JP5013363B2 - Nondestructive inspection equipment - Google Patents

Description

  TECHNICAL FIELD The present invention relates to a nondestructive inspection apparatus capable of measuring internal corrosion of pipes and the like, and dents such as holes, and a coil design method for the nondestructive inspection apparatus. The present invention relates to a nondestructive inspection apparatus capable of measuring a dent with high accuracy and a coil design method of the nondestructive inspection apparatus.

An apparatus for inspecting piping in a non-destructive manner is described in, for example, Japanese Patent Laid-Open No. 2000-329890 (Patent Document 1). According to the publication, a traveling rail is provided in the vicinity of a bent portion of a pipe, and a traveling carriage having an ultrasonic probe is installed thereon, and the thickness of the bent portion of the pipe is measured.
JP 2000-329890 A (paragraph number 0011, etc.)

  Conventional wall thickness measurement has been performed as described above. If it is a bent part where insulation is not wound on the pipe, the contact can be directly contacted with the pipe, so the wall thickness can be measured. There was a problem that measurement was not possible because contact was not possible.

  The present invention has been made in view of the above-described problems, and the object to be measured such as a pipe provided with a heat insulating material or the like as a heat insulating material is also non-contacted and corroded in its inside, such as a hole. It is an object of the present invention to provide a nondestructive inspection apparatus capable of measuring the dimensions of the coil and a coil design method for the nondestructive inspection apparatus.

  A nondestructive inspection apparatus according to the present invention drives a SQUID probe placed on a measurement object via a heat insulating material and a SQUID probe with respect to the measurement object covered with the heat insulating material. The SQUID probe is capable of measuring the dimensions of the object at a predetermined distance away from the SQUID probe based on the change in the magnetic field. And calculating means for calculating a minute hole of the object to be measured, that is, an internal dimension.

  Since the SQUID probe can measure the internal dimensions of the object to be measured at a predetermined distance based on the change in the magnetic field, for example, the SQUID probe can be used for a pipe or the like that is kept warm by a heat insulating material. A non-destructive inspection apparatus can be provided that can measure the size of a dent such as corrosion and holes inside a non-contacting object.

  Preferably, the apparatus further includes means for moving the SQUID probe along the object to be measured on the heat insulating material.

  In one embodiment of the present invention, a plurality of SQUID probes are placed on the object to be measured with a space between each other, and the moving means places a plurality of SQUID probes with a space between each other. Scan the object to be measured.

  In another embodiment of the present invention, a plurality of SQUID probes are prepared, at least one SQUID probe is placed on the object to be measured, and at least one other SQUID probe is provided. Measures the magnetic field of the environment around the object to be measured.

  More preferably, the object to be measured is a pipe, and the internal dimension of the object to be measured is the thickness of the corroded portion of the pipe or the dimension of the pipe.

  According to an embodiment of the present invention, the nondestructive inspection apparatus further includes means for applying a magnetic field from the outside.

  The predetermined distance y and the dimension d of the pipe hole that can be measured for the pipe are expressed by d> (y / 1000).

  Note that the SQUID probe preferably includes two coils arranged close to each other.

  In another aspect of the present invention, a coil design method for a non-destructive inspection apparatus for remotely inspecting a defect of an object to be measured using a SQUID probe having a built-in coil includes: a SQUID probe and an object to be measured; In accordance with the distance between the current value and at least one of the value of the current flowing through the coil and the number of turns.

  In still another aspect of the present invention, the design of a coil for a nondestructive inspection apparatus that remotely inspects a defect of an object to be measured using a SQUID probe containing a pair of coils arranged adjacent to each other. The method increases the diameter of the coil in accordance with the distance between the SQUID probe and the object to be measured.

    In still another aspect of the present invention, the design of a coil for a nondestructive inspection apparatus that remotely inspects a defect of an object to be measured using a SQUID probe containing a pair of coils arranged adjacent to each other. According to the method, the coil width between the pair of coils is increased according to the distance between the SQUID probe and the object to be measured.

  Hereinafter, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 shows that the inventor of the nondestructive inspection apparatus according to the present invention can detect the presence or absence of iron powder 25 by using a SQUID probe (hereinafter referred to as “sensor”) 11. It is a figure which shows the state which performed the experiment which measures this, and is a figure which shows the positional relationship of the sensor 11 and the iron powder 25 as an example of a to-be-measured object. Here, the illustration of the SQUID driving device and the arithmetic device is omitted.

  Referring to FIG. 1, when the distance between the iron powder 25 having a diameter of dmm and the sensor 11 is y, the inventor performs a plurality of experiments, and the diameter d of the iron powder 25 having a measurable dimension is d> (y / 1000) is obtained.

  In other words, for example, if the distance y is 1 m or less, the iron powder 25 having a diameter larger than 1 mm can be measured.

  When this is applied, it can be used for measurement of internal corrosion of pipes covered with a heat insulating material, dents and the like. Next, this principle will be described. FIG. 2 shows a state in which the iron powder 26 having a diameter of about 1 mm has fallen off at the inner diameter portion of the pipe 21 covered with the heat insulating material 22. In this case, as described above, the sensor 11 (not shown) can measure up to the dimension of d = (y / 1000) including the inner diameter of the pipe 21 and the thickness of the heat insulating material 22. If the sum of the thicknesses of the inner diameters of the pipe 21 and the pipe 21 is 100 mm, d> 100/1000 and d = dropping of iron powder having a diameter larger than 0.1 mm (that is, the hole size of the pipe or the thickness of the corroded portion) This means that it can be detected.

  Based on the above knowledge, the outline | summary of the nondestructive inspection apparatus of the piping covered with the heat insulating material using the sensor 11 is demonstrated. Non-destructive inspection apparatuses using the sensor 11 include a non-inductive system that does not use a coil and an inductive system that uses a coil. In both the non-inductive system and the inductive system, there is a system with one sensor and a system with two sensors. In addition, in the two-sensor system, each sensor is scanned over the object to be measured at intervals, and one sensor scans the object to be measured, and the other sensor detects ambient environmental noise. There is a method to do. This will be specifically described below.

(1) Non-inductive method FIG. 3 is a block diagram showing a schematic configuration of the nondestructive inspection apparatus 10 when one sensor 11 is used in the non-inductive method. Referring to FIG. 3, the nondestructive inspection apparatus 10 includes a circumferential direction of the pipe 20 as indicated by an arrow in the drawing along the outer surface of the object to be measured 20 (including the pipe 21 and the heat insulating material 22 here). A sensor 11 that is provided so as to be movable in the direction (indicated by B in the figure) and the length direction (indicated by A in the figure) and that detects the thickness of the inside of the pipe 21, and a measuring device 19 that operates as a SQUID driving device. Including.

  Here, the sensor 11 scans the object to be measured by being moved by an arbitrary moving means (not shown).

  As described above, the sensor 11 can detect the distance to the inner surface of the pipe 21 even if it is not in direct contact with the pipe 21 constituting the object to be measured 20, so the pipe 21 covered with the heat insulating material 22. Detects a decrease in wall thickness due to corrosion, etc.

  The measuring device 19 is a personal computer connected to the SQUID driving circuit 12 and the SQUID driving circuit 12 for driving the sensor 11 and serving as a calculation means for calculating the corrosion inside the pipe, the thickness reduction of the hole diameter, etc. (Hereinafter referred to as “PC”) 18.

  In this non-induction method, the sensor 11 detects a change in the magnetic field using only the magnetic field of the pipe. Therefore, even if the heat insulating material 22 is covered with a metal shield such as aluminum, a reduction in thickness due to corrosion or the like inside the pipe 21 can be detected.

  Next, a measurement method using two sensors in the non-induction method will be described. FIG. 4 is a diagram showing a measurement method in this case. FIG. 4A is a diagram showing an example of scanning the object to be measured with the two sensors 11a and 11b spaced apart from each other by a fixed distance a, and FIG. 4B shows the two sensors 11a and 11b. Among these, it is a figure which shows the example in the case of measuring the to-be-measured object with one sensor 11a, and detecting the surrounding environmental noise with the other sensor 11c. 4A and 4B, only the sensors 11a and 11b are shown, and the measurement apparatus is not shown.

  Referring to FIG. 4A, the two sensors (sensors 11a and 11b) are moved at a predetermined speed v in the direction of the arrow at a predetermined speed v. That is, the sensors 11a and 11b are moved by being shifted by Δt = a / v, the synchronization of both signals obtained at that time is detected, and if both signals are synchronized and abnormal, the abnormality indicates a defect. It is determined by the personal computer 18 that it is present.

  Next, referring to FIG. 4B, in this case, a sensor 11a that detects a defect along the object to be measured 20 and a sensor 11b that is provided separately and detects only ambient disturbance noise, The signal obtained by the sensor 11a is subtracted from the signal obtained by the sensor 11a to cancel the disturbance noise.

(2) Guidance method Next, the guidance method will be described. FIG. 5 is a diagram showing a nondestructive inspection device 30 when a single sensor 11 is used in the guidance method. Referring to FIG. 5, in the guidance method,
A current is applied to the object to be measured using a coil, and measurement is performed using a magnetic field generated by the current. For this purpose, means for applying a magnetic field from the outside (magnetic field applying coil described later) and a device for removing ambient environmental noise (cancelling coil described later) are added.

  Referring to FIG. 5, this embodiment includes a sensor unit 24 and a measuring device 29. The sensor unit 24 includes the sensor 11, the magnetic field application coil 17, and the canceling coil 23. In addition to the SQUID driving circuit 12 and the personal computer 18 for driving the sensor 11, which are elements in the previous embodiment, the measuring device 29 includes a lock-in amplifier 13 connected to the SQUID driving circuit 12, and a lock-in amplifier. 13, a voltage / current converter 15, 16 connected to the transmitter 14, a magnetic field application coil 17 connected to the voltage / current converter 15, and a voltage / current converter 16. Connected canceling coil 23.

  The voltage output from the transmitter 14 is exchanged for current by the voltage / current converter 15, and an alternating current is passed through the magnetic field application coil 17 provided in the vicinity of the sensor 11. The magnetic field application coil 17 through which the alternating current flows applies a magnetic field to the pipe 21. An eddy current perpendicular to the passage of the magnetic flux flows through the pipe 21 to which the magnetic field is applied, due to electromagnetic induction. The sensor 11 detects a magnetic flux signal corresponding to the sum of the magnetic flux from the magnetic field application coil 17 and the magnetic flux from the eddy current, and transmits it to the personal computer 18 via the lock-in amplifier 13. Calculate internal dimensions such as minute hole diameters.

  The voltage output from the transmitter 14 is exchanged into a current by the voltage / current converter 16, and an alternating current is passed through a canceling coil 23 provided in the vicinity of the sensor 11. The canceling coil 23 through which the alternating current has passed removes the magnetic field applied to the sensor 11 by the coil 17. Therefore, a signal from which the magnetic field by the coil 17 has been removed is input to the personal computer 18 through the lock-in amplifier 13 and a defect inside the pipe 21 is detected.

  The nondestructive inspection apparatus 30 according to this embodiment has a higher detection accuracy of the object to be measured 20 than the non-inductive method.

  Next, a measurement method using two sensor units in the guidance method will be described. FIG. 6 is a diagram showing a measurement method in this case. FIG. 6A is a diagram illustrating an example of scanning the object to be measured with the two measuring devices 29a and 29b spaced apart from each other by a fixed distance a, and corresponds to FIG. 4A in the non-inductive method. FIG. FIG. 6B is a diagram corresponding to FIG. 4B in the non-induction method. In FIG. 6A, the measurement object is scanned by the two sensor units 24a and 24b, whereas in FIG. 6B, the measurement object is measured by one sensor unit 24a. The other sensor unit 24c detects ambient environmental noise.

  In this case, a difference circuit 41 for taking a difference between signals acquired by the measuring devices 29a and 29b is provided, and the output is input to a personal computer. By acquiring the difference signal, as shown in FIG. 6A, when the sensor is moved at the speed v, the difference signal steadily removes noise at the time of surface measurement, and time t = a / v By capturing only the signals that are synchronized with each other, highly accurate measurement is possible.

  In FIG. 6B, the environmental noise captured by the sensor 24c can be identified with the environmental noise around the sensor 24a, the environmental noise can be removed by the difference circuit 42, and the measurement accuracy can be improved.

  Here, the signals acquired by the measuring devices 29a and 29b are signals output from the lock-in amplifier 13 to the personal computer 18 in FIG. 5, and these signals are processed by the personal computer.

  Next, another embodiment of the guidance method will be described. FIG. 7 is a block diagram showing another embodiment of the guidance method. With reference to FIG. 7, in this embodiment, a measuring device 49 is connected to a sensor unit 31 for detecting a corroded part of a pipe existing inside the DUT 20. Coils 36a and 36b are provided before and after the scanning direction (direction indicated by an arrow in the drawing) of the sensor unit 31 for detecting a corroded portion of a pipe existing inside the measurement object 20.

  Referring to FIG. 7, this measuring device 49 has the same configuration as measuring device 29, and is a voltage for driving SQUID driving circuit 12, lock-in amplifier 13, transmitter 14, and canceling coil 23 (not shown). In addition to the current / current converter 16 and the PC 18, an AC magnetic field application circuit 35 for applying an AC magnetic field to the DUT 20 is provided. The AC magnetic field application circuit 35 applies an AC magnetic field to the coils 36 a and 36 b that are movably provided around the sensor unit 31 in order to apply the AC magnetic field to the entire object to be measured 20. .

  Note that. For example, a magnetic field having a low frequency of 150 Hz or less is applied as a specific AC magnetic field in this induction method. In a normal measuring apparatus, the frequency is increased to increase the measurement sensitivity. Normally, when the frequency is increased, only the surface of the object to be measured can be detected. On the other hand, in a sensor using SQUID, when the frequency is lowered, it is possible to detect corrosion or the like inside the pipe covered with the heat insulating material (for example, when the heat insulating material is 10 mm and the pipe thickness is about 10 mm). become.

  Next, a measurement method using two sensor units shown in FIG. 7 will be described. FIG. 8 is a diagram showing a measurement method in this case. FIG. 8A is a diagram showing an example in which the object to be measured 20 is scanned with the two sensor portions 31a and 31b spaced apart from each other by a fixed distance a. FIG. It is a corresponding figure. FIG. 8B corresponds to FIG. 6B in the above embodiment. In FIG. 8A, the measurement object 20 is scanned by the two sensor units 31a and 31b, while in FIG. 8B, the measurement object 20 is measured by one sensor unit 31a. The measurement is performed and the ambient sensor noise is detected by the other sensor unit 31c as in the previous embodiment.

  Note that the coil shown in FIG. 8A is provided for each sensor. However, as shown in FIG. 8C, the coils 37a and 37b may be provided at both ends of the pipe to be shared.

  As a matter of course, 50 Hz and 60 Hz are avoided as the frequency of the AC magnetic field to be applied. Further, in the above embodiment, the case where the nondestructive inspection apparatus is applied to the measurement of the thickness of the corroded portion inside the pipe covered with the heat insulating material has been described. , It becomes possible to measure dimensions such as thickness without contact.

  In the above-described embodiment, two sensors are used to scan the object to be measured with the two sensors spaced apart from each other, or the object to be measured is scanned with one sensor and the rest Although the example of measuring the ambient environment with the sensor of the above has been described, the present invention is not limited to this, but the object to be measured is scanned with three or more sensors, the sensor is scanned with two sensors, and the ambient with another sensor The environment may be measured.

  Further, in the above embodiment, the case of detecting the dimension reduction in the pipe due to the corrosion in the pipe or the drop-out due to the contact of the foreign substance has been described. However, the present invention is not limited to this. Detecting the thickness of the part where the property of the part has changed due to (for example, the part of the material where the non-magnetic material has gained magnetism due to fatigue, etc.) You may apply to the detection of.

  Next, an experimental example according to an embodiment of the present invention will be described. FIG. 9A is a diagram for explaining the basic principle of this experiment. With reference to FIG. 9A, when the sensor 51 is brought close to the object to be measured 60, an eddy current 53 is generated on the surface of the object to be measured 60 by the magnetic field 52 by the coil of the sensor 51, thereby induction by the eddy current 53. A magnetic field 59 is generated. By moving the sensor 51 in the direction indicated by A in the figure, the eddy current is distorted by the concave portion existing on the object 60 to be measured, and the induced magnetic field 59 is also changed. This change is detected by the sensor 51. A coil 54 for applying an alternating magnetic field is provided on the measured object 60 side of the sensor 51.

  As shown in FIG. 9B, the coil 54 has a shape in which two D-shaped coils are arranged so that their linear portions face each other, and is hereinafter referred to as a WD coil 54. When a current is passed through the WD coil 54, the magnetic field generated from the coil is canceled on the center line 51a, and the influence from the transmission signal of the sensor 51 can be ignored on the center line 51a. As shown in FIG. 9B, a current flows through the WD coil 54 in the direction indicated by the arrow 56 in the drawing, and a magnetic field is formed in the direction indicated by the arrow 55.

  FIG. 10 is a diagram showing a measuring instrument used in this experiment. The basic configuration is the same as that shown in FIG. 5 and 10, sensor 51 is connected to SQUID drive circuit 12 shown in FIG. 5, WD coil 54 is connected to voltage / current converter 15 shown in FIG. Is connected to the voltage / current converter 16 shown in FIG. Here, the canceling coil 57 is provided in order to cancel the sensor 51 because the magnetic field directly from the transmission coil of the sensor 51 enters when the sensor 51 is slightly displaced from the center line 51a of the WD coil 54. Provided.

  FIG. 11 is a diagram showing a positional relationship among the sensor 51, the WD coil 54, and the DUT 60 used in this experiment. Referring to FIG. 11, WD coil 54 is provided at a distance of 5 mm from sensor 51. Using this measuring apparatus, a WD coil 54 is positioned at a distance h on a measurement object 60 having a diameter of 10 mm and a through hole 61 having a diameter of 10 mm, and the distance h is changed. Detected magnetic field. The number of turns of the WD coil 54 used here is 3, the current is 150 mA, the frequency is 200 Hz, and the coil width (the distance between the centers of the two coils constituting the WD coil 54) is about 0.1 mm. The result is shown in FIG.

  FIGS. 12A to 12C are graphs showing the magnetic field detection results (upper stage) when the distance h is changed to 1 mm, 5 mm, and 10 mm, and how the magnetic field changes with the distance. As shown in FIG. 12, the change of the magnetic field by the through-hole is detected in each. In this experiment, it was possible to detect the presence of the through-hole until h was about 15 mm, but it was impossible to measure when it exceeded 20 mm.

  Furthermore, as described above, in the WD coil, since h cannot be measured when h exceeds 20 mm, the shape of the coil includes two normal coils (W coil, W), rather than the WD coil (FIG. 13A). It can be seen that FIG. 13B is preferable.

  Next, coil parameters were examined. As the coil parameter, the effect was examined by changing the number of turns of the coil and the value of the current flowing through the coil.

  FIG. 14 is a diagram showing experimental results in this case. Here, the relationship between the distance h and the magnetic field when the number of turns of the coil is changed to 3, 10, and 30 and the current value is changed to 0.15A, 0.3A, and 0.45A is shown. FIG. Referring to FIG. 14, if the current value × number of turns shown by ■ is 0.45, the distance h is 20 mm at most, whereas the current value × number of turns shown by a black diamond is 13.5. Then, it can be seen that detection is possible even when the distance h is 50 mm. It can also be seen that the signal intensity tends to increase as the current value × the number of turns increases.

   From the above, as a specification of the pair of coils constituting the sensor 51, in order to enable measurement from a further distance h, the number of turns of the coil is increased and the current value is increased to increase the excitation magnetic field (number of turns × It can be seen that it is better to increase the current value. In other words, it can be seen that the specification of the pair of coils used when designing the sensor 51 may be any one of changing the number of turns of the coil and the current value according to the distance h used.

  Next, the coil when the diameter of each coil is changed while the distance (coil width) d between the centers of the W coils is constant when the W coil 58 including two normal coils 58a and 58b is used. The relationship between the diameter and the acquisition signal will be described. FIG. 15A is a diagram illustrating a state in which the diameter φ of the coils 58a and 58b is changed while the coil width d between the W coils 58a and 58b is constant.

  FIG. 16 is a diagram showing the measurement results in this case, and the W coil 58 and the DUT 60 when the diameters of the individual coils 58a and 58b constituting the W coil 58 are changed to 10 mm, 20 mm, and 30 mm. It is a figure which shows the relationship between the distance h between and an acquisition signal (magnetic field). Referring to FIG. 16, it can be seen that the signal intensity tends to increase as the coil diameter increases.

  Next, the relationship between the coil width d between the W coils and the acquired signal when the diameters φ of the coils 58a and 58b are constant will be described. The measurement state in this case is shown in FIG. As shown in FIG. 15B, the distance h between the W coils 58a and 58b and the DUT 60 was changed, and the distance (coil width) d between the W coils 58a and 58b was changed.

  The measurement result in this case is shown in FIG. FIG. 17 shows the coil width d and the acquired signal (magnetic field) when the distance h is changed to 20 mm, 30 mm, and 40 mm when the W coil 58 has 30 turns, the current value is 300 mA, and the coil diameter is 30 mm. It is a figure which shows the relationship.

  Referring to FIG. 17, it can be seen that as the distance h increases, the signal intensity increases as the coil width d of the W coil is increased.

  As mentioned above, although embodiment of this invention was described with reference to drawings, this invention is not limited to the thing of embodiment shown in figure. Various modifications and variations can be made to the illustrated embodiment within the same range or equivalent range as the present invention.

It is a figure which shows the positional relationship of a sensor and the iron powder which is a to-be-measured object. It is a figure which shows the state which the iron powder fell out in the internal diameter of piping. It is a block diagram which shows schematic structure of the nondestructive inspection apparatus using a non-induction system. It is a block diagram which shows the measuring method at the time of using a several sensor in a non-induction system. It is a block diagram which shows schematic structure of the nondestructive inspection apparatus using a guidance system. It is a block diagram which shows the measuring method at the time of using two or more nondestructive inspection apparatuses shown in FIG. It is a block diagram which shows schematic structure of the nondestructive inspection apparatus using the induction system which wound the induction coil around piping. It is a block diagram which shows the measuring method at the time of using two or more nondestructive inspection apparatuses shown in FIG. It is a figure for demonstrating the experiment example which concerns on one embodiment of this invention. It is a figure which shows the measuring apparatus used in this experiment. It is a figure which shows the relationship between the WD coil of the sensor used in this experiment, and a to-be-measured object. It is the graph which showed the change state of the magnetic field by the detection result (upper stage) of the magnetic field at the time of changing the distance h of a sensor and a to-be-measured object, and distance. It is a figure which shows a WD coil and a W coil. It is a figure which shows the experimental result in the case of changing the number of turns of a coil, and the electric current value sent through a coil. It is a figure which shows the state which changes the distance between W coils, the diameter of a coil, and a coil width. It is a figure which shows the measurement result at the time of changing the diameter of a coil, making the width | variety of W coil constant. It is a figure which shows the relationship between a coil width when the distance of W coil and a to-be-measured object is changed, and an acquisition signal.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Nondestructive inspection apparatus, 11,51 sensor, 24 sensor part, 12 SQUID drive device, 13 Lock-in amplifier, 14 Transmitter, 15,16 Voltage / current converter, 17 Magnetic field application coil, 18 PC, 19 Measuring device, 20, 60 object to be measured, 21 piping, 22 heat insulating material, 23 canceling coil, 25 iron powder, 29 measuring device 30 nondestructive inspection device, 31 sensor unit 35 AC magnetic field application circuit, 36, 37 coil, 41, 42 difference Circuit, 49 measuring device, 54 WD coil, 58 W coil.

Claims (6)

For a measurement object covered with a heat insulating material, a SQUID probe placed on the measurement object via the heat insulating material;
A SQUID driving device for driving the SQUID probe,
The SQUID probe can measure the dimension of the object to be measured at a predetermined distance based on a change in magnetic field,
The SQUID driving device includes an operation unit that analyzes an output from the SQUID probe and calculates an internal dimension of the object to be measured.
The calculation means calculates the internal dimensions of the measurement object covered with the heat insulating material based on the data obtained without passing an electric current through the measurement object,
The object to be measured is a pipe, and the dimension of the hole of the object to be measured is the dimension of the hole of the pipe.
The predetermined distance y and the dimension d of the pipe hole capable of measurement of the pipe are:
A non-destructive inspection device represented by d> y / 1000 . The nondestructive inspection apparatus according to claim 1, further comprising moving means for moving the SQUID probe along the object to be measured on the heat insulating material. A plurality of the SQUID probes are placed on the object to be measured with a space between each other, and the moving means places the plurality of SQUID probes on the object to be measured with a space between each other. The nondestructive inspection apparatus according to claim 1, wherein scanning is performed. A plurality of the SQUID probes are prepared,
At least one SQUID probe is mounted on the object to be measured;
The nondestructive inspection apparatus according to any one of claims 1 to 3 , wherein at least one other SQUID probe measures a magnetic field in an environment around the object to be measured. External further comprising means for applying a magnetic field from the non-destructive inspection device according to any one of claims 1 to 4. The SQUID probe comprises two coils disposed close to each other, non-destructive inspection apparatus according to any one of claims 1 to 5.