Eddy current testing method (or eddy current testing device)

Abstract

To provide an eddy current flaw detection method capable of detecting defects in a material to be inspected while identifying differences in shape depending on the type of defect without using a special device, and such an eddy current flaw detection device.SOLUTION: An eddy current flaw detection device performs a first inspection step in which eddy current flaw detection is performed with a transmission and reception direction directed in a first direction, in which the transmission and reception direction is a direction that connects a transmission coil that applies a magnetic field to the surface of an inspected material to generate eddy currents, and a reception coil that detects the magnetic field generated by the eddy currents; a second inspection step in which eddy current flaw detection is performed with the transmission and reception direction directed in a second direction that intersects the first direction; and a determination step in which at a position where a defect is detected in at least one of the first inspection step and the second inspection step on the surface of the inspected material, a difference between signal strength detected in the first inspection step and signal strength detected in the second inspection step is calculated, and it is determined that the larger the difference, the higher the anisotropy of the detected defect shape.SELECTED DRAWING: Figure 4

Description

Technical Field

[0001] The present invention relates to an eddy current flaw detection method and an eddy current flaw detection apparatus, and more particularly to an eddy current flaw detection method and an eddy current flaw detection apparatus capable of detecting defects on the surface of a test material and making a determination related to the shape of the defects.

Background Art

[0002] Eddy current flaw detection is used as a non-destructive method for detecting defects such as scratches generated on the surface or near the surface of a metal material. In an eddy current flaw detection apparatus, an alternating magnetic field is generated by a coil disposed near the surface of the test material, and defects on the surface of the test material can be detected by detecting disturbances in the eddy current generated on the surface of the test material. Thus, eddy current flaw detection can sensitively detect various defects by utilizing magnetic changes associated with disturbances in the eddy current on the surface of the test material. On the other hand, for defects that cause magnetic changes on the surface of the test material, it may be difficult to distinguish the types of these defects even if different types of defects are mixed.

[0003] For example, when the test material is made of a ferromagnetic material, if a non-uniform magnetic permeability distribution occurs on the surface of the test material due to material modifications such as oxidation and carbonization, the magnetic change due to the magnetic permeability distribution can be detected as a defect. However, in many cases of performing eddy current flaw detection, it is desired to selectively detect physical scratches, and a non-uniform magnetic permeability distribution on the material surface is not desired to be detected as a defect. If these non-uniform magnetic permeability distributions are detected as defects without being distinguished from scratches, it will result in false detection in the detection of scratches.

[0004] In eddy current testing of ferromagnetic materials, a method has been proposed to reduce false detections caused by magnetic changes due to factors other than defects by performing eddy current testing after magnetically saturating the material under inspection. For example, in the eddy current testing apparatus disclosed in Patent Document 1, a magnetic field forming magnet is placed outside the detection unit for detecting changes in eddy currents, and the magnetic flux density of the magnetic field generated by the magnetic field forming magnet is set to a predetermined range. In addition, a method has been devised in which the material under inspection, which is made of a ferromagnetic material, is heated to a temperature above the Curie point to demagnetize it before performing eddy current testing. Furthermore, a method has been proposed to prevent false detections in eddy current testing of ferromagnetic materials without changing the magnetic state of the material under inspection by magnetization or demagnetization. For example, Patent Document 2 discloses an eddy current testing apparatus equipped with a cross coil and a pancake coil, which determines the presence of defects in the material under inspection by comparing the intensity and phase angle of the response signals of both against a threshold. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2019-060723 [Patent Document 2] Japanese Patent Publication No. 2015-225068 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] When performing eddy current testing, special equipment configurations are often required to distinguish between detected defects and to selectively detect specific types of defects, as shown in Patent Documents 1 and 2. It would be desirable to be able to identify defect types with a simple equipment configuration using a general-purpose eddy current probe, without the need for such special equipment. Among various defects, some have different shapes depending on their nature and origin, and it may be possible to identify defect types by focusing on differences in shape.

[0007] The problem that the present invention aims to solve is to provide an eddy current testing method and an eddy current testing apparatus that can detect defects in a material under inspection while identifying differences in shape depending on the type of defect, without using special equipment. [Means for solving the problem]

[0008] To solve the above problems, the eddy current testing method and eddy current testing apparatus according to the present invention have the following configuration. [1] The present invention provides an eddy current testing method for detecting surface defects of a material to be inspected using an eddy current testing probe equipped with a plurality of coils, comprising: a first inspection step in which eddy current testing is performed with the transmission and reception direction set to a first direction, with the transmission and reception direction set to the direction connecting a transmitting coil that applies a magnetic field to the surface of the material to be inspected to generate eddy currents and a receiving coil that detects the magnetic field generated by the eddy currents; a second inspection step in which eddy current testing is performed with the transmission and reception direction set to a second direction intersecting the first direction; and a determination step in which, at least one of the first and second inspection steps on the surface of the material to be inspected, the difference between the signal intensity detected in the first inspection step and the signal intensity detected in the second inspection step is calculated at the location where a defect is detected, and the larger the difference, the higher the anisotropy of the shape of the detected defect is determined to be.

[0009] [2] In the embodiment of [1] above, the first inspection step and the second inspection step may be carried out without magnetizing or demagnetizing the material to be inspected.

[0010] [3] In the embodiment of [1] or [2] above, if there are candidates for defects that may be formed on the surface of the material to be inspected, namely a first type defect with high anisotropy of shape and a second type defect with lower anisotropy of shape than the first type defect, in the determination step, if the difference is greater than or equal to a threshold, it is determined that the detected defect is a first type defect, and if the difference is less than the threshold, it is determined that the detected defect is a second type defect. [4] In the embodiment of [3] above, the first type of defect may be a scratch, and the second type of defect may be a non-uniform distribution of magnetic permeability.

[0011] [5] In any one embodiment of [1] to [4] above, the eddy current testing probe has a coil array in which a plurality of coil rows, each having a plurality of coils arranged in a row, are arranged in a coil array, and in the first inspection step, eddy current testing is performed on two coils included in different coil rows in the coil array as the transmitting coil and the receiving coil, and in the second inspection step, eddy current testing is performed on two coils included in the same coil row in the coil array as the transmitting coil and the receiving coil.

[0012] [6] In any one embodiment of [1] to [5] above, eddy current testing may be performed at different frequencies in the first inspection step and the second inspection step.

[0013] [7] The eddy current testing apparatus according to the present invention comprises the eddy current testing probe and a calculation unit that performs the determination step, and performs any one of the eddy current testing methods described in [1] to [6] above. [Effects of the Invention]

[0014] [1] In the eddy current testing method according to the above invention, the eddy current testing process includes a first inspection step in which the transmission and reception direction connecting the transmitting coil and the receiving coil is directed in a first direction, and a second inspection step in which the transmission and reception direction is directed in a second direction. In eddy current testing, the larger the area occupied by the defect formed on the surface of the material to be inspected in the transmission and reception direction, the larger the amount of change in the magnetic field detected by the receiving coil tends to be. Therefore, if the defect has a large anisotropy in shape, a large difference in signal intensity will occur between the first inspection step and the second inspection step, which have different transmission and reception directions. On the other hand, if the defect has a shape with low anisotropy, a large difference will not occur between the signal intensity obtained in the two inspection steps. Therefore, the larger the difference in signal intensity detected in the two inspection steps, the higher the anisotropy of the detected defect's shape can be determined, and defects that give shapes with different degrees of anisotropy can be detected separately. The first and second inspection processes, which involve different transmission and reception directions, can be easily implemented by using an eddy current testing probe with three or more coils or by changing the arrangement of the eddy current testing probe. They can be easily performed without the need for special equipment, simply by performing predetermined calculations on the inspection results obtained using a general-purpose eddy current testing probe.

[0015] [2] When the first and second inspection steps are performed without magnetizing or demagnetizing the material under inspection, the equipment and steps required for magnetizing or demagnetizing the material under inspection can be omitted, and eddy current testing can be easily performed. In the eddy current testing method according to the present invention, the degree of anisotropy of the defect shape can be determined by taking the difference between the detection signals of the first and second inspection steps. Therefore, even if at least a part of the material under inspection is composed of a ferromagnetic material, if a non-uniform distribution of magnetic permeability occurs on the surface of the ferromagnetic material due to material modification, etc., and such a distribution of magnetic permeability and other types of surface defects have shapes with different degrees of anisotropy, the two can be easily distinguished and detected by eddy current testing without causing a change in magnetic state due to magnetization or demagnetization.

[0016] [3] When there are two types of defects that can form on the surface of the material being inspected, a first-class defect with high anisotropy in shape and a second-class defect with lower anisotropy in shape than the first-class defect, the determination process can be performed such that if the difference is greater than or equal to a threshold, the detected defect is determined to be a first-class defect, and if the difference is less than the threshold, the detected defect is determined to be a second-class defect. This allows for easy distinction and detection of the two types of defects with different anisotropies in shape. Furthermore, by recognizing only the type of defect to be detected as a defect and not the other type, false detections can be reduced while selectively detecting the type of defect to be detected.

[0017] [4] In this case, if the first type of defect is a scratch and the second type of defect is a non-uniform distribution of magnetic permeability, scratches often take the form of a linear, highly anisotropic shape on the metal surface, while areas where magnetic permeability is non-uniformly distributed due to surface modification such as oxidation or charring are often formed as regions with low anisotropy. Therefore, the eddy current testing method according to the present invention can easily and accurately distinguish between the two. When non-destructive testing of metal materials by eddy current testing, scratches are often to be detected as defects, while surface modification such as oxidation or charring is often not to be recognized as defects. In such cases, scratches, which are first type defects where the difference in detection signals between the first and second inspection steps is large, can be distinguished from the non-uniform distribution of magnetic permeability, which is a second type defect, and recognized as defects, thereby selectively detecting scratches while reducing false detections.

[0018] [5] When an eddy current testing probe has a coil array, which is a series of coil arrays arranged in rows, and in the first inspection step, two coils in different coil arrays are used as transmitting and receiving coils, and in the second inspection step, two coils in the same coil array are used as transmitting and receiving coils, the first and second inspection steps, which have different transmitting and receiving directions, can be easily switched between each other by electrically selecting which coils in the coil array will be used as transmitting and receiving coils. Furthermore, such inspections can be easily performed over a wide area of ​​the surface of the material being inspected. As a result, eddy current testing that distinguishes defects with different degrees of anisotropy can be easily and accurately performed. Eddy current testing probes equipped with coil arrays are commercially available and generally available.

[0019] [6] When eddy current testing is performed at different frequencies in the first and second inspection processes, even defects with an anisotropic shape extending in an intermediate direction between the first and second directions can be detected with high sensitivity.

[0020] [7] In the eddy current testing apparatus according to the above invention, a first inspection step and a second inspection step are performed using an eddy current testing probe with mutually different transmission and reception directions. The calculation unit then calculates the difference in signal intensity obtained in the two inspection steps, and determines that the larger the difference, the higher the anisotropy of the detected defect shape. By performing eddy current testing with different transmission and reception directions and then performing calculations on the inspection results, it is possible to distinguish and detect defects having shapes with different degrees of anisotropy. Since there is no need to use a special eddy current testing probe to distinguish between types of defects, nor is there a need to provide special components other than the eddy current testing probe and the calculation unit, the configuration of the eddy current testing apparatus and the process of eddy current testing using the apparatus can be simplified. [Brief explanation of the drawing]

[0021] [Figure 1] It is a schematic diagram for explaining eddy current flaw detection using the eddy current flaw detection device according to an embodiment of the present invention. [Figure 2] It is a diagram for explaining two measurement modes in the eddy current flaw detection method according to an embodiment of the present invention. (a) shows the axial mode, and (b) shows the transverse mode. [Figure 3] It is a diagram for explaining the spatial distribution of the detection signal obtained in the above eddy current flaw detection method. (a) shows the detection results in the axial mode, (b) shows the detection results in the transverse mode, and (c) shows the difference between them. [Figure 4] It is a C-scope image showing the measurement results obtained in the example. (a) shows the inspection results in the axial mode, (b) shows the inspection results in the transverse mode, and (c) shows the difference between them.

Mode for Carrying Out the Invention

[0022] Hereinafter, the eddy current flaw detection method and the eddy current flaw detection device according to the embodiment of the present invention will be described.

[0023] [Eddy Current Flaw Detection Device] First, the eddy current flaw detection device according to an embodiment of the present invention will be briefly described. FIG. 1 shows a schematic configuration of the eddy current flaw detection device 1 according to the present embodiment. The eddy current flaw detection device 1 includes an eddy current flaw detection probe 10 and an arithmetic unit 20.

[0024] As the eddy current testing probe 10, a known eddy current testing probe capable of detecting surface defects by eddy current testing can be applied to a material to be inspected M made of a conductive material such as a metal material. The eddy current testing probe 10 is equipped with a plurality of coils 11. With the eddy current testing probe 10 positioned above the surface of the material to be inspected M, an alternating current is passed through one of the coils 11, which acts as the transmitting coil, thereby applying an alternating magnetic field to the surface of the material to be inspected M and generating eddy currents on the surface of the material to be inspected M. The magnetic field generated by the eddy currents on the surface of the material to be inspected M is then detected by one of the coils 11, which acts as the receiving coil. If there are defects such as scratches on the surface of the material to be inspected M, the eddy currents are disturbed by the defects, causing a change in the magnetic field detected by the receiving coil compared to the surrounding area without defects. By detecting this change in the magnetic field due to the disturbance of the eddy currents, defects formed on the surface of the material to be inspected M can be detected. Note that the coil 11 is positioned on the eddy current testing probe 10 opposite the surface of the material M being inspected. However, in Figure 1, the coil 11 is shown extended to the opposite side of the surface of the material M being inspected, in order to make its position easier to understand.

[0025] The type of eddy current testing probe 10 is not particularly limited, but as shown in the figure, one equipped with a coil array can be preferably used. In the coil array, multiple rows (two in this case) of coil rows A1 and A2, each consisting of multiple coils 11 arranged in a row, are arranged side by side. In adjacent coil rows A1 and A2, the coils 11 are arranged alternately, and in one coil row A1, the coil 11 constituting the adjacent coil row A2 is positioned midway between two adjacent coils 11. In the eddy current testing probe 10 having a coil array, the state of each coil 11 can be switched by a control circuit (not shown) equipped with a multiplexer, and the coil 11 that functions as a transmitting coil and the coil 11 that functions as a receiving coil can be electrically switched.

[0026] Typical measurement modes obtained by switching coil 11 include the axial mode and the transverse mode. The state of coil 11 in each measurement mode is shown in Figures 2(a) and (b). In the figures, the position where coil 11 is positioned above the surface of the material under inspection M is shown by a circle, and the eddy current EC generated on the surface of the material under inspection M is shown by a dashed line. In addition, the transmitting coil 11 is indicated by the symbol "T", the receiving coil 11 is indicated by the symbol "R", and the transmission and reception direction, which is the direction connecting the transmitting and receiving coils, is indicated by an arrow.

[0027] In axial mode, as shown in Figure 2(a), coils 11 included in different coil rows A1 and A2 are used as the transmitting coil and the receiving coil. Specifically, measurements are performed by rapidly switching between two states: a first state in which the transmitting coil T is set in the first coil row A1, and the coil 11 adjacent to the transmitting coil T among the coils 11 constituting the adjacent second coil row A2 is set as the receiving coil R; and a second state in which the coil that was the receiving coil R in the first state is set as the transmitting coil T2, and the coil adjacent to the coil that was the transmitting coil T in the first coil row A1 is set as the receiving coil R2. In this case, the transmitting and receiving direction is close to the direction in which the multiple coil rows A1 and A2 are arranged (x direction) intersecting the direction in which each coil row A1 and A2 extends (y direction) (first direction).

[0028] On the other hand, in the lateral mode, as shown in Figure 2(b), the coils 11 included in the same coil row A1 or A2 are used as the transmitting coil and the receiving coil. Specifically, measurements are performed by rapidly switching between a first state in which the transmitting coil T and the receiving coil R are set adjacent to each other in the first coil row A1, and a second state in which the transmitting coil T2 and the receiving coil R2 are set adjacent to each other in the second coil row A2. In this case, the transmitting and receiving direction is along the direction in which each coil row A1, A2 extends (y-direction) (second direction).

[0029] Thus, by using the eddy current testing probe 10 equipped with a coil array, measurements can be performed with the transmission and reception directions intersecting in a first direction and measurements with the transmission and reception directions in a second direction, and these can be switched solely by electrical control at the same measurement position. Furthermore, since the numerous coils 11 form coil rows A1 and A2, measurements can be performed simultaneously over a wide area along coil rows A1 and A2. In addition, by moving the eddy current testing probe 10 on the surface of the material to be inspected M (motion S), two-dimensional flaw detection can be performed over a wide area of ​​the surface of the material to be inspected M.

[0030] Furthermore, the eddy current testing probe 10 does not have to be a coil array type, as long as it is capable of performing eddy current testing measurements with the transmission and reception directions oriented in a first and second direction that intersect with each other. If it has three or more coils 11, measurements can be performed with the transmission and reception directions oriented in the first and second directions by switching between the transmitting and receiving coils by electrical control. Moreover, even with an eddy current testing probe that has only two coils 11, measurements can be performed with the transmission and reception directions oriented in the first and second directions by changing the direction in which the eddy current testing probe is positioned on the surface of the material M under inspection.

[0031] In the eddy current flaw detection apparatus 1 according to this embodiment, the calculation unit 20 receives a detection signal obtained from the eddy current flaw detection probe 10, that is, a signal indicating the magnitude of the magnetic field detected by the receiving coil, and performs calculations based on the detection signal. The calculation unit 20 can be configured as a computer or the like. The calculation unit 20 may also serve as a control unit that controls eddy current flaw detection measurement by the eddy current flaw detection probe 10.

[0032] The eddy current testing apparatus 1 according to this embodiment may include, in addition to the eddy current testing probe 10 and calculation unit 20 described above, and a control circuit for electrically controlling each coil 11 of the eddy current testing probe 10, it may also include various components necessary for eddy current testing, such as a moving means for moving the eddy current testing probe 10 along the surface of the material M under inspection. However, in order to simplify the structure of the eddy current testing apparatus 1, it is preferable that components not necessary for executing the eddy current testing method according to the embodiment of the present invention described below, such as a magnet for magnetizing the material M under inspection, are not provided in the eddy current testing apparatus 1.

[0033] [Eddy current testing method] Next, an eddy current testing method according to one embodiment of the present invention will be described. The eddy current testing method according to this embodiment can be carried out using the eddy current testing apparatus 1 described above.

[0034] In this eddy current testing method, an eddy current testing measurement is performed on the surface of the material M under inspection using an eddy current testing probe 10. This involves a first inspection step where eddy current testing is performed with the transmission / reception direction directed in a first direction, and a second inspection step where eddy current testing is performed with the transmission / reception direction directed in a second direction. Here, the second direction is a direction intersecting the first direction, and both are in-plane directions on the surface of the material M under inspection. Furthermore, the calculation unit 20 performs a determination step, which will be explained in detail later, based on the detection signal obtained in the first inspection and the detection signal obtained in the second inspection. When using an eddy current testing probe 10 equipped with a coil array, the first inspection step may be performed in the axial mode as shown in Figure 2(a), and the second inspection step may be performed in the lateral mode as shown in Figure 2(b).

[0035] In the determination process, the type of defect present on the surface of the material M under inspection is distinguished by utilizing the relationship between the anisotropy of the shape of the defect on the surface of the material M under inspection and the anisotropy of the signal intensity detected by eddy current testing. In eddy current testing, if a defect such as a scratch on the surface of the material M under inspection has a shape that intersects the direction of the eddy current EC at a right angle or close to it, the disturbance that the defect exerts on the eddy current EC will be large, and the change in the magnetic field generated by the eddy current EC at the location of the defect will be large. On the other hand, if a defect such as a scratch on the surface of the material M under inspection has a shape that is aligned with the direction of the eddy current EC, or a direction close to it, the disturbance that the defect exerts on the eddy current EC will be kept small, and the change in the magnetic field generated by the eddy current EC at the location of the defect will be small.

[0036] Therefore, when an eddy current EC is generated in the transmitting coil and the magnetic field generated by the eddy current EC is detected in the receiving coil, the amount of change in the magnetic field detected in the receiving coil becomes large when a defect formed on the surface of the material under inspection M occupies a large area in the direction along the transmission / reception direction connecting the transmitting and receiving coils, or in a direction close to it. In other words, when using an eddy current flaw detection probe 10 equipped with a coil array, as shown in Figure 2(a), if a defect S1 such as a scratch has a shape extending in a direction perpendicular to the coil rows A1 and A2 (x direction) or in a direction close to it, the amount of change in the detected magnetic field becomes large in axial mode inspection. On the other hand, in lateral mode inspection, the amount of change in the detected magnetic field remains within a small range. In contrast, as shown in Figure 2(b), if a defect S2 such as a scratch has a shape extending in the direction along the coil rows A1 and A2 (y direction) or in a direction close to it, the amount of change in the detected magnetic field becomes large in lateral mode inspection. On the other hand, in axial mode inspection, the amount of change in the detected magnetic field remains within a small range.

[0037] Thus, when defects formed on the surface of the material under inspection M have a highly anisotropic shape, as shown by defects S1 and S2 in Figures 2(a) and (b), if eddy current testing is performed with the transmission and reception directions facing two different directions at the location of these defects, anisotropy will occur in the intensity of the detection signals obtained from those measurements. The amount of change in signal intensity due to the defect will be greater when the transmission and reception direction is closer to the direction in which the defect extends. On the other hand, when defects formed on the surface of the material under inspection M have a less anisotropic shape, the amount of change in the detection signal when eddy current testing is performed with different transmission and reception directions will not differ significantly depending on the direction. This is because there is little difference in the length of the region in which the eddy current EC crosses the defect depending on the direction.

[0038] Therefore, in the eddy current testing method according to this embodiment, in at least one of the first inspection step (e.g., axial mode inspection) and the second inspection step (e.g., lateral mode inspection) in which eddy current testing measurements are performed with mutually different transmission and reception directions, the difference between the signal intensity detected in the first inspection step and the signal intensity detected in the second inspection step is calculated at the location where a defect is detected as a change in the detection signal, and a difference signal is obtained. The larger the difference signal, the higher the anisotropy of the detected defect's shape is determined to be. Furthermore, many defects exhibit different degrees of anisotropy in the shape that appears on the surface of the material M under inspection, depending on their type, i.e., characteristics and origin. However, based on the magnitude of the difference signal obtained in the detection step, the type of defect can be distinguished using the anisotropy of the shape as an indicator. Preferably, the detection signals acquired in the first and second inspection processes are C-scope images (a two-dimensional display in grayscale of the peak top values ​​of the alternating magnetic field detected at each position on the surface of the material M under inspection), and as difference signals, a two-dimensional image is created by calculating the difference in signal intensity between the first and second inspection processes at each point of these C-scope images.

[0039] As shown in Figure 1, the eddy current testing method according to this embodiment will be specifically explained using the example of a material M made of a ferromagnetic material on which physical scratches S1 and S2 and a non-uniform distribution of magnetic permeability (magnetic unevenness) U are formed on the surface. Note that the material M is not necessarily a ferromagnetic material; it may be a non-magnetic material with residual scale or other substances with different magnetic properties on its surface. In metal materials constructed as rolled materials, scratches are often formed before rolling, and these scratches are stretched during the rolling process, resulting in highly anisotropic scratches that extend linearly in one direction. In the illustrated example, linear scratches S1 and S2 are formed on the surface of the material M. In this example, it is assumed that both scratches S1 extending in the x direction (horizontal direction in Figure 1) and scratches S2 extending in the y direction (depth direction in Figure 1) are present. After rolling, the surface of rolled material is often shot-blasted to remove film-like oxides, carbides, and other compounds that have formed on the surface during previous processing steps. However, due to unevenness in the shot-blasting, areas where oxides, carbides, and other compounds are not sufficiently removed locally may occur. These areas contain materials with different magnetic permeability from the surrounding areas, thus contributing to the formation of magnetic unevenness. Because this magnetic unevenness is formed after rolling and tends to occupy a certain area, its shape on the metal surface is often low in anisotropy. In the example shown in Figure 1, the magnetic unevenness U takes the shape of an island-like area that occupies a certain area in the xy plane.

[0040] Figures 3(a) and 3(b) schematically show C-scope images obtained by performing eddy current testing in the axial mode and transverse mode, respectively, on the surface of the material M under inspection shown in Figure 1, using the eddy current testing probe 10. In the images, areas with higher signal intensity detected at each location on the surface of the material M under inspection are shown in darker colors. In both the axial mode and the transverse mode, two defects S1 and S2 and one magnetic irregularity U are detected as regions that give a different signal intensity from the surrounding area. The locations where defects S1, S2 and magnetic irregularity U appear are the same in the axial mode and the transverse mode, but the signal intensities given by these defects S1, S2, and U differ between the two. Specifically, defect S1 extending in the x-direction is observed with a high signal intensity in the axial mode image, but with a lower signal intensity in the transverse mode image. Conversely, the defect S2 extending in the y-direction shows a low signal intensity in the axial mode image, while it is observed with a high signal intensity in the transverse mode image. On the other hand, magnetic unevenness U is observed with a similarly high signal intensity in both the axial mode and transverse mode images.

[0041] Thus, defects formed as linearly extending, highly anisotropic shapes yield high signal strength in one of the axial mode inspections and low signal strength in the other, depending on the direction of extension. In the illustrated situation, the transmission and reception direction in the axial mode inspection is oriented in a direction close to the x-direction of the surface of the material M being inspected, and a defect S1 extending in the x-direction, which is close to that transmission and reception direction, is observed with high signal strength in the axial mode inspection. On the other hand, the transmission and reception direction in the lateral mode inspection is along the y-direction of the material M being inspected, and a defect S2 extending in the y-direction, which coincides with that transmission and reception direction, is observed with high signal strength in the lateral mode inspection. In this way, when highly anisotropic defects are observed by eddy current testing, a high signal strength is obtained when the transmission and reception direction is close to the direction in which the defect extends, and the signal strength decreases when the transmission and reception direction is farther from the direction in which the defect extends. On the other hand, magnetic irregularities formed as defects with low anisotropy do not tend to produce a large difference in signal strength depending on the transmission and reception direction. Thus, in eddy current testing, it becomes possible to identify the type of defect based on the difference in signal intensity depending on the transmission and reception direction, using the anisotropy of the defect shape on the surface of the material M being inspected as a mediator.

[0042] In the eddy current testing method of this embodiment, as a method for sensitively detecting the difference in signal intensity depending on the transmission and reception direction as described above, the difference is taken between the signal intensities obtained from inspections in different transmission and reception directions and the difference signal is calculated. Figure 3(c) shows the difference image obtained by taking the difference in signal intensity between the C-scope image in axial mode in Figure 3(a) and the C-scope image in lateral mode in Figure 3(b). Let I1 and I2 be the signal values ​​of each position (each pixel) in the axial mode and lateral mode, respectively, and the value ΔI of each pixel in the difference image is expressed as shown in equation (1) below. ΔI = |I1 - I2| (1) Furthermore, when using a general-purpose eddy current flaw detection probe 10 having a coil array, the spatial resolution in the direction along coil rows A1 and A2 in the image obtained in the lateral mode is half that of the axial mode. Therefore, the number of pixels in the C-scope images of both modes should be made equal by linear interpolation or the like before performing the calculation in equation (1).

[0043] In the difference image in Figure 3(c), images corresponding to the two scratches S1 and S2 are clearly displayed. This is because the anisotropy of the shapes of scratches S1 and S2 causes a large difference in the signal intensity detected in the axial mode and the transverse mode. As shown in Figures 3(a) and (b), the scratch S1 extending in the x direction has a high signal intensity in the axial mode, and the scratch S2 extending in the y direction has a high signal intensity in the transverse mode. However, as shown in equation (1), the difference image takes the absolute value of the difference in signal intensity obtained in both modes, so in the difference image, scratches S1 and S2 extending in either direction appear equally clear with strength in the positive direction. On the other hand, although the image corresponding to the magnetic unevenness U is clearly observed in the C-scope images of the axial and transverse modes in Figures 3(a) and (b), as shown in Figure 3(c), it does not give a signal intensity sufficient to be clearly distinguishable in the difference image. This is because the signal intensity corresponding to magnetic unevenness U is almost the same in the axial mode and the transverse mode, resulting in the difference signal having no intensity or showing only a very low intensity.

[0044] Thus, in difference images, defects with high anisotropy in shape, such as physical scratches, give high signal strength, while defects with low anisotropy in shape, such as magnetic irregularities, have virtually no signal strength or only a low signal strength. In other words, if a defect is observed in at least one of the axial or transverse mode observation images, the larger the value displayed at the location corresponding to that defect in the difference image, the more likely it is that a highly anisotropic defect exists at that location. While the anisotropy of a defect itself can be determined by performing image analysis on the C-scope image in the axial or transverse mode to analyze the shape of the defect, the degree of anisotropy in the shape of a defect can be easily and clearly determined without two-dimensional image analysis by obtaining a difference image by taking the difference between the C-scope images obtained from measurements in two directions and simply determining the magnitude of the numerical values ​​at each location in that difference image. Furthermore, in difference images, the signal contrast between the defect-free region and the image itself is higher than in the image obtained from measurements in two directions, thus increasing the sensitivity in defect recognition. Furthermore, since the difference image displays the difference value as an absolute value, even in cases of defects with anisotropy in shape, such as defects S1 and S2 in Figure 1, where the direction of anisotropy is different, a similar signal intensity can be obtained regardless of the direction, allowing it to be determined that the defects have a similar degree of anisotropy.

[0045] As an application of the method for determining the anisotropy of defect shape using difference images, the type of defect can be determined based on the difference image. As explained above, physical scratches tend to have a highly anisotropic shape, while magnetic irregularities tend to have a less anisotropic shape. In many cases, the degree of anisotropy in shape differs depending on the type of defect. In such cases, if the signal intensity of the difference image is within a predetermined range, it can be determined that a specific type of defect has been formed. For example, as shown in the example in Figure 1, if there are candidate defects that can be formed on the surface of the material under inspection M, namely type I defects with high shape anisotropy (scratches S1 and S2 here) and type II defects with low shape anisotropy (magnetic irregularities U here), then if the difference value obtained at a position corresponding to a certain defect in the difference image is above a predetermined threshold, it can be determined that the detected defect is a type I defect. If the difference signal is below the threshold, it can be determined that the detected defect is a type II defect.

[0046] Furthermore, if the system is to selectively detect only Type 1 defects or only Type 2 defects as defects, and not detect the other even if it exists, then it is sufficient to recognize a defect only if the difference signal is above or below a predetermined threshold. For example, in metal materials such as steel, if there are scratches on the surface, it may affect the quality of the steel, so it is necessary to detect them by eddy current testing and take countermeasures such as removal. However, even if local oxidation or carbonization that causes magnetic unevenness occurs, the impact on the quality of the steel is small, so it is not necessary to detect magnetic unevenness as a defect. In such cases, it is preferable to determine that a defect that should be recognized as a defect exists only if the difference value in the obtained difference image is above a predetermined threshold, and to notify the inspector from the calculation unit 20. If the difference value in the difference image remains below that threshold, the system does not recognize the existence of a defect, and no notification is given to the worker.

[0047] To extract difference values ​​above a threshold, for example, a high-pass filter can be applied to the difference image to remove the offset, and the region where the signal remains can be recognized as a defect with a difference above the threshold. The threshold set by the high-pass filter should be higher than the difference value corresponding to low-anisotropy defects that do not need to be recognized, and lower than the difference value corresponding to high-anisotropy defects that should be recognized. If low-anisotropy defects that do not need to be recognized have a nearly isotropic shape and the difference value is substantially zero, then the threshold should be set to exceed the noise generated in the difference value in the region where such defects occur, or in the region where no defects occur. Conversely, if it is desired to ignore high-anisotropy defects such as scratches and recognize only low-anisotropy defects such as magnetic irregularities as defects, then only the region that gives a difference value below a predetermined threshold by applying a low-pass filter, etc., among the regions in which defects are recognized as existing in at least one of the two-direction eddy current testing, should be recognized as defects.

[0048] As described above, by using the eddy current testing method according to this embodiment, it is possible to distinguish and detect different types of defects on the surface of the material M under inspection that have different degrees of anisotropy in shape. Since the identification of the type of defect is based solely on the difference in signal intensity between measurements in two directions caused by the anisotropy in shape, even if the material M under inspection is a ferromagnetic material, there is no need to perform operations that change the state of the defect itself, such as magnetization or demagnetization. Furthermore, as long as it is possible to perform eddy current testing by changing the transmission and reception directions in two ways, there is no need to use special equipment for eddy current testing, and a general-purpose eddy current testing probe 10, including coil array type probes, can be suitably used. Therefore, the eddy current testing apparatus and eddy current testing method can be simple.

[0049] When using an eddy current flaw detection probe 10 equipped with a coil array, when performing eddy current flaw detection by changing the transmission and reception direction, such as between axial and transverse modes, the location where defects are detected hardly shifts even when the transmission and reception direction is changed, and if there is no anisotropy in the shape of the defect, there is almost no difference in signal strength. Therefore, when calculating the difference signal based on the signal values ​​obtained from inspections in two transmission and reception directions, the number of pixels in the inspection image can be matched by using linear interpolation or the like as appropriate, and then simply taking the absolute value of the difference between the two signal values ​​I1 and I2 as shown in equation (1) above, the anisotropy of signal strength caused by the anisotropy of the defect shape can be evaluated with high accuracy. However, if there is a large difference in the location where defects are detected when the transmission and reception direction is changed due to the influence of the specific equipment condition or measurement method, or if there is a large difference in signal strength even if there are no defects with anisotropy in shape, then the difference may be taken after making appropriate corrections to the signal position and signal strength.

[0050] When performing eddy current testing in two transmission and reception directions, specific conditions such as the applicable frequency (frequency of the AC magnetic field generated by the transmitting coil and detected by the receiving coil) should be set appropriately to accurately and sensitively detect the defect to be detected. For example, when it is desired to detect elongated defects such as scratches S1 and S2 on the surface of the metal material described above, it is preferable to perform eddy current testing at different frequencies for the axial mode inspection and the lateral mode inspection. If the defect extends in a direction midway between the two transmission and reception directions, or in a direction close to it, performing eddy current testing with different transmission and reception directions at the same frequency may result in a small difference in the signal difference obtained from the two inspection directions, as shown in equation (1), making it difficult to recognize the defect as having high anisotropy in shape, even though the anisotropy of the defect shape is high. However, if eddy current testing in the two transmission and reception directions is performed at different frequencies, the degree to which the presence of the defect affects the eddy current EC will differ depending on the frequency, which tends to result in a large difference in the obtained signal strength. As a result, when the difference is taken as in equation (1), a relatively large value is obtained as the difference value, making it easier to recognize it as a defect with high anisotropy in shape.

[0051] Furthermore, by taking the difference in signal values ​​obtained by eddy current testing in two transmission and reception directions, it is possible to selectively detect defects with anisotropic shape, regardless of the specific shape or dimensions of the defect. However, if the length of the defect (the dimension in the direction in which the defect extends) is too small relative to the diameter of the coil 11, the difference in signal values ​​due to anisotropy becomes small, which may make selective detection of the defect difficult. From the perspective of improving the accuracy of recognizing defects with anisotropy, the eddy current testing probe 10 used should be selected such that the expected length of the defect is more than half the coil diameter, or even greater than the coil diameter. [Examples]

[0052] The present invention will be specifically described below with reference to examples. However, the present invention is not limited to these examples.

[0053] [Test Method] As a test sample, a rolled material made of carbon steel was prepared. After rolling, the surface coating was removed by shot blasting, but an uneven distribution of the shot blast finish was intentionally created. As a result, a sample was obtained in which the coating remained on some parts of the surface, giving it magnetic unevenness. Furthermore, linear scratches, 10 mm long and 0.5 mm deep, were artificially created at two locations offset from the magnetic unevenness. The direction of the two scratches was the same.

[0054] Eddy current testing was performed on the sample prepared as described above. A 32-channel coil array was used as the eddy current probe. Measurements were performed in two modes: axial and transverse. The eddy current probe was positioned relative to the test sample so that the direction of the coil rows in the coil array was perpendicular to the length of the defect. The material under inspection was not magnetized or demagnetized before the eddy current testing.

[0055] [Test Results] Figures 4(a) and 4(b) show C-scope images obtained by eddy current testing in axial and lateral modes. Each image is displayed in grayscale, with areas of high signal intensity being brighter and areas of low signal intensity being darker. In Figure 4(a), as shown in positions A, B, and C, there are three locations in the axial mode image where the signal intensity is higher than the surrounding area. Comparing this with the results of visual inspection of the sample surface, scratches are formed along the lateral direction of the image at positions A and B, and magnetic irregularities due to shot blasting irregularities are formed at position C. In other words, in axial mode, both scratches and magnetic irregularities are detected as regions with higher signal intensity than the surrounding area. The regions corresponding to magnetic irregularities are observed as faintly brighter over a wider area than the regions corresponding to scratches, confirming that the magnetic irregularities have lower anisotropy in shape than the scratches.

[0056] Next, looking at the lateral mode image in Figure 4(b), we can see that in positions A, B, and C, which were observed as regions with higher signal intensity than the surrounding area in the axial mode, regions with different signal intensity from the surrounding area are also observed. This indicates that scratches and magnetic irregularities are detected in the lateral mode measurements as well. However, the behavior of these signal intensities when compared to the surrounding area differs from that in the axial mode. Specifically, the signal intensity at positions A and B, where scratches were formed, was higher than the surrounding area in the axial mode, whereas in the lateral mode it is lower than the surrounding area. At position C, where magnetic irregularities were formed, the signal intensity is higher than the surrounding area in the lateral mode as well, similar to the axial mode.

[0057] Linear interpolation was performed on the image in the lateral mode so that the number of pixels in the vertical direction (coil row direction) doubled, matching the number of pixels in the axial mode. Then, as shown in equation (1), the difference image obtained by calculating the difference in signal intensity at each pixel is shown in Figure 4(c). At positions A and B corresponding to scratches, high intensity is obtained in the difference image in Figure 4(c), corresponding to the fact that the behavior of signal intensity compared to the surrounding area is reversed in the axial mode and lateral mode. The contrast with the surrounding area is also higher than that of the axial mode image in Figure 4(a). On the other hand, at position C corresponding to magnetic unevenness, no distribution of positive signal intensity that is clearly different from the surrounding area beyond the noise level is observed in the difference image. This is because at position C, higher signal intensity than the surrounding area is obtained in both the axial mode and lateral mode, and when the difference is taken, these changes in signal intensity cancel each other out.

[0058] Thus, in the difference image, scratches, which are defects with high anisotropy in shape, are obtained as regions with clear contrast from the surrounding area, whereas magnetic irregularities, which are defects with low anisotropy in shape, are not recognized as regions that are clearly distinguishable from the surrounding area. In this way, by taking the difference in signal intensity obtained from two measurements with different transmission and reception directions, it is possible to identify the type of defect based on its anisotropy in shape, and the larger the difference value, the higher the anisotropy in shape of that defect can be determined. In the difference image of Figure 4(c), by setting a threshold level slightly higher than the noise occurring in areas where no defects are formed, and extracting the regions where a difference value larger than that threshold is obtained, it is possible to clearly identify the regions where scratches occur, distinguishing them from the regions where magnetic irregularities occur.

[0059] Although embodiments of the present invention have been described in detail above, the present invention is not limited to the above embodiments, and various modifications are possible without departing from the spirit of the invention. [Explanation of Symbols]

[0060] 1 Eddy current flaw detection device 10. Eddy current testing probe 11 coils 20 Arithmetic section A1, A2 coil row EC eddy current M Inspected material R,R2 receiving coils S1,S2 scratches T,T2 transmitting coils U Magnetic unevenness

Claims

1. In an eddy current testing method for detecting surface defects of a material under inspection using an eddy current testing probe equipped with multiple coils, Of the plurality of coils, the direction connecting the transmitting coil, which applies a magnetic field to the surface of the material under inspection to generate eddy currents, and the receiving coil, which detects the magnetic field generated by the eddy currents, is defined as the transmitting and receiving direction. A first inspection step in which eddy current testing is performed with the aforementioned transmission and reception direction directed towards the first direction, A second inspection step involves performing eddy current testing in a second direction that intersects the first direction with the transmission and reception direction, A determination step is performed in which, on the surface of the material to be inspected, at a location where a defect is detected in at least one of the first inspection step and the second inspection step, the difference between the signal intensity detected in the first inspection step and the signal intensity detected in the second inspection step is calculated, and the larger the difference, the higher the anisotropy of the shape of the detected defect is determined to be. When there are candidates for defects that may form on the surface of the material under inspection, namely, a first-type defect which is a scratch with high anisotropy in shape, and a second-type defect which is a non-uniform distribution of magnetic permeability with lower anisotropy in shape than the first-type defect, In the determination step, if the difference is greater than or equal to a threshold, it is determined that the detected defect is a type one defect, and if the difference is less than the threshold, it is determined that the detected defect is a type two defect.

2. The eddy current testing method according to claim 1, wherein the first inspection step and the second inspection step are performed without magnetizing or demagnetizing the material to be inspected.

3. The eddy current testing probe has a coil array, which is made up of multiple rows of coils arranged in a line, In the first inspection step, two coils included in different coil rows in the coil array are designated as the transmitting coil and the receiving coil, and eddy current testing is performed on them. The eddy current testing method according to claim 1, wherein in the second inspection step, eddy current testing is performed on two coils included in the same coil row in the coil array, designated as the transmitting coil and the receiving coil.

4. The eddy current testing method according to claim 1, wherein eddy current testing is performed at different frequencies in the first inspection step and the second inspection step.

5. An eddy current testing method for detecting surface defects of a material to be inspected using an eddy current testing probe equipped with multiple coils, Of the plurality of coils, the direction connecting the transmitting coil, which applies a magnetic field to the surface of the material under inspection to generate eddy currents, and the receiving coil, which detects the magnetic field generated by the eddy currents, is defined as the transmitting and receiving direction. A first inspection step in which eddy current testing is performed with the aforementioned transmission and reception direction directed towards the first direction, A second inspection step in which eddy current testing is performed at a different frequency from the first inspection step, with the transmission and reception direction directed in a second direction intersecting the first direction, An eddy current testing method comprising: a determination step of calculating the difference between the signal intensity detected in the first inspection step and the signal intensity detected in the second inspection step at a location on the surface of the material to be inspected where a defect is detected in at least one of the first inspection step and the second inspection step, and determining that the larger the difference, the higher the anisotropy of the shape of the detected defect.

6. The eddy current testing probe and, It has a calculation unit that performs the aforementioned determination step, An eddy current testing apparatus for performing the eddy current testing method according to any one of claims 1 to 5.

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