Development of SQUID-Based System for ... - IEEE Xplore

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Development of SQUID-Based System for Nondestructive Evaluation R. Nagendran, M. P. Janawadkar, M. Pattabiraman, Dipak Kumar Baisnab, J. Jayapandian, R. Baskaran, L. S. Vaidhyanathan, Y. Hariharan, A. Nagesha, M. Valsan, K. Bhanu Sankara Rao, and Baldev Raj

Abstract—This article describes the development of a superconducting quantum interference device (SQUID)-based system for nondestructive evaluation. The setup incorporates an in-house developed thin-film-based Nb SQUID with readout flux locked loop electronics and consists of a liquid helium cryostat with adjustable stand-off distance, a precision XY 0  scanner for studying both flat and cylindrical samples, and a data acquisition system. The system has been used for the detection of artificially engineered subsurface defects in aluminum plates and to track magnetic-to-nonmagnetic phase transformation in stainless steel [grade 316L(N)] weldment specimens subjected to low cycle fatigue deformation. Index Terms—Eddy current testing, fatigue, nondestructive evaluation, superconducting quantum interference device (SQUID), weldment.

I. INTRODUCTION

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ONVENTIONAL eddy current testing is the popular nondestructive evaluation (NDE) technique for detecting subsurface flaws in conducting structures like aluminum, stainless steel, etc. In this technique, the strength of the eddy current generated in the material by the application of time varying magnetic field exponentially decays as a function of depth measured from the surface and, therefore, the depth of the defect detecof the material. In convention is limited by the skin depth tional eddy current testing, the signal decreases if the excitation frequency is decreased in order to increase the skin depth. The other way to increase the signal is to increase the coil diameter and use a larger number of turns. In this case, the spatial resolution will be poor. Thus, there is a compromise between the signal-to-noise ratio and the spatial resolution of the system [1]. The system based on the superconducting quantum interference device (SQUID) as a detector promises better signal-to-noise ratio at low operating frequencies. Since the sensitivity of the SQUID device is independent of the operating frequency from near dc to several kilohertz (white noise regime), it is possible to detect subsurface defects. The pickup loop can be made of superconducting wire with miniature size; unlike normal metal pickup loops for which induced voltage is proportional to rate of change of flux, the superconducting pickup loop directly senses the change in magnetic flux and, hence, its use is crucial in retaining the requisite low frequency sensitivity. Several groups

Manuscript received October 17, 2006; revised March 16, 2007. This paper was recommended by Associate Editor M. Mueck. The authors are with the Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, India. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TASC.2007.902112

have successfully utilized this feature to detect subsurface defects not detectable by conventional eddy current techniques [1]–[6]. Fatigue damage is one of the most important problems for a number of high-temperature components in power plants particularly in welded regions. A major consideration during the welding of 316L(N) stainless steel is its resistance to hot cracking. The presence of an optimum amount of delta ferrite in the austenitic weld metal is desirable to prevent hot cracking in the weldment. However, the -ferrite structure is highly unstable during high-temperature service and transforms to carbides and brittle intermetallic phases, e.g., sigma phase. Since the -ferrite is a magnetic phase, one can evaluate the -ferrite content by measuring remanent magnetization of the weldment sample. This paper describes the construction of a SQUID-based NDE system which comprises of a SQUID insert housed in a liquid helium cryostat, precision XY- scanner for the sample movement and the data acquisition module to acquire the SQUID output signal with respect to sample coordinates for the flat plate as well as cylindrical samples. This SQUID system has been used for the measurement of subsurface defects by inducing eddy currents in conducting materials at relatively low frequencies and also for the measurement of extremely low content of -ferrite in the 316L(N) stainless steel weldment specimens. II. EXPERIMENTAL DETAILS The SQUID is a highly sensitive detector of magnetic flux or any other physical quantity that can be converted into magnetic flux. The SQUID sensor is a thin-film superconducting device fabricated by using thin-film microfabrication techniques. The SQUID sensor produces a measurable periodic output voltage for tiny changes in input magnetic flux which are too small to be detected by any other means. This periodic output can be linearized by using flux locked loop (FLL) electronics. In the NDE system developed in our laboratory, we have used a homebuilt dc SQUID sensor and the associated FLL electronics. The Hz. The intrinsic noise of the SQUID system used is development of the SQUID sensor and the FLL electronics have been described in detail elsewhere [7]. A liquid helium cryostat with a capacity of 12 L has been fabricated using Stainless Steel (grade 316) as the construction material. The liquid helium cryostat has an access of about 40 mm for inserting the SQUID sensor and the superconducting gradiometer. The evaporation rate of the liquid helium cryostat is about 250 cc/hr. The liquid helium cryostat has been mounted on a nonmagnetic cantilever stand of all stainless steel construction. A flexible bellow has been provided at the tail end of

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Fig. 1. Schematic view of the lower portion of the liquid helium cryostat.

the cryostat to reduce the stand-off distance between the superconducting gradiometric pickup loop and the room-temperature specimen to as low a value as possible (Fig. 1). A thin copper wire, electrically insulated from the liquid helium vessel, is attached to the bottom flange of the liquid helium vessel. When the bottom plate touches the liquid helium vessel by adjusting the flexible bellow there is an electrical continuity between the wire and the bottom plate. In this way, one can adjust and maintain minimum stand off distance between the superconducting gradiometer and the specimen which is crucial for improving the spatial resolution. The warm-to-cold distance of the cryostat is estimated to be about 10 mm. A double “D” excitation coil has been used to excite eddy currents in the specimen at room temperature. This double “D” configuration for the excitation coil enables realization of a wide dynamic range since the direct coupling of magnetic flux to the pickup loop is a minimum when the axis of the excitation coil coincides with that of the pickup loop. A first order gradiometer made of superconducting wire is used as a pickup loop which is inductively coupled to the SQUID device. It consists of two loops of 4-mm diameter wound in opposition and separated by a baseline of 30 mm. This configuration enables discrimination against distant sources of magnetic noise which produce equal and opposite response from the two loops constituting the gradiometer. The central axes of the excitation coil and the pickup loop were adjusted to coincide and held stationary during the measurement. The setup was calibrated by measuring the system response to the magnetic field produced by a large circular coil and the calibration constant was inferred to be 20 nT/cm per flux quantum coupled to the SQUID. The material which is to be evaluated is kept at room temperature at a stand-off distance of about 10 mm and is scanned under the liquid helium cryostat. Eddy currents were excited at a relatively low frequency of about 200 Hz using a double “D” coil having an overall diameter of 20 mm carrying a current of 100 mA (root-mean-square). Changes in the induced eddy current flow associated with the presence of a defect manifest as changes of the flux signal detected by the SQUID device. The SQUID, in turn, produces an output voltage corresponding to these changes in magnetic flux. This output is phase-sensitively detected by a

Fig. 2. Schematic view of the experimental setup.

lock-in amplifier. The schematic diagram of the experimental setup is shown in Fig. 2. In the case of remanent magnetization measurements, the double “D” coil is not used and the SQUID output is directly read by data logger and recorded by the computer with respect to the coordinates of the sample as it is scanned under the stationary cryostat. The XY scanner has been designed for the scanning of flat plate samples with a positional accuracy of 0.025 mm and repeatability of 0.1 mm. The maximum stroke length is about 300 mm along each axis and the scanning speed can be varied from 1 to 50 mm/s. A separate sample stage has been incorporated to enable investigations on cylindrical tube specimens with diameter ranging between 20 and 120 mm. The major components of the XY scanner are computer controlled XY stage, supporting platform which moves smoothly over a frictionless table and nonmetallic and nonmagnetic sample holder. The whole assembly has been mounted on a single frame. Necessary vibration isolation has been provided to minimize the transmission of floor vibrations. Care has been taken to isolate the SQUID system from the magnetic noise generated by the high torque stepper motors. The development of the XY- scanner has been described in detail elsewhere [8]. III. RESULTS We have taken two different kinds of samples such as aluminum plate with artificially engineered flaws for subsurface defect detection by using the SQUID-based eddy current technique and fatigue cycled stainless steel weldment specimens to detect the -ferrite content through remanent magnetization measurement by the SQUID system.

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Fig. 4. Weld pad geometry used for the weld joint specimens.

Fig. 3. (a) Aluminum plate with artificially engineered defects. (b) Magnetic anomalies associated with the defects buried at a depth of 9 mm below the surface recorded by the SQUID.

A. Subsurface Defect Detection An aluminum plate with a length of 300 mm, width of 100 mm, and thickness of 10 mm with artificially engineered defects, as shown in Fig. 3(a), has been fabricated. Two rectangular defects have been engineered with a separation of 150 mm. One defect has a length of 50 mm, width of 1 mm, and height of 1 mm, and another defect has a length of 50 mm, width of 1 mm, and height of 0.5 mm for simulation of subsurface defect characterization. This aluminum plate with defects on its bottom surface was scanned under the SQUID probe and the changes of magnetic field associated with the defects were recorded with respect to the positional coordinates. Fig. 3(b) shows the magnetic anomalies associated with these defects recorded using a SQUID. It may be noted that in this experiment, a SQUID output voltage of 15 mV corresponds to a flux quantum coupled to the SQUID loop. The potential of the system for detection of subsurface defects using low-frequency eddy current excitation is evident from this data. B. Stainless Steel 316L(N) Weldment Specimens The 316L(N) stainless steel weldment specimens were prepared by welding 316L(N) base metal with 316N electrodes by a manual metal-arc welding process. Welding was carried out on a 25-mm-thick plate with a double-V configuration with an included angle of 70 (Fig. 4). Welds were radio-graphed and only sound joints were taken for fabrication of the specimens. The low cycle fatigue (LCF) tests were conducted at a strain amplitude of 0.6% using an Instron servohydraulic fatigue testing machine under total axial strain control mode at 600 C. s was employed for the test. The A strain rate of magnetic flux signal arising from the remanent magnetization of the sample was coupled to the SQUID device via a superconducting flux-transformer shaped in the form of a first-order gradiometer. As the sample was scanned under the cryostat, SQUID output revealed a characteristic magnetic anomaly when the center of the gauge length of the sample passed under the pickup loop indicating the presence of a magnetic phase at the

Fig. 5. SQUID output for the virgin and the fatigue fractured sample.

location of the weld. To show the system capability in measuring the virgin sample which was strongly magnetic and fractured sample which was magnetically very weak, the two samples were kept at a separation of 150 mm and scanned under the SQUID at a stand-off distance of about 25 mm. The choice of this relatively high stand-off distance was on account of the need to be able to measure the magnetically strong virgin sample and the magnetically weak fractured sample simultaneously in a single scan. The scanning started far away from the sample and as the sample approached the pickup loop, SQUID output increased and it subsequently decreased as the sample moved away from the pickup loop. Fig. 5 shows the variation in the SQUID output for the virgin and the fatigue fractured sample. The scale on the right shows the inferred magnetic field gradient at the location of the pickup loop. For the virgin sample, the amplitude of the magnetic anomaly was measured to be , whereas for the sample which was subjected to fatigue testing at 600 C until fracture, the amplitude of the magnetic anomaly under identical meawas measured to be as small as surement conditions. As a complementary study, the measurements of -ferrite in the weldment specimen were also carried out using a magne-gauge1 along the 25-mm gauge length of both the virgin and the sample subjected to LCF. The experiments clearly show that the magnetic phase present in the virgin sample transforms to a nonmagnetic phase ( or carbides, etc.) 1American

Instrument Aminco Brenner Magne-Gauge (model 5-660).

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Fig. 6. SQUID output for the virgin and fatigue cycled weldment specimen.

Fig. 7. SQUID output versus number of fatigue cycles for the weldment specimen subjected to fatigue deformation at 600 C.

during the fatigue deformation at 600 C. The transformation of -ferrite to brittle -phase has been known to influence the cracking behavior and thereby the fatigue life in the weld metals and weld joints of 316 and 316L(N) stainless steels [9]–[11]. The amount of transformation of the -ferrite into brittle phases has also been found to be a strong function of the temperature and frequency of testing [11]. A series of experiments has been carried out in the stainless steel weldment specimens to evaluate the transformation of magnetic phase to nonmagnetic phases at different levels of fatigue deformation. In this study, a single weld joint was selected for remanent magnetization measurements in the virgin state and subsequently after every 50 fatigue cycles. At every stage prior to fatigue cycling and remanent magnetization measurement, the weld joint was properly demagnetized by subjecting it to low-frequency alternating magnetic field to eliminate the influence of the past history and then remagnetized by applying a preset dc magnetizing field (35 Oe) for a preset time (300 s) before the measurement of remanent magnetization using the SQUID-based setup commenced. The peak value of demagnetization field was kept slightly higher than the magnetizing field. To evaluate the relative changes in the magnetic content of the weldment specimen, the parameters of the measurement setup such as stand-off distance, FLL gain, etc., were maintained constant throughout the whole series of measurements. The virgin , weldment specimen gave a maximum SQUID signal of when the sample was subwhich decreased rapidly to jected to LCF for 50 cycles. Thereafter, no significant changes in the maximum SQUID signal could be observed up to 150 cycles. However, a marked decrease in the SQUID signal was noticed when the sample was subjected to LCF for 200 cycles accompanied by the initiation of a crack at the boundary of the weldment specimen. Micro cracks were seen at the boundary of the weldment specimen when the specimen was examined through the microscope. The magnetic profile of the weldment specimen scanned under the SQUID probe is shown in Fig. 6 in the virgin state as well as after subjecting the specimen to different levels of fatigue deformation. Fig. 7 shows the variation in the maximum SQUID signal as a function of the number of cycles of fatigue loading and portrays the transformation of magnetic

-ferrite to nonmagnetic phases when the weldment specimen is subjected to LCF at 600 C. It may be noted that although the initial signals were large, SQUID-based measurements were intended to look for small magnetic anomalies, if any, which could be correlated with the residual life of the weldment specimen. Further experiments are underway to clarify these issues. IV. CONCLUSION A SQUID-based system for NDE based on a precision XYscanner and a computer controlled data acquisition system has been developed. The system has been used to detect subsurface defects in relatively thick conducting plates which are not ordinarily detectable by conventional eddy current techniques owing to the skin depth limitations. Quantitative characterization of magnetic anomalies associated with artificially engineered defects is important in the context of the inverse problem relating the observed anomalies to the position, size, and orientation of the subsurface defects. The system has also been used to characterize the -ferrite content in SS 316L(N) weldment specimen subjected to fatigue at 600 C. The ability of the system to detect very small changes in remanent magnetization makes it possible to identify small magnetic anomalies, if any, prior to the complete failure of the weld. Possibilities exist for developing the technique further to forecast the residual life of a weld undergoing fatigue damage. ACKNOWLEDGMENT The authors would like to thank R. Saha, A. V. Thanikai Arasu, and G. Chinnamma for fabrication of SQUID sensors, R. Mallika for development of SQUID electronics, K. Gireesan for characterization of SQUID sensors, and M. K. Ranganathan and N. Chinnasamy for overall technical support. REFERENCES [1] W. G. Jenks, S. S. H. Sadeghi, and J. P. Wikswo, “SQUIDs for nondestructive evaluation,” J. Phys. D, vol. 30, pp. 293–323, 1997. [2] Y. Tavrin, M. Siegel, and J. H. Hinken, “Standard method for detection of magnetic defects in aircraft engine discs using a HTS SQUID gradiometer,” IEEE Trans. Appl. Supercond., vol. 9, no. 2, pp. 3809–3812, Jun. 1999.

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[3] P. Chen, L. Chen, J. Li, and C. K. Ong, “Novel excitation coils for nondestructive evaluation of non-magnetic metallic structures by high-T c dc SQUID,” Supercond. Sci. Technol., vol. 15, pp. 855–858, 2002. [4] H. Weinstock, “A review of SQUID magnetometry applied to nondestructive evaluation,” IEEE Trans. Magn., vol. 27, no. 2, pt. 4, pp. 3231–3236, Mar. 1991. [5] M. Mück, M. Korn, C. Welzel, S. Grawunder, and F. Schölz, “Nondestructive evaluation of various materials using a SQUID-based eddycurrent system,” IEEE Trans. Appl. Supercond., vol. 15, no. 2, pt. 1, pp. 733–736, Jun. 2005. [6] U. Klein, M. E. Walker, C. Carr, D. M. McKirdy, C. M. Pegrum, G. B. Donaldson, A. Cochran, and H. Nakane, “Integrated low-temperature superconductor squid gradiometers for nondestructive evaluation,” IEEE Trans. Appl. Supercond., vol. 7, no. 2, pp. 3037–3039, Jun. 1997. [7] M. P. Janawadkar, R. Baskaran, R. Saha, K. Gireesan, R. Nagendran, L. S. Vaidhyanathan, J. Jayapandian, and T. S. Radhakrishnan, “SQUIDs highly sensitive magnetic sensors,” Current Sci., vol. 77, pp. 759–769, 1999. [8] R. Nagendran, M. P. Janawadkar, M. Pattabiraman, D. K. Baisnab, R. Baskaran, L. S. Vaidhyanathan, Y. Hariharan, B. Raj, A. Nagesha, M. Valsan, and K. B. S. Rao, “Development of SQUID based non destructive evaluation system for detecting transformation of  -ferrite to non-magnetic phases during fatigue,” Nondestructive Testing and Evaluation Int., vol. 40, pp. 215–219, 2007. [9] M. Valsan, D. Sundararaman, K. B. S. Rao, and S. L. Mannan, “A comparative evaluation of low cycle fatigue behaviour of type 316LN base metal, 316 weld metal and 316LN/316 weld joint,” Metall. Trans., vol. 26A, pp. 1207–1219, 1995. [10] M. Valsan, A. Nagesha, K. B. S. Rao, and S. L. Mannan, “High temperature low cycle fatigue and creep fatigue interaction behaviour of 316 and 316(N) weld metals and their weld joints,” Trans. Ind. Inst. Met., vol. 55, no. 5, pp. 341–348, 2002. [11] A. Nagesha, M. Valsan, K. B. S. Rao, and S. L. Mannan, “Strain rate effect on the low cycle fatigue behaviour of type 316L(N) SS base metal and 316SS weld metal,” in Proc. Int. Welding Conf. IWC’99, New Delhi, India, Feb. 15–17, 1999, vol. 2, pp. 696–703.

R. Nagendran received the M.Sc. degree in materials science from Anna University, Chennai, India, in 1993, and M.Tech. degree in solid state technology from Indian Institute of Technology, Chennai, India, in 1994. He is working as a Scientific Officer, Materials Science Division, Indira Gandhi Centre for Atomic Research, Kalpakkam, India. His current research interests are in the area of development of SQUID-based measuring systems.

Dipak Kumar Baisnab received the B.Sc. degree (phys. hons.) in 1997 from Bankura Christian College, India, and M.Sc. degree in physics, in 1999, from Burdwan University, Burdwan, W.B., India. He joined the Indira Gandhi Centre for Atomic Research, Kalpakkam, India, in 2001. He was initially involved in developing SQUID-based magnetometer and NDE. Currently he is working towards developing high-temperature SQUID sensors.

J. Jayapandian received the M.Sc. degree in physics from University of Madras, Chennai, India, in 1979, and the M.Phil degree from Bharathidasan University, Trichirapalli, India, in 1983. He is currently working in the Materials Science Division, Indira Gandhi Center for Atomic Research, Kalpakkam, India, and heads the Design, Development and Services Section. His research interests are in the area of embedded designs, sensors based on MEMS technology, and instrumentation.

R. Baskaran received the M.Sc. degree in physics from Madras University, Chennai, India, in 1984. In 1984, he joined the Materials Science Division, Indira Gandhi Centre for Atomic Research, Kalpakkam, India. His research interests are in the areas of Superconductivity, SQUID sensor fabrication, inversion problem in the context of nondestructive evaluation of materials, and magnetoencephalography (MEG).

L. S. Vaidhyanathan received the Doctoral degree in physics from the Indian Institute of Technology, Chennai, India, in 1992. Since then, he has worked in the Materials Science Division, Indira Gandhi Centre for Atomic Research, Kalpakkam, India. His current research interests are in the areas of superconductivity, superconducting thin-film devices, and the development of SQUID devices for applications such as NDE and biomagnetism.

M. P. Janawadkar received the M.Sc. degree in physics from Karnatak University, India, in 1976. He joined the Materials Science Division of the Indira Gandhi Centre for Atomic Research in 1977 and has been working in the area of superconductivity since then. Currently, he is engaged in the program on development and utilization of SQUID devices.

Y. Hariharan received the M.Sc. degree in physics from the Indian Institute of Technology, Bombay, India, in 1972, and the Ph.D. degree from the Indian Institute of Technology, Chennai, India, in 1988. He is currently working in the Materials Science Division, Indira Gandhi Center for Atomic Research, Kalpakkan, India, and heads the Superconductivity Research and Applications Section. His research interests are in the areas of cryogenics, superconductivity, low-temperature physics, and SQUID sensors.

M. Pattabiraman received the M.Sc. degree in physics from Indian Institute of Technology, Chennai, India, in 1997, and the Ph.D. degree in physics from Indian Institute of Technology, Chennai, India, in 2002. He is an Assistant Professor in the Department of Physics, Indian Institute of Technology, Chennai, India. His current research interests are in low-temperature condensed matter physics.

A. Nagesha received his B.Sc. degree in engineering from REC, Rourkela, India, and the M.Tech. degree from IIT, Madras. He joined the Indira Gandhi Centre for Atomic Research, Kalpakkam, India, in 1993. His current research interest are thermomechanical fatigue behavior of 316L(N) Austenitic Stainless Steel, low cycle fatigue of modified 9Cr-1Mo Ferritic Steel—base and welds, life prediction under LCF and TMF cycling, etc.

NAGENDRAN et al.: DEVELOPMENT OF SQUID-BASED SYSTEM FOR NDE

M. Valsan received the B.Tech. degree from Mysore University, Mysore, India, in 1980, and the Ph.D. degree from the Indian Institute of Science, Bangalore, India, in 1991. He is currently working in the Mechanical Metallurgy Division, Indira Gandhi Center for Atomic Research, Kalpakkam, India, and heads the Fatigue Studies Section. His research interests are in the areas of Mechanical Properties such as creep, fatigue, creep–fatigue interactions, microstructure-mechanical properties correlation, deformation mechanisms, dynamic strain ageing and constitutive relations.

K. Bhanu Sankara Rao received the B.E degree from VRCE, Nagpur, India, the M.Tech. degree from IIT, Bombay, India, and the Ph.D. degree from the University of Madras, India. He heads the Mechanical Metallurgy Division of the Indira Gandhi Centre for Atomic Research, Kalpakkam, India. His current research interests are mechanical metallurgy, physical metallurgy, welding science and technology, and powder metallurgy. He has published over 300 research papers and has been the recipient of a number of awards including

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Best Metallurgist, Binani and SAIL Gold Medals of Indian Institute of Metals, Materials Research Society of India Medal, and the NASA Appreciation award.

Baldev Raj received the B.E. degree from Ravishankar University, India, and the Ph.D. degree from the Indian Institute of Science, Bangalore, India. He is a Distinguished Scientist and Director of the Indira Gandhi Centre for Atomic Research, Kalpakkam, India, and steers the science and technology programs aimed at developing world-class technology for fast breeder reactors. His specializations include materials characterization linked to performance, NDE and quality management, testing and evaluation using nondestructive evaluation methodologies, materials development and performance assessment, and technology management. He has more than 650 publications, has co-authored 11 books and monographs, and has co-edited 27 books and special volumes of journals. He has 5 Indian standards and 19 patents to his credit. He has won a number of major national and international awards for his pioneering research work and was recently awarded the prestigious Padmashree by the Government of India in recognition of his contributions.