx and Jy, conductivities of FSW areas were estimated. Due to the difference in the FSW conditions, the conductivity distributions varied dramatically. From these results, it was suggested that the conductivities in FSW areas should be varied due to the temperature heated by the friction between the milling tool and the materials.

Eddy-Current-Based SQUID-NDE for Detection of Surface Flaws on Copper Tubes

An eddy-current-based SQUID-NDE system has been developed to detect shallow surface flaws of less than 50 mum in depth on heat-exchanger copper tubes using an HTS-SQUID gradiometer and an Helmholtz-coil-type inducer. In this study, detectable flaw sizes on the tubes were investigated by experiments and simulation. Copper tube specimens with flaws of various sizes were inspected by the system with an excitation field of 5.6 muT at 3 kHz. A magnetic anomaly due to the shallowest flaw of 10 mum depth, 100 mum width and 15 mm length was successfully detected. The experimental results showed that the magnetic signal amplitude due to a flaw was proportional to the effective flaw size given by the product of flaw depth, width and length. A numerical simulation was carried out to calculate the magnetic signal from a flaw on a copper tube to determine the dependence on the flaw size. It is concluded that a flaw with a volume of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> mum <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> should be detectable by improving the system parameters.

Non-destructive inspection using HTS–SQUID on aluminum liner covered by CFRP

An eddy-current-based SQUID non-destructive inspection (NDI) system to detect deep-lying cracks in multi-layer composite-Al vessels was developed taking advantage of the uncontested sensitivity of HTS–SQUID in low-frequency range. An HTS–SQUID gradiometer was mounted in a pulse tube cryocooler. A pair of differential coils with C-shaped ferrite cores was employed to induce an enhanced eddy current in an Al vessel wrapped in a carbon fiber reinforced plastic (CFRP) cover. Ellipsoidal dome-shaped Al liners containing through cracks, which were made by pressure cycle tests, in the CFRP covers with total thickness of 6mm (CFPR 3mm, and Al 3mm) were inspected by the system. While inducing eddy currents in the vessels with excitation fields at 100Hz or 7kHz, the vessels were rotated under the HTS–SQUID. Above the cracks, anomalous signals due to the cracks were clearly detected at both frequencies. These results suggested the SQUID-NDI technique would be a possible candidate for inspection of high-pressure multi-layer composite-Al vessels.

SQUID NDE for in situ inspection of copper heat exchanger tubes

An eddy-current-based SQUID NDE system was constructed for in situ inspectionof copper heat exchanger tubes using an HTS SQUID gradiometer and aHelmholtz-type-coil inducer. Thin copper tubes of 6.35 mm in diameter and0.8 mm in thickness were selected from products as specimens. Artificial flaws of100 µm in width, 15–25 mmin length and 10–50 µm in depth were made on the surfaces of the tubes. The tubes were moved at a velocity of1.6–32 m min−1 by a motor through the Helmholtz-type coil, which generated an excitation field of10 µT at 3 kHz. The SQUID NDE system could detect an anomalous magnetic response due to a flaw of10 µm in depth on thetube moving at 32 m min−1.

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, 6.35mm in outer diameter and 0.825mm in thickness, was demonstrated using the SQUID-NDI system. With excitation field of 1.6μT at 5kHz, 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.

Mobile Cryocooler-Based SQUID NDE System Utilizing Active Magnetic Shielding

A mobile cryocooler-based SQUID NDE system was developed for realization of NDE of fixed targets that can't be moved or rotated. In order to move the SQUID in an ambient field, an active magnetic shielding technique employing a compensation coil and feedback circuit including a band elimination filter was introduced in a SQUID NDE system. The HTS SQUID gradiometer was cooled by a coaxial pulse tube cryocooler to 74 K /spl plusmn/ 0.04 K. The flux noise caused by moving the SQUID at 32 mm/s was well suppressed. Unlocking of the flux-locked loop (FLL) circuit or a significant increase in noise did not occur during the motion. Detection of hidden slots in fixed single and double-layered carbon/carbon composites in ambient field was demonstrated by moving the HTS SQUID gradiometer.