WO1995001574A1

WO1995001574A1 - Non-destructive testing system using high temperature superconducting squid

Info

Publication number: WO1995001574A1
Application number: PCT/US1994/005681
Authority: WO
Inventors: Kai Wai Wong; Chang-Xin Fan; Dongfeng Lu
Original assignee: Midwest Superconductivity Inc
Current assignee: Midwest Superconductivity Inc
Priority date: 1993-07-01
Filing date: 1994-05-20
Publication date: 1995-01-12
Anticipated expiration: 1996-01-01
Also published as: AU7138094A

Abstract

A non-destructive testing system includes a gradiometer (16) fabricated of high temperature superconducting material, a low frequency current generator (38) for producing a test current in a test body (36), and a lock-in amplifier (46). The preferred gradiometer (16) includes a two-hole (20a, b), rf-SQUID. The generator (38) causes a test current to flow through the test body (36) to produce corresponding magnetic fields. Structural defects in the test body (36) distort the current flow and its corresponding magnetic field, creating a change in magnetic field and its gradient as compared to the field at non-defective region, which is detected by the gradiometer (16). The lock-in amplifier (46) derives the gradient signal associated with the frequency of the generator (38) and rejects noise.

Description

NON-DESTRUCTIVE TESTING SYSTEM USING HIGH TEMPERATURE SUPERCONDUCTING SQUID

Background of the Invention

1. Field of the Invention

The present invention is concerned with the field of non-destructive testing of conductive materials using superconducting quantum interference devices (SQUIDS) . More particularly, the invention is concerned with a non-destructive testing system including a SQUID fabricated of high temperature superconducting material as a gradiometer sensor and including a generator for apply¬ ing a test current to a test body so that magnetic field anomalies associated with structural defects in the test body can be detected by the sensor.

2. Description of the Prior Art

The use of low temperature superconducting gradiometers (those being cooled to their critical te per- ature by liquid helium) as sensors in non-destructive testing systems is known. Non-destructive testing systems are used to detect surface and subsurface metal faults and defects. In one prior art system, a current is applied or induced in the material to be tested, resulting in a corresponding magnetic field around the material. Struc¬ tural defects or anomalies distort the current flow and its corresponding magnetic field, creating a change in the magnetic field and magnetic field gradient. An assembly including a superconducting pick-up coil is placed near the tested material to detect such a change caused by the defect in the material. Low temperature superconducting gradiometers suffer from an inherent limitation in that they must be cooled to a few degrees Kelvin to become superconductive. To reach this temperature, delicate cryostats using liquid helium must be used, which add significant bulk to the non-destructive testing systems and defeat portability and thereby utility. Furthermore, low temperature rf-SQUIDs are inoperative in the presence of certain high frequency noises. This requires that low temperature rf-SQUID be well shielded from ambient magnetic fields and requires a superconducting flux transformer with its pick-up coil located outside the shielding.

To measure the magnetic gradient, the coil consists of two parts with equal areas but wound in opposite directions. It is insensitive to uniform magnet- ic field but sensitive to field gradient. With the low temperature superconductors that are metals or alloys, the two parts of the pick-up coil can be adjusted to a high degree of symmetry, therefore effectively reducing the background noise. Those skilled in the art have con- sidered it necessary to have such a transformer, even with the high temperature superconductors, for the NDT system. However, a flux transformer with low noise, high efficien¬ cy, and good stability is very difficult to fabricate using the high temperature, ceramic superconductors. This is why there have not been NDT products based on the high temperature superconductors.

Summary of the Invention

The present invention overcomes the problems outlined above and provides a distinct advance in the state of the art. More particularly, the non-destructive testing system hereof provides for highly sensitive non¬ destructive testing of conductive bodies while eliminating the need for liquid helium, magnetic shielding and superc- onductive transformers.

The preferred apparatus includes a detector assembly and test current generator. The detector assem¬ bly includes a high temperature (above 63 K) superconduct¬ ing rf-SQUID as a gradiometer operable to detect the change in magnetic field gradients caused by defects in a test material in which current flow is produced by the generator. Such an rf-SQUID apparatus is capable of working without any magnetic shielding and can be used as a sensor, thus eliminating the necessity of a supercond- ucting transformer.

In preferred forms, the non-destructive testing apparatus includes an AC current generator to apply current to the test body and a lock-in amplifier for selecting the signal corresponding to frequency of the applied current and rejecting ambient noises. In addi¬ tion, the apparatus includes a data acquisition system for receiving and storing test results.

Brief Description of the Drawings Figure 1 is a block diagram of the non-destruc¬ tive testing apparatus of the present invention shown in use on a test body;

Fig. 2 is a block diagram of a second embodiment of the non-destructive testing apparatus using external coils for inducing an eddy current in the test body;

Fig. 3 is a perspective view of the preferred two-hole rf-SQUID gradiometer and associated electronics of the preferred apparatus;

Fig. 4 is another perspective view of the rf- SQUID gradiometer of Fig. 3;

Fig. 5 is a graph of a sample output of the magnetic gradient responses detected by the apparatus of Fig. 1 with a test body illustrated below the graph in correspondence with test body defects; Fig. 6 is a graph of another sample output of the magnetic gradient responses detected by the apparatus of Fig. 1 with a test body illustrated below the graph in correspondence with test body defects;

Fig. 7 is a plan view of the rf-SQUID gradi- ometer of the apparatus of Fig. 1; Fig. 8 is a plan view of a thin film SQUID assembly useful in the invention; and

Fig. 9 is a side sectional view of the assembly of Fig. 8.

Detailed Description of the Preferred Embodiments

Turning now to the drawings, and particularly Fig. 1, the non-destructive testing apparatus 10 broadly includes a detector assembly 12, test current generator 38, and the lock-in amplifier 46 shown in Figs. 1 and 2, detector assembly 12 includes a symmetric two-hole rf- SQUID gradiometer 16 immersed in a dewar 17 filled with liquid nitrogen and further includes SQUID electronics 34. As illustrated in Figs. 3, 4 and 7, gradiometer 16 is a generally disk-shaped block of material that becomes superconductive at a temperature above 63 K, preferably fabricated of bulk polycrystalline high temper¬ ature superconducting materials such as yttrium 123 compound or thallium 2223 compound. Gradiometer 16 in- eludes walls 28a and 28b defining a pair of spaced aper¬ tures 20a and 20b with an interconnecting gap 22 therebe¬ tween defined by spaced walls 24a and 24b. Apertures 20a and 20b are of equal area and in preferred forms have diameters of 1 mm, separated by gap 22. The center-to- center distance of the apertures is 2 mm. As illustrated in Figs. 4 and 7, weak link 26, formed of the block material, spans gap 22 and intercouples spaced walls 24a,b. Link 26 has approximate dimensions of 0.1 mm³. Part of the link 26 has to be further cut until a good rf- SQUID transfer function pattern was obtained.

In order to become superconductive, gradiometer 16 must be cooled below its transition temperature. Accordingly, gradiometer 16 is immersed in dewar 17 filled with liquid nitrogen, preferably the dewar with a rigid or flexible tail shown in Fig. 1. Alternatively, a commer- cial cryostat may be used, such as Model U177-0 cryogenic cooling system operable using gas manufactured by MMR Technologies, Inc.

As illustrated in Fig. 3, the detector assembly 12 includes a resonance tank circuit 18 that composes inductor coil 30 and capacitor 32. In preferred forms, inductor coil 30 is a coil of approximately 60 turns made of #36 copper wire having an approximate diameter of .8 mm and positioned in and coaxially with aperture 20a, al- though those skilled in the art will appreciate that coil 30 functions equivalently in aperture 20b. In this position, coil 30 inductively couples with apertures 20a, .

Capacitor 32 (220 pf) is electrically coupled in parallel with inductor coil 30 to form a tank circuit that oscillates at the tuned frequency depending upon the particular values of coil 30 and capacitor 32. SQUID electronics 34 is connected to inductor coil 30 and capacitor 32 and includes a conventional SQUID measuring system such as the model 330 SQUID electronics manufac¬ tured by Biomagnetic Technologies, Inc. In particularly preferred forms, the components of tank circuit 18 are tuned near the resonance frequency of 19 MHz.

Low frequency test current generator 38 (below 10 KHz) provides a means for causing a test current to flow through a test body 36 and includes two alternative embodiments. In the first embodiment illustrated in Fig. 1, an AC generator 38 directly produces an alternating current flow through test body 36. Generator 38 and the test body 36 form a closed series loop causing current to flow through the test body 36. AC generator 38 is a conventional unit such as model 3030 Function Generator manufactured by BK Precision.

In the second embodiment of generator 38 illus- trated in Fig. 2, current generator 38 supplies a low frequency current (less than 10 KHz) to coils 44 and 42 with the result that eddy currents are generated in test body 36. This eddy current embodiment presents an advan¬ tage in that no physical contact need be made to the test body in order to provide a test current. This embodiment is preferred if some non-metallic material intervenes between the test body and the gradiometer (e.g. paint or cloth) . Magnetic field generator 40 includes generator 38, inner balancing coil 42 and outer driving coil 44 aligned concentrically about gradiometer. Driving coil 44 carries an alternating current supplied by generator 38. The drive coil current transmits an oscillating magnetic field that induces eddy currents in the test body. Current oscillating in a circular loop parallel to the surface of a uniform conductor induces eddy current in that conductor that flow in concentric circles about the axis of the drive loop. Balancing coil 42 carries an alternating current in the opposite direction of the current in the drive coil 44 to suppress the magnetic field directed towards the gradiometer.

In particularly preferred forms, a lock-in amplifier 46 is used to process output signals for deriv¬ ing and amplifying only magnetic gradient signals associ¬ ated with the frequency of the applied current. Ambient magnetic fields which can interfere with the gradiometer sensor are usually greatest at multiples of 60 Hz. Thus, it is desirable to select a test current of 30 or 90 Hz and to amplify the output of magnetic gradient signals associated with the selected frequency only. Additional- ly, a data acquisition system 48 consisting of a simple plotter or computer is provided in the preferred embodi¬ ment to record the output signal provided by the lock-in amplifier 46. Any commercially available plotter with two-dimensional plotting capability is sufficient. In use, the non-destructive testing system of the present invention detects subsurface structural defects in conductive test bodies by sensing the change in magnetic gradient fields caused by those defects. In the operation of apparatus 10, test current generator 38 causes evenly distributed, alternating current to flow in test body 36. With eddy current generator 40, current oscillating in a circular loop parallel to the surface of test body 36 induces eddy currents that flow in concentric circles about the axis of drive loop 44.

The current flowing through test body 36, whether applied or induced, creates a corresponding test- current magnetic field. If the test body 36 is uniform and free of subsurface defects, the magnetic fields or the magnetic filed gradients detected will be the same as long as the relative configuration remains the same. When a metal defect such as a crack is present in the test body, however, the applied or induced current is distorted at the site of the defect and the corresponding test current- magnetic field is changed in the vicinity of the defect and such a change can be detected by this nondestructive testing system of the present invention.

In the testing mode, the two-hole rf-SQUID gradiometer 16 is immersed in dewar 17. Apertures 20a and 20b of gradiometer 16 are subjected to the test magnetic field generated by test body 36 and receive corresponding magnetic fluxes. As a result, corresponding aperture currents, illustrated by the arrows in Fig. 7, circulate about apertures 20a,b. Because the areas of apertures 20a and 20b are equal, the flux difference is proportional to the mean magnetic field difference sensed by the aper¬ tures.

If test body 36 contains subsurface defects, however, the test current and its associated test magnetic field is distorted and thereby presents a change in gradient in the flux through apertures 20a and 20b. SQUID electronics 34 detects this change and provides an output signal proportional to the change in the magnetic gradient emanating from test body 36. Fig. 5 is a graph of the output 48 during non¬ destructive testing of test body 36a having various defects 50a, 50b, 50c, 50d, and 50e. These defects result in respective magnetic field gradients in the vicinities thereof which are detected by detector assembly 12 as it passes along the length of test body 36. These magnetic field gradients in turn produce corresponding peaks 52a, 52b, 52c, 52d and 52e in output 48 as illustrated. As shown, the height of each peak 52a-e is related to the significance and orientation of the defect in the direc- tion of the base-line of the gradiometer, which is the line connecting the centers of the apertures.

Fig. 6 is another graph similar to Fig. 5, illustrating output 54 having peaks 56a, 56b, 56c and 56d corresponding to defects 58a, 58b, 58c and 58d in test body 36b. As can be observed in Fig. 6, the heights of peaks 56a-d are related to the nature of the detected defects. For example, defect 58a produces a greater magnetic field gradient due to its orientation than the magnetic field gradient produced by defect 58c, which is the same size but oriented perpendicular to the direction of the base line of the planar gradiometer.

The SQUID assembly of the invention can also be of dc-SQUID type or be made of thin films. Fig. 8 shows a sample of thin film dc-SQUID fabricated on a 4 x 4 mm² LaA10₃ substrate using multi-layer design. The substrate is first conditioned with the deposition of a 100 nm-thick film of SrTi0₃. The dc-SQUID is made based on edge junctions. First, a MgO seed layer and a Ce0₂ buffer layer are deposited. The thin seed and buffer layers function as an insulator layer as well as defining Josephson junctions. Then a layer 60, composed of 250 nm of YBCO, comprises the body of the SQUID and the Josephson junc¬ tions 59a and 59b defined by grain boundaries induced by the previously patterned seed layer. This layer can then be patterned by dry etching by the Ar ion beam to have a designed SQUID pattern shown in Fig. 8, except the strip 64. The width of YBCO line is 10 μm and the two areas enclosed by the line is 40 x 40 μm², respectively. Then a 200 nm-thick layer of SrTi0₃ is deposited on top of YBCO pattern. To finish the "8" shaped SQUID pattern, a window has to be opened on the SrTiθ₃ by dry etching with an incident angle of 60° at the sites 61a and 61b. Then a strip 64 of YBCO thin layer is deposited to connect 61a and 61b. At the cross point 62, the strip 64 is insulated from the first layer 60 of YBCO by SrTi0₃ film. Since the areas enclosed by the two squares are equal but one runs clockwise and the other counterclockwise,this pattern serves as a gradiometer. Of course, a passivation layer of SrTi0₃ is needed to protect the YBCO. Finally, a thin layer defines Ag contact pads using a liftoff process with Ar ion-beam cleaning in situ before Ag sputter deposition. As will now be appreciated, the present inven¬ tion provides a highly sensitive, non-destructive, testing system for detecting subsurface defects in conductive test bodies using high temperature superconductive materials enabling practical utility not present in the prior art. As those skilled in the art will appreciate, the present invention encompasses many variations in the preferred embodiments described herein. For example, gradiometers fabricated of various superconductive materials can be used and still fall within the scope of the present invention.

Having thus described the preferred embodiments of the present invention the following is claimed as new and desired to be secured by Letters Patent:

Claims

Claims ;

1. An apparatus for detecting a defect in a test body comprising: a SQUID assembly including a SQUID operable at a temperature greater than 63 Kelvin for sensing an ambient magnetic field or magnetic field gradient and responsive thereto for generating corresponding output signals; means, including generating means for generating a driving current of preset frequency of less than

10 KHz in the test body, for creating a test magnetic field corresponding to the configura¬ tion of the test body and said driving current, wherein a defect in the test body changes the magnetic field distribution in the vicinity of the defect and produces a difference in said test field or gradient thereof as compared to said test field or gradient thereof at a non- defective region; and processing means operably coupled with said SQUID assembly for processing said output signals for deriving test signals corresponding to the frequency of said driving current or harmonic thereof, and for producing a test result from said test signals indicative of the defect in the test body.

2. The apparatus as set forth in claim 1, said SQUID assembly not including a superconducting transform- er, said SQUID being fabricated of bulk polycrystalline 2223 thallium material.

3. The apparatus as set forth in claim 1, said SQUID including structure defining two apertures of substantially equal area and having diameters of less than about 1 mm with said SQUID being free of magnetic shield- ing by superconductive shields and free of magnetic shielding by mu-metal shields.

4. The apparatus as set forth in claim 1, said processing including a lock-in amplifier operable for producing an amplifier output corresponding to said difference in said test magnetic field or gradient there¬ of.

5. The apparatus as set forth in claim 4, said processing means including means for recording said amplifier output.

6. The apparatus as set forth in claim 1, said generating means including a generator electrically coupled with said test body.

7. The apparatus as set forth in claim 1, said generating means including a magnetic driving field generator positioned near said test body for inducing eddy currents in said test body.

8. The apparatus as set forth in claim 7, said magnetic field generator including inner and outer concen¬ tric coils located axially relative to said SQUID, said outer coil carrying an oscillating electric current for producing a corresponding driving magnetic field to induce eddy currents in said test body, said inner coil including means for carrying an oscillating electric current so as to produce a balancing magnetic field to suppress magnetic field directed towards the axis of said inner and outer coils.

9. The apparatus as set forth in claim 1, said SQUID presenting a transition temperature above 63 Kelvin, said apparatus further including cooling means for cooling said SQUID to a temperature below said transition tempera¬ ture.

10. The apparatus as set forth in claim 9, said cooling means including a liquid nitrogen dewar, which may have a rigid or flexible tail.

11. The apparatus as set forth in claim 9, said cooling means including a cryostat operable for using gas.

12. A method of detecting a defect in the test body comprising the steps of: providing a SQUID assembly including a SQUID operable at a temperature greater than 63 Kelvin for sensing an ambient magnetic field or magnetic field gradient and responsive thereto for ge¬ nerating corresponding output signals; generating a driving current of preset frequency of less than 10 KHz in the test body for creating a test magnetic field corresponding to the configuration of the test body and said driving current, wherein a defect in the test body changes the magnetic field distribution in the vicinity of the defect and produces a difference in said test field or gradient thereof as com¬ pared to said test field or gradient thereof at a non-defective region; and processing said output signals for deriving test signals corresponding to the frequency of said driving current or harmonic thereof, and for producing a test result from said test signals indicative of the defect in the test body.

13. The method as set forth in claim 12, further including the step of providing said SQUID assem¬ bly to exclude a superconducting transformer, said SQUID being fabricated of bulk polycrystalline 2223 thallium material.

14. The method as set forth in claim 12, further including the step of configuring said SQUID to include structure defining two apertures of substantially equal area and having diameters of less than about 1 mm with said SQUID being free of magnetic shielding by superconductive shields and free of magnetic shielding by mu-metal shields.

15. The method as set forth in claim 12, said processing step including the step of using a lock-in amplifier operable for producing an amplifier output corresponding to said difference in said test magnetic field or gradient thereof.

16. The method as set forth in claim 15, said processing step including the step of recording said amplifier output.

17. The method as set forth in claim 12, said generating step including the step of using a generator electrically coupled with said test body.

18. The method as set forth in claim 12, said generating step including the step of using a magnetic driving field generator positioned near said test body for inducing eddy currents in said test body.

19. The method as set forth in claim 18, further including the step of providing said magnetic field generator to include inner and outer concentric coils located axially relative to said SQUID, said outer coil carrying an oscillating electric current for produc¬ ing a corresponding driving magnetic field to induce eddy currents in said test body, said inner coil including means for carrying an oscillating electric current so as to produce a balancing magnetic field to suppress magnetic field directed towards the axis of said inner and outer coils.

20. The method as set forth in claim 12, said method further including the step of cooling said SQUID to a temperature below said transition temperature.

21. The method as set forth in claim 20, said cooling step including the step of using a liquid nitrogen dewar, which may have a rigid or flexible tail.

22. The method as set forth in claim 20, said cooling step including the step of using a cryostat operable for using gas for cooling.