The present application claims priority from Japanese Patent Application JP 2017-034332 filed on Feb. 27, 2017, the content of which is hereby incorporated by reference into this application.
The present invention relates to an eddy-current testing method and an eddy-current testing device.
Eddy-current testing is a method in which an eddy-current testing probe (hereinafter simply called “probe”) is brought into close contact with a conductive object to scan the object and induce it to generate an eddy current by an AC magnetic field generated by a coil of the probe, and it is decided whether the object has a flaw or not according to change in the impedance of the coil which is caused by an eddy current turbulence due to a flaw. When a small narrow portion of the object is inspected by eddy-current testing, it is difficult for the probe to scan it stably, and liftoff, a phenomenon that the probe leaves the surface of the object (inspection surface) and is not in contact with the inspection surface, occurs, resulting in generation of a liftoff signal depending on the distance between the probe and the object (amount of liftoff). Since a liftoff signal is a noise, in order to measure an eddy-current testing signal (flaw signal) due to a flaw accurately, it is necessary to suppress liftoff and distinguish a liftoff signal from a measured signal.
JP 2012-141238 discloses an eddy-current testing method which can evaluate the amount of liftoff accurately even if a curved surface is inspected using a flexible probe. In the eddy-current testing method described in JP 2012-141238, maximum signal value V0 before placement of an eddy current probe on the inspection surface and signal value V1 at the time of the placement are compared in terms of magnitude and according to the magnitude relation between these signal values, a liftoff value is determined from liftoff characteristics using any of a value of a detection signal generated upon contact of the eddy current probe with the inspection surface, characteristics of liftoff, signal value V0 and signal value V1.
In eddy-current testing, if liftoff, a phenomenon that the eddy-current testing probe leaves the surface of the object, occurs, a liftoff signal is detected as a noise and there are cases that the flaw signal to be detected is misidentified or cannot be detected. Since liftoff often occurs despite efforts to suppress liftoff, there is a demand for a method which separates and distinguishes a flaw signal and a liftoff signal. In the related art described in JP 2012-141238, for example, comparison is made between signals in terms of magnitude to determine the liftoff value. However, the liftoff signal phase differs according to the liftoff mode (how the probe leaves the inspection surface) and thus there is a problem that it is difficult to accurately separate and distinguish a flaw signal and a liftoff signal merely by comparing them in terms of magnitude without consideration for the signal phase.
An object of the present invention is to provide an eddy-current testing method and eddy-current testing device which can separate and distinguish a flaw signal and a liftoff signal.
An eddy-current testing method of the present invention includes: liftoff signal measuring step of using an eddy-current testing probe including one or more coils, giving liftoff to the eddy-current testing probe, performing eddy-current testing on a reference specimen simulating an object, and measuring a liftoff signal of the reference specimen, an eddy-current testing signal generated by liftoff being defined as the liftoff signal; phase recording step of recording a phase of a Lissajous figure obtained from the liftoff signal of the reference specimen; eddy-current testing signal measuring step of performing eddy-current testing on the object using the eddy-current testing probe and measuring an eddy-current testing signal of the object; and decision step of deciding that the eddy-current testing signal detected in the eddy-current testing signal measuring step is a liftoff signal if the eddy-current testing probe detects the eddy-current testing signal in all the coils in the eddy-current testing signal measuring step and a phase of a Lissajous figure obtained from the eddy-current testing signal detected in the eddy-current testing signal measuring step coincides with the phase recorded in the phase recording step for all the coils.
According to the present invention, an eddy-current testing method and eddy-current testing device is provided which can separate and distinguish a flaw signal and a liftoff signal.
In the eddy-current testing method and eddy-current testing device according to the present invention, when an object is tested and a flaw in the object is detected, even if liftoff, a phenomenon that the eddy-current testing probe leaves the inspection surface as the surface of the object, occurs, an eddy-current testing signal (flaw signal) due to a flaw in the object and an eddy-current testing signal (liftoff signal) generated by the liftoff can be separated and distinguished and thus the flaw signal can be detected without fail. The object to be tested should be a conductor and, for example, it may be a rotor disc of a steam turbine (in particular, a small narrow portion such as a groove engaged with a moving blade).
First, a Lissajous figure which is used in the eddy-current testing methods according to the embodiments is explained here. The present invention focuses attention on the fact that the phase of a Lissajous figure obtained from a liftoff signal is different from the phase of a Lissajous figure obtained from a flaw signal. The phase of a liftoff signal is expressed as a phase of a Lissajous figure obtained from the liftoff signal, and the phase of a flaw signal is expressed as a phase of a Lissajous figure obtained from the flaw signal.
If the object 8 has no flaw, the eddy current which occurs in the object 8 due to the AC magnetic field generated by the probe 10 has a uniform distribution regardless of a position of the probe 10 and thus the probe 10 does not detect an eddy-current testing signal. On the other hand, if the object 8 has a flaw, the eddy current which occurs in the object 8 has a distorted distribution and thus the probe 10 detects an eddy-current testing signal (flaw signal). The probe 10 detects not only a flaw signal but also an eddy-current testing signal (liftoff signal) due to liftoff which is a phenomenon that the probe 10 leaves the inspection surface 12. Liftoff occurs in several modes which are different depending on how the probe 10 leaves the inspection surface 12.
There are three modes in liftoff of the probe 10, parallel mode 13, widthwise inclination mode 14, and lengthwise inclination mode 15. The parallel mode 13 is a liftoff mode where the probe 10 leaves the inspection surface 12 with the contact surface of the probe 10 parallel to the inspection surface 12. The widthwise inclination mode 14 is a liftoff mode where the probe 10 leaves the inspection surface 12 with the contact surface of the probe 10 inclined with respect to the inspection surface 12 in the width direction (rotated around the length direction as a rotation axis). The lengthwise inclination mode 15 is a liftoff mode where the probe 10 leaves the inspection surface 12 with the contact surface of the probe 10 inclined with respect to the inspection surface 12 in the length direction (rotated around the width direction as a rotation axis).
In the lengthwise inclination mode 15, the probe 10 leaves the inspection surface 12 with the contact surface 28 inclined with respect to the inspection surface 12 in the length direction.
Since the probe 10 is solidified with resin, etc., substantially no distortion or bending occurs. The detecting coils of the detecting coil section 9 are integrated and all of the detecting coils move almost in the same way with each other along with the movement of the probe 10. Therefore, even if liftoff of the probe 10 occurs in the widthwise inclination mode 14, parallel mode 13, and lengthwise inclination mode 15, there is no possibility that some of the detecting coils of the probe 10 contact the object 8, and all the detecting coils (namely, all the channels) leave the inspection surface 12 of the object 8. If liftoff of the probe 10 occurs, all the detecting coils leave the inspection surface 12 of the object 8, so a liftoff signal is detected in all the channels.
In plotting a Lissajous figure, the initial phases of the real number part (x component) and imaginary number part (y component) can be arbitrarily set. In the examples shown in
The phase 21 of the Lissajous
As shown in
Now, an eddy-current testing method and an eddy-current testing device according to embodiments of the present invention will be described.
In S1, eddy-current testing is performed on a reference specimen using the probe 10 to measure liftoff signals of the reference specimen. Liftoff signals are measured in all the channels (all the detecting coils of the detecting coil section 9) of the probe 10. The reference specimen is a trial object which simulates the real object 8. The reference specimen and the object 8 may have a magnetic property, for example, made of a ferromagnetic substance such as iron, or they may not have a magnetic property, for example, made of a diamagnetic substance such as copper.
A liftoff signal is measured by giving liftoff to the probe 10 using a tool or equipment to let the probe 10 leave the reference specimen and performing eddy-current testing on the reference specimen. The probe 10 is given liftoff in three modes, liftoff in the parallel mode 13, liftoff in the widthwise inclination mode 14, and liftoff in the lengthwise inclination mode 15. In liftoff in the widthwise inclination mode 14, liftoff signals in two types of inclinations (two types of rotations around the length direction as a rotation axis) are measured. In liftoff in the lengthwise inclination mode 15, liftoff signals in two types of inclinations (two rotations around the width direction as a rotation axis) are measured. In short, five types of liftoff are given to the probe 10 and the liftoff signal is measured for each of the five types of liftoff.
For each channel, a plurality of liftoff distances are given to each of the five types of liftoff and each liftoff signal is measured. The liftoff distance is a distance between a detecting coil and the reference specimen. The plural liftoff distances for measurement of liftoff signals may be determined arbitrarily depending on the characteristics of the object 8 and the probe 10 within the range of liftoff distances which can occur actually.
In S2, the phases of Lissajous figures obtained from the liftoff signals in each channel, which have been measured in S1, are recorded in a data table.
In S3, eddy-current testing is performed on the real object 8 using the probe 10 and an eddy-current testing signal of the object 8 is measured. Eddy-current testing signals are measured in all the channels (all the detecting coils of the detecting coil section 9) of the probe 10.
In S4, a decision is made as to whether the probe 10 has detected an eddy-current testing signal in eddy-current testing on the object 8 in S3. If the probe 10 has not detected an eddy-current testing signal in any channel, it means that a flaw signal and a liftoff signal have not been detected and thus the object 8 is sound without a flaw (S8) and it is decided that no liftoff has been detected in the eddy-current testing in S3. If the probe 10 has detected an eddy-current testing signal in at least one channel, the process proceeds to S5.
In S5, a decision is made as to whether the probe 10 has detected eddy-current testing signals in all the channels in eddy-current testing on the object 8 in S3. If there is any one channel which has not detected an eddy-current testing signal, it is decided that the eddy-current testing signal detected in S3 is a flaw signal and the object 8 has a flaw (S9). Specifically, if the probe has a plurality of channels (detecting coils) and has detected an eddy-current testing signal in one or more channels and has not detected an eddy-current testing signal in one or more channels, it is decided that the eddy-current testing signal detected in S3 is a flaw signal. If the probe 10 has detected eddy-current testing signals in all the channels, the process proceeds to S6.
In S6, the phases of the Lissajous figures obtained from the eddy-current testing signals detected by eddy-current testing on the object 8 (phases of the Lissajous figures obtained from the eddy-current testing signals of the object 8) are compared with the phases recorded in the data table in S2 (phases of the Lissajous figures obtained from the liftoff signals of the reference specimen) for all the channels of the probe 10. Specifically, all the channels of the probe 10 are checked as to whether the phases of the liftoff signals of the reference specimen as recorded in the data table include a phase which coincides with the phase of an eddy-current testing signal of the object 8.
In S7, if in all the channels of the probe 10 the phases of the Lissajous figures obtained from the eddy-current testing signals detected in eddy-current testing on the object 8 coincide with the phases recorded in the data table in S2 (namely, if the phases of the liftoff signals of the reference specimen include a phase which coincides with the phase of an eddy-current testing signal detected in eddy-current testing on the object 8), it is decided that the eddy-current testing signal of the object 8 as detected in S3 is a liftoff signal (S10). If in at least one channel of the probe 10, the phase of a Lissajous figure obtained from an eddy-current testing signal detected in eddy-current testing on the object 8 does not coincide with any phase recorded in the data table in S2, it is decided that the eddy-current testing signal detected in S3 is a flaw signal and the object 8 has a flaw (S11).
As explained so far, liftoff in one of the three modes, parallel mode 13, widthwise inclination mode 14, and lengthwise inclination mode 15, occurs in the probe 10, all the detecting coils of the detecting coil section 9 can detect a liftoff signal. The phase of a liftoff signal is different from the phase (90 degrees) of a flaw signal and varies depending on detecting coils (channels). The eddy-current testing method according to this embodiment takes advantage of such phase characteristics of liftoff signals to separate and distinguish a flaw signal and a liftoff signal.
If the object 8 has a magnetic property, an evaluation is conventionally made as to whether the object 8 has a flaw or not, for example, by a magnetic particle inspection (MT, magnetic particle testing). In the magnetic particle inspection, the object 8 is magnetized and magnetic particles are attached to the magnetized object 8, then the magnetic particle pattern is visually examined to find a flaw of the object 8, and then the object 8 is demagnetized and the magnetic particles are removed from the demagnetized object 8.
In the eddy-current testing method according to this embodiment, even if the object 8 has a magnetic property, Lissajous figures such as ones shown in
In S2 in
The computing device 43 performs the process of S2 in
Furthermore, the computing device 43 performs the process of S4 and S5 in
Furthermore, the computing device 43 performs the process of S6 and S7 in
In this embodiment, the structure of the eddy-current testing device has been briefly explained. The eddy-current testing device according to this embodiment can separate and distinguish a flaw signal and a liftoff signal by using the computer 40 including the memory 41 and the computing device 43.
The present invention is not limited to the above embodiments and may be embodied in other various aspects.
The above embodiments have been explained in detail for easy understanding of the invention and embodiments of the invention need not include all the above elements. Some elements of an embodiment may be replaced by elements of another embodiment. Also, some elements of an embodiment may be added to another embodiment. Also, addition of other elements or deletion or replacement of some elements may be made for each embodiment.
Number | Date | Country | Kind |
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2017-034332 | Feb 2017 | JP | national |