Eddy current flaw detection device and eddy current flaw detection method

Abstract

To provide an eddy current flaw detection technique which does not impair reliability of a flaw detection result even when a conductive object is present at a position opposite an object under inspection.SOLUTION: An eddy current flaw detection device 10 is provided, comprising: a first coil 11 configured to induce eddy current on a surface of an object 15 under inspection by imparting an alternating magnetic field and detect a reaction magnetic field generated by the eddy current; and a ferromagnetic body 17 provided between the first coil 11 and a conductive body 16 located opposite the object 15 under inspection across the first coil 11.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] Embodiments of the present invention relate to eddy current flaw detection technology for non-destructive inspection of a test object.

Background Art

[0002] Eddy current flaw detection targeting a conductive material as a test object supplies an alternating current to a coil to induce an eddy current near the surface of the test object, and detects the reaction magnetic field generated by this eddy current with the coil. When there is a defect near the surface of the test object, the flow of the eddy current changes due to this defect. By utilizing the change in the intensity and distribution of the reaction magnetic field formed by the eddy current accordingly, the presence or absence of a defect is detected.

[0003] In eddy current flaw detection, when a conductor exists near the test object, this conductor may have an adverse effect such as noise on the detection signal, reducing the reliability of the flaw detection result. To address such a problem, a device configuration has been shown that devises the coil arrangement to suppress the influence of the presence of the conductor.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, the improvement effect can be obtained by such a countermeasure only in cases where the shapes and positions of the test object, the conductor, and the coil are in a specific relationship. As an example where it is difficult to obtain an improvement effect by such a countermeasure, there is a case where a coil is inserted into the gap with an opposing conductor to inspect near the surface of the test object. In this case, due to a change in the shape or a defect of another conductor existing near the test object, an adverse effect such as noise on the detection signal cannot be avoided, and a decrease in the reliability of the flaw detection result cannot be avoided.

[0006] Embodiments of the present invention have been made in consideration of such circumstances, and an object thereof is to provide an eddy current flaw detection technique that does not impair the reliability of flaw detection results even when a conductor exists at a position facing the object to be inspected. [Means for Solving the Problems]

[0007] In an eddy current flaw detection apparatus according to an embodiment, a first coil that applies an alternating magnetic field from a detection surface to induce an eddy current on the surface of an object to be inspected and detects a reaction magnetic field generated by the eddy current, and a ferromagnetic material provided between a conductor facing the object to be inspected with the first coil interposed therebetween and the opposite surface of the first coil located on the side opposite to the detection surface. A second coil disposed between the ferromagnetic material and the conductor, inducing an eddy current on the surface of the conductor, and detecting a reaction magnetic field generated by this eddy current; It is provided with. [Effects of the Invention]

[0008] According to the embodiments of the present invention, an eddy current flaw detection technique that does not impair the reliability of flaw detection results is provided even when a conductor exists at a position facing the object to be inspected. [Brief Description of the Drawings]

[0009]

Figure 1

Figure 2

Figure 3

Figure 4

[0010] (First Embodiment) Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. FIG. 1 is a cross-sectional view of an eddy current flaw detector 10A (10) according to a first embodiment of the present invention. The eddy current flaw detector 10A includes a first coil 11 that applies an alternating magnetic field from a detection surface 21 to induce an eddy current on the surface of a test object 15 and detects a reaction magnetic field generated by this eddy current, a conductor 16 facing the test object 15 with the first coil 11 interposed therebetween, and a ferromagnetic material 17 provided between the first coil 11 and the opposite surface (first opposite surface) 25 of the first coil 11 located on the side opposite to the detection surface 21.

[0011] The eddy current flaw detector 10 configured as described above is inserted into the gap formed by the test object 15 and the conductor 16 and scanned. As a result, the alternating magnetic field supplied from the detection surface 21 of the first coil 11 is applied to the test object 15 to induce an eddy current. When the eddy current flaw detector 10 passes through the surface where the defect 18a exists, the eddy current changes and is detected by the first coil 11 as a change in the reaction magnetic field. The position of the eddy current flaw detector 10 where the change in the reaction magnetic field is detected is recognized as the position of the defect 18a existing in the test object 15.

[0012] An alternating magnetic field is also supplied from the opposite surface 25 of the first coil 11, but this alternating magnetic field is shielded by the ferromagnetic material 17, and the application to the conductor 16 is limited. As a result, an eddy current is induced in the ferromagnetic material 17, but hardly induced in the conductor 16. Thus, even when the eddy current flaw detector 10 passes through the surface of the conductor 16 where the defect 18b exists, the eddy current induced in the ferromagnetic material 17 does not change. Therefore, the change in the reaction magnetic field on the opposite surface 25 of the first coil 11 is not detected, and the defect 18b existing in the conductor 16 is not misrecognized as existing in the test object 15.

[0013] The first coil 11 shown in each drawing discloses a self-induction type in which an induction coil for inducing an eddy current and a detection coil for detecting a reaction magnetic field generated by this eddy current are commonly configured. However, although the description is omitted, it is also possible to adopt a mutual induction type in which an induction coil having a function of inducing an eddy current and a detection coil having a function of detecting a reaction magnetic field are separately configured.

[0014] The function of the induction coil of the first coil 11 is to generate an alternating magnetic field in the vertical direction from both the inspection surface 21 and the opposite surface 25 thereof when an alternating current is supplied. When this alternating magnetic field is applied to the inspection object 15 which is also a conductor and the ferromagnetic body 17, alternating eddy currents are induced near the surfaces of both to cancel out this alternating magnetic field. The eddy currents induced in this way have a certain distribution with respect to the first coil 11 as long as the inspection object 15 and the ferromagnetic body 17 are homogeneous as materials.

[0015] The function of the detection coil of the first coil 11 is to interlink with the alternating reaction magnetic field created by the alternating eddy current and induce an electromotive force in this first coil 11. Then, the distribution of the eddy current that changes due to the defect 18a existing on the surface of the inspection object 15 and the material inhomogeneity is detected as a change in the impedance of the first coil 11.

[0016] In addition to the case where the ferromagnetic body 17 is scanned together with the first coil 11, the ferromagnetic body 17 may be fixed from the side of the inspection object 15 or the conductor 16. In either case, since the eddy current induced in the ferromagnetic body 17 always has a certain distribution with respect to the first coil 11, no impedance change occurs even when the first coil 11 is scanned. Also, since the alternating magnetic field generated from the opposite surface 25 of the first coil 11 is shielded by the ferromagnetic body 17 and the supply to the conductor 16 is suppressed, no eddy current is induced in this conductor 16. For this reason, even when the first coil 11 is at the position of the defect 18b of the conductor 16, the impedance does not change, and the defect 18b of the conductor 16 is not misdetected as a defect of the inspection object 15.

[0017] The thickness t1 of the ferromagnetic body 17 is formed thicker than the penetration depth d based on the skin effect of the eddy current induced by the first coil 11 (d < t1). Here, the skin effect is a phenomenon in which when an alternating current flows through a conductor, the current density is high at the surface of the conductor and low as it moves away from the surface. Here, the penetration depth d defined such that the current value of the eddy current becomes 1 / e (about 0.37) of the surface current is expressed by the following formula (1).

[0018] d = √(2ρ / (ωμ)) ··· (1) Here, ρ: resistivity of the ferromagnetic material 17, ω: angular frequency of the alternating current supplied to the coil = 2π × frequency, μ: absolute permeability of the ferromagnetic material 17

[0019] By forming the thickness t1 of the ferromagnetic material 17 to be thicker than the penetration depth d in this way, the alternating magnetic field applied from the opposite surface 25 of the first coil 11 will be confined within the ferromagnetic material 17. As a result, this alternating magnetic field is effectively shielded by the ferromagnetic material 17, and the induction of eddy currents on the surface of the conductor 16 is suppressed.

[0020] Furthermore, the width W of the ferromagnetic material 17 is formed to be larger than the outer diameter D of the first coil 11. Thereby, the alternating magnetic field from the opposite surface 25 of the first coil 11 can be effectively shielded, and the application to the conductor 16 can be further suppressed.

[0021] (Second Embodiment) Next, the second embodiment of the present invention will be described with reference to FIG. 2. FIG. 2 is a cross-sectional view of the eddy current flaw detector 10B(10) according to the second embodiment. In FIG. 2, parts having the same configuration or function as those in FIG. 1 are denoted by the same reference numerals, and redundant explanations are omitted.

[0022] The eddy current flaw detector 10B is configured such that the first coil 11 and the ferromagnetic material 17 are integrated. In the eddy current flaw detector 10B having such a configuration, even if the gap between the test object 15 and the conductor 16 is a narrower constricted portion, it can be inserted. Also, when scanning the surface of the test object 15 with the eddy current flaw detector 10B, the ferromagnetic material 17 can be moved together with the first coil 11. Thereby, using a more compact ferromagnetic material 17, the influence of the defect 18b of the conductor 16 on the side opposite to the test object 15, which is the purpose of flaw detection, can be suppressed.

[0023] (Third Embodiment) Next, a third embodiment of the present invention will be described with reference to FIG. 3. FIG. 3 is a cross-sectional view of an eddy current flaw detector 10C(10) according to the third embodiment. In FIG. 3, parts having the same configuration or function as those in FIGS. 1 and 2 are denoted by the same reference numerals, and redundant explanations are omitted.

[0024] The eddy current flaw detector 10C includes a first coil 11 and a ferromagnetic body 17 in the same manner as in the first embodiment, and further includes a second coil 12. This second coil 12 is disposed between the ferromagnetic body 17 and the conductor 16, induces eddy currents on the surface of the conductor 16, and detects the reaction magnetic field created by these eddy currents.

[0025] With the eddy current flaw detector 10C configured in this way, the defect 18a of the test object 15 is detected by the first coil 11, and the defect 18b of the conductor 16 is detected by the second coil 12. As a result, in a single scan, not only can the defect 18a of the test object 15 be detected, but also the defect 18b existing in the conductor 16 can be simultaneously separated and detected.

[0026] Furthermore, in the third embodiment, the thickness t2 of the ferromagnetic body 17 is formed thicker than the deeper one of the first penetration depth d1 based on the skin effect of the eddy currents induced by the first coil 11 and the second penetration depth d2 based on the skin effect of the eddy currents induced by the second coil 12 (d1 < d2 < t2 or d2 < d1 < t2). As understood from the above formula (1), if the frequencies of the alternating currents supplied to the first coil 11 and the second coil 12 are the same, the thickness t2 of the ferromagnetic body 17 in the third embodiment is desirably the same as or greater than the thickness t1 of the ferromagnetic body 17 in the first embodiment.

[0027] By forming the thickness t2 of the ferromagnetic body 17 in the third embodiment thicker than the deeper one of the penetration depth d1 and the penetration depth d2 in this way, the alternating magnetic field applied from the opposite surface (first opposite surface) 25 of the first coil 11 and the alternating magnetic field applied from the opposite surface (second opposite surface) 26 of the second coil 12 are both confined within the ferromagnetic body 17.

[0028] As a result, the alternating magnetic fields supplied from the opposite surfaces 25 and 26 of each of the first coil 11 and the second coil 12 are effectively shielded by the ferromagnetic body 17. Then, the alternating magnetic fields supplied from the detection surfaces 21 and 22 of each of the first coil 11 and the second coil 12 are supplied to the surfaces of the test object 15 and the conductor 16, respectively, and the defects 18a and 18b can be detected separately.

[0029] (Fourth Embodiment) Next, a fourth embodiment of the present invention will be described with reference to FIG. 4. FIG. 4 is a cross-sectional view of an eddy current flaw detector 10D(10) according to the fourth embodiment. In FIG. 4, parts having the same configuration or function as those in FIGS. 1, 2, and 3 are denoted by the same reference numerals, and redundant descriptions are omitted.

[0030] The eddy current flaw detector 10D is configured by integrating the first coil 11, the ferromagnetic body 17, and the second coil 12. In the eddy current flaw detector 10D having such a configuration, even if the gap between the test object 15 and the conductor 16 is a narrower constricted portion, it can be inserted. Further, when the eddy current flaw detector 10D is scanned on the surface of the test object 15, the first coil 11, the second coil 12, and the ferromagnetic body 17 can all be moved. As a result, with the compact eddy current flaw detector 10D, the defects 18a and 18b existing in the opposing test object 15 and conductor 16 can be accurately separated and detected.

[0031] According to the eddy current flaw detector of at least one of the embodiments described above, by providing a ferromagnetic body on the side opposite to the detection surface of the coil, even if a conductor exists at a position opposing the test object, the reliability of the flaw detection result is not impaired.

[0032] Although some embodiments of the present invention have been described, these embodiments are presented by way of example and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, replacements, changes, and combinations can be made without departing from the gist of the invention. These embodiments and their modifications are included in the scope and gist of the invention, as well as in the invention described in the claims and the equivalent scope thereof.

Explanation of Reference Numerals

[0033] 10(10A, 10B, 10C, 10D)…Eddy current flaw detector, 11…First coil, 12…Second coil, 15…Specimen to be inspected, 16…Conductor, 17…Ferromagnetic material, 18a, 18b…Defects, 21…Inspection surface, 25…Opposite surface of the first coil (first opposite surface), 26…Opposite surface of the second coil (second opposite surface).

Claims

1. A first coil that applies an alternating magnetic field from a detection surface to induce eddy currents on the surface of the test object and detects the reaction magnetic field generated by these eddy currents; A ferromagnetic material provided between a conductor facing the test object with the first coil interposed therebetween and the opposite surface of the first coil located on the opposite side of the detection surface; An eddy current flaw detector comprising a second coil disposed between the ferromagnetic material and the conductor, inducing eddy currents on the surface of the conductor, and detecting the reaction magnetic field generated by these eddy currents.

2. The eddy current flaw detector according to claim 1, wherein the first coil and the ferromagnetic material are integrally formed.

3. The eddy current flaw detector according to claim 1, wherein the first coil, the ferromagnetic material, and the second coil are integrally formed.

4. The eddy current flaw detector according to claim 1 or claim 2, wherein the thickness of the ferromagnetic material is greater than the penetration depth based on the skin effect of the eddy currents induced by the first coil.

5. The eddy current flaw detector according to claim 1 or claim 3, wherein the thickness of the ferromagnetic material is greater than the greater of the first penetration depth based on the skin effect of the eddy currents induced by the first coil and the second penetration depth based on the skin effect of the eddy currents induced by the second coil.

6. The eddy current flaw detector according to any one of claims 1 to 5, wherein the width of the ferromagnetic material is greater than the outer diameter of the first coil.

7. The eddy current flaw detector according to any one of claims 1 to 6, The eddy current flaw detector is such that the first coil is of a self-induction type in which an induction coil for inducing the eddy current and a detection coil for detecting the reaction magnetic field are commonly configured, or of a mutual-induction type in which the induction coil and the detection coil are separately configured.

8. A step of applying an alternating magnetic field from the detection surface of the first coil to induce an eddy current on the surface of the object to be inspected, and detecting, with the first coil, the reaction magnetic field generated by this eddy current; A step of providing a ferromagnetic material between a conductor facing the object to be inspected with the coil interposed therebetween and the opposite surface of the coil located on the opposite side of the detection surface, to suppress the induction of an eddy current on the surface of the conductor; A step of applying an alternating magnetic field from the detection surface of a second coil disposed between the ferromagnetic material and the conductor to induce an eddy current on the surface of the conductor, and detecting, with the second coil, the reaction magnetic field generated by this eddy current. An eddy current flaw detection method including these steps.

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