JP7693597B2 - Eddy current inspection device and eddy current inspection method - Google Patents

Description

An embodiment of the present invention relates to eddy current testing technology for performing non-destructive testing of an object to be inspected.

Eddy current testing, which tests conductive materials, involves supplying an alternating current to a coil to induce eddy currents near the surface of the test piece, and then using the coil to detect the reactive magnetic field created by these eddy currents. If a defect is present near the surface of the test piece, this defect will cause a change in the flow of eddy currents. The presence or absence of a defect can be detected by utilizing the resulting changes in the strength and distribution of the reactive magnetic field formed by the eddy currents.

In eddy current testing, if a conductor is present near the object being inspected, this conductor may have a detrimental effect on the detection signal, such as noise, reducing the reliability of the inspection results. To address this issue, an equipment configuration has been presented that uses innovative coil arrangements to suppress the effects of the presence of conductors.

Patent No. 3981965

However, such measures can only produce improvement effects in cases where the shapes and positions of the object to be inspected, the conductor, and the coil are in a specific relationship. An example of a case where it is difficult to produce improvement effects with such measures is when a coil is inserted into the gap between the opposing conductor to inspect the vicinity of the surface of the object to be inspected. In this case, adverse effects such as noise on the detection signal cannot be avoided due to changes in shape or defects in another conductor located near the object to be inspected, and a decrease in the reliability of the inspection results is unavoidable.

The embodiment of the present invention has been made in consideration of these circumstances, and aims to provide an eddy current flaw detection technology that does not impair the reliability of the flaw detection results, even when a conductor is present at an opposing position to the object being inspected.

In the eddy current flaw detection device of the embodiment, there is provided a first coil that applies an alternating magnetic field from the detection surface, induces eddy currents on the surface of the object being inspected, and detects the reaction magnetic field created by this eddy current, a ferromagnetic body that is provided between a conductor facing the object being inspected across the first coil and the opposite surface of the first coil that is located on the opposite side of the detection surface, and a second coil that is disposed between the ferromagnetic body and the conductor, induces eddy currents on the surface of the conductor, and detects the reaction magnetic field created by the eddy currents .

Embodiments of the present invention provide eddy current inspection technology that does not compromise the reliability of inspection results, even when a conductor is present at an opposing position to the object being inspected.

1 is a cross-sectional view of an eddy current flaw detector according to a first embodiment of the present invention. FIG. 5 is a cross-sectional view of an eddy current flaw detector according to a second embodiment. FIG. 11 is a cross-sectional view of an eddy current flaw detector according to a third embodiment. FIG. 13 is a cross-sectional view of an eddy current flaw detector according to a fourth embodiment.

First Embodiment
Hereinafter, an embodiment 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 AC magnetic field from a detection surface 21 to induce eddy currents on the surface of an object to be inspected 15 and detects a reaction magnetic field generated by the eddy currents, and a ferromagnetic body 17 that is provided between a conductor 16 that faces the object to be inspected 15 across the first coil 11 and an opposite surface (first opposite surface) 25 of the first coil 11 that is located on the opposite side of the detection surface 21.

The eddy current inspection device 10 configured in this manner is inserted into the gap between the object to be inspected 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 object to be inspected 15, inducing eddy currents. When the eddy current inspection device 10 passes over the surface on which 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 inspection device 10 that detects this change in the reaction magnetic field is recognized as the position of the defect 18a existing in the object to be inspected 15.

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 its application to the conductor 16 is limited. As a result, eddy currents are induced in the ferromagnetic material 17, but are hardly induced in the conductor 16. As a result, even if the eddy current flaw detector 10 passes over the surface of the conductor 16 in which the defect 18b exists, the eddy current induced in the ferromagnetic material 17 does not change. Therefore, no change in the reaction magnetic field on the opposite surface 25 of the first coil 11 is detected, and the defect 18b existing in the conductor 16 is not mistakenly recognized as existing in the inspected object 15.

The first coil 11 shown in each drawing is a self-induction type in which an induction coil that induces eddy currents and a detection coil that detects the reaction magnetic field created by these eddy currents are commonly configured. However, although not described, it can also be a mutual induction type in which an induction coil with the function of inducing eddy currents and a detection coil with the function of detecting the reaction magnetic field are separately configured.

The function of the induction coil of the first coil 11 is to generate an AC magnetic field in the vertical direction from both the detection surface 21 and the opposite surface 25 when an AC current is supplied. When this AC magnetic field is applied to the test object 15 and the ferromagnetic body 17, which are also conductors, it induces AC eddy currents near the surfaces of both, so as to cancel out the AC magnetic field. The eddy currents induced in this way have a certain distribution in the first coil 11, so long as the test object 15 and the ferromagnetic body 17 are homogeneous materials.

The function of the detection coil of the first coil 11 is to link the AC reaction magnetic field created by the AC eddy current and induce an electromotive force in the first coil 11. The distribution of eddy currents, which changes due to defects 18a present on the surface of the inspected object 15 and unevenness in the material, is detected as a change in the impedance of the first coil 11.

In addition to the case where the ferromagnetic material 17 is scanned together with the first coil 11, it may also be fixed from the side of the inspected object 15 or the conductor 16. In either case, the eddy current induced in the ferromagnetic material 17 always has a constant distribution with respect to the first coil 11, so that scanning the first coil 11 does not cause an impedance change. In addition, the AC magnetic field generated from the opposite surface 25 of the first coil 11 is shielded by the ferromagnetic material 17 and its supply to the conductor 16 is suppressed, so that no eddy current is induced in this conductor 16. Therefore, even if the first coil 11 is located at the position of the defect 18b in the conductor 16, the impedance does not change, and the defect 18b in the conductor 16 is not erroneously detected as a defect in the inspected object 15.

The thickness _t1 of the ferromagnetic body 17 is formed to be thicker than the skin 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 on the surface of the conductor and decreases away from the surface. Here, the skin depth d, which is defined so that the current value of the eddy current is 1/e (approximately 0.37) of the surface current, is expressed by the following formula (1).

d=√(2ρ/(ωμ))...(1)
Here, ρ is the electrical resistivity of the ferromagnetic body 17, ω is the angular frequency of the AC current supplied to the coil=2π×frequency, and μ is the absolute permeability of the ferromagnetic body 17.

By forming the thickness _t1 of the ferromagnetic body 17 to be thicker than the penetration depth d in this manner, the AC magnetic field applied from the opposite surface 25 of the first coil 11 is confined within the ferromagnetic body 17. As a result, this AC magnetic field is effectively shielded by the ferromagnetic body 17, and the induction of eddy currents on the surface of the conductor 16 is suppressed.

Furthermore, the width W of the ferromagnetic body 17 is formed to be larger than the outer diameter D of the first coil 11. This effectively blocks the AC magnetic field from the opposite surface 25 of the first coil 11, further suppressing its application to the conductor 16.

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

In the eddy current flaw detection device 10B, the first coil 11 and the ferromagnetic body 17 are integrally configured. In the eddy current flaw detection device 10B having such a configuration, the device can be inserted even into a narrower gap between the object to be inspected 15 and the conductor 16. In addition, when the eddy current flaw detection device 10B is scanned over the surface of the object to be inspected 15, the ferromagnetic body 17 can be moved together with the first coil 11. This allows the use of a more compact ferromagnetic body 17 to suppress the effect of the defect 18b in the conductor 16 on the opposite side to the object to be inspected 15, which is the target of the flaw detection.

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 Figs. 1 and 2 are indicated by the same reference numerals, and duplicated explanations will be omitted.

The eddy current flaw detector 10C includes a first coil 11 and a ferromagnetic body 17, 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, and induces eddy currents on the surface of the conductor 16, and detects the reaction magnetic field created by these eddy currents.

By configuring the eddy current flaw detector 10C in this manner, the defect 18a in the object 15 is detected by the first coil 11, and the defect 18b in the conductor 16 is detected by the second coil 12. This makes it possible to not only detect the defect 18a in the object 15, but also to simultaneously isolate and detect the defect 18b in the conductor 16 with a single scan.

Furthermore, in the third embodiment, the thickness _t2 of the ferromagnetic body 17 is formed to be thicker than either the first penetration depth _d1 based on the skin effect of the eddy current induced by the first coil 11 or the second penetration depth d2 based on the skin effect of the eddy current induced by the second coil ₁₂ ( _d1 < d2 < _t2 or _d2 < _d1 < _t2 ). As can be understood from the above formula ( ₁ ), if the frequencies of the AC currents supplied to the first coil 11 and the second coil 12 are the same, it is desirable that the thickness _t2 of the ferromagnetic body 17 in the third embodiment be the same as or greater than the thickness _t1 of the ferromagnetic body 17 in the first embodiment.

In this way, the thickness _t2 of the ferromagnetic material 17 in the third embodiment is formed to be thicker than the greater of the penetration depths _d1 and _d2 , so that the AC magnetic field applied from the opposite surface (first opposite surface) 25 of the first coil 11 and the AC magnetic field applied from the opposite surface (second opposite surface) 26 of the second coil 12 are both confined in the ferromagnetic material 17.

As a result, the AC magnetic field supplied from the opposite surfaces 25, 26 of the first coil 11 and the second coil 12 is effectively shielded by the ferromagnetic material 17. The AC magnetic field supplied from the detection surfaces 21, 22 of the first coil 11 and the second coil 12 is then supplied to the surfaces of the test object 15 and the conductor 16, respectively, allowing the defects 18a and 18b to be detected separately.

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 Figs. 1, 2, and 3 are indicated by the same reference numerals, and duplicated explanations will be omitted.

The eddy current flaw detector 10D is configured with the first coil 11, ferromagnetic body 17, and second coil 12 integrated together. The eddy current flaw detector 10D having such a configuration can be inserted even into narrower gaps between the object to be inspected 15 and the conductor 16. In addition, when the eddy current flaw detector 10D is scanned over the surface of the object to be inspected 15, the first coil 11, the second coil 12, and the ferromagnetic body 17 can be moved together. This allows the compact eddy current flaw detector 10D to accurately detect defects 18a, 18b that exist in the opposing object to be inspected 15 and conductor 16.

According to at least one embodiment of the eddy current flaw detector described above, by providing a ferromagnetic body on the opposite side of the coil's detection surface, the reliability of the flaw detection results is not compromised even if a conductor is present in the opposing position of the object being inspected.

Although several embodiments of the present invention have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, substitutions, modifications, and combinations can be made without departing from the spirit of the invention. These embodiments and their modifications are within the scope of the invention and its equivalents as set forth in the claims, as well as the scope and spirit of the invention.

10 (10A, 10B, 10C, 10D)... Eddy current flaw detector, 11... First coil, 12... Second coil, 15... Test object, 16... Electrical conductor, 17... Ferromagnetic material, 18a, 18b... Defect, 21... Detection surface, 25... Opposite surface of first coil (first opposite surface), 26... Opposite surface of second coil (second opposite surface).

Claims (8)

a first coil for applying an AC magnetic field from a detection surface to induce eddy currents on the surface of the object to be inspected, and detecting a reaction magnetic field generated by the eddy currents;
a ferromagnetic body provided between a conductor facing the device under test across the first coil and an opposite surface of the first coil located on the opposite side of the detection surface;
An eddy current flaw detection device comprising : a second coil disposed between the ferromagnetic material and the conductor, which induces an eddy current on the surface of the conductor and detects a reaction magnetic field created by the eddy current . The eddy current flaw detector according to claim 1,
The first coil and the ferromagnetic body are integrally configured as an eddy current flaw detector. The eddy current flaw detector according to claim 1 ,
The first coil, the ferromagnetic body, and the second coil are integrated into an eddy current flaw detector. The eddy current flaw detector according to claim 1 or 2,
An eddy current flaw detector, wherein the thickness of the ferromagnetic body is greater than the penetration depth based on the skin effect of the eddy current induced by the first coil. The eddy current flaw detector according to claim 1 or 3 ,
An eddy current flaw detection device in which the thickness of the ferromagnetic material is thicker than either a first penetration depth based on the skin effect of the eddy current induced by the first coil or a second penetration depth based on the skin effect of the eddy current induced by the second coil, whichever is greater. The eddy current flaw detector according to any one of claims 1 to 5 ,
An eddy current flaw detector, wherein the width of the ferromagnetic body is greater than the outer diameter of the first coil. The eddy current flaw detector according to any one of claims 1 to 6 ,
The first coil is an eddy current flaw detection device of a self-induction type in which an induction coil that induces the eddy current and a detection coil that detects 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. applying an alternating magnetic field from a detection surface of a first coil to induce eddy currents on the surface of the object to be inspected, and detecting a reaction magnetic field generated by the eddy currents with the first coil;
a step of providing a ferromagnetic body between a conductor facing the test object across the coil and an opposite surface of the coil located on the opposite side of the detection surface, thereby suppressing induction of an eddy current on a surface of the conductor;
applying an alternating magnetic field from a detection surface of a second coil arranged between the ferromagnetic material and the conductor to induce eddy currents on the surface of the conductor, and detecting a reaction magnetic field created by the eddy currents with the second coil .

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