JP2004117041A - Elastic wave detection method, its apparatus, and inspection method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an elastic wave detection method and an elastic wave detecting apparatus for detecting AE with high detection precision by using an optical fiber. <P>SOLUTION: The elastic wave detecting apparatus comprises a light source 2; a splitter 4 for dispersing light from the light source 2; first and second optical fibers 5, 6 for guiding light that is dispersed by the splitter 4 while at least one of them is installed to a target to be inspected; a coupler 7 for overlapping light that is guided from one end of the first optical fiber 5 to the other end; a photodetector 8 for detecting intensity in the overlapped light; and a treating apparatus 9 for extracting time and frequency information from the detection signal of the photo detector 8 and for detecting elastic waves that are generated in the target to be inspected. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an inspection method and apparatus for inspecting the state of an object to be inspected by detecting an elastic wave generated in the object to be inspected such as a structure or the generated elastic wave.
[0002]
[Prior art]
AE (Acoustic Emission) is an ultrasonic region (frequency of 50 kHz to 1 MHz) generated when phenomena such as minute deformation, fracture such as cracks, phase transformation, or movement of crystal grain boundaries occur inside a solid such as a metal material. ), In a broad sense, includes an elastic wave in a frequency band of several hundreds kHz to several tens kHz due to cracks in rock or fluctuations in the underground ground.
By detecting the above-described AE in real time, it is possible to detect corrosion of a structure, fatigue failure, and the like.
[0003]
[Problems to be solved by the invention]
For the detection of AE, for example, a piezoelectric element (PbTiZrO₃) Is used. An ultrasonic sensor using a piezoelectric element has a high detection sensitivity and can accurately detect a weak AE generated from a structure or the like.
However, an ultrasonic sensor using a piezoelectric element requires a device that amplifies the detection signal or converts the impedance, requires a coaxial cable to transmit the detection signal, is limited in the place of use, and is susceptible to electromagnetic interference. Disadvantages such as increased weight.
[0004]
As another method of detecting AE, a method of detecting the AE using an optical fiber is known. In the method of detecting AE using an optical fiber, the AE can be detected at a relatively low cost without being limited by the place of use.
Specifically, for example, a so-called Michelson interferometer is configured using an optical fiber. When the AE enters the optical fiber, the refractive index of the core of the optical fiber changes according to the strength of the AE. By detecting the change in the refractive index as a change in the phase of light transmitted through the optical fiber, the intensity of the AE can be specified.
The Michelson interferometer needs to cut an optical fiber flat and deposit a reflective film such as gold on the cut end face. The AE detection accuracy of the Michelson interferometer depends on the accuracy of the end face of the optical fiber.
However, it is difficult to process the end face with high precision, and it is difficult to deposit a reflective film on the end face. For this reason, it was difficult for the Michelson-type interferometer to obtain high AE detection accuracy.
[0005]
The present invention has been made in view of the above-described problem, and has as its object to detect AE generated from an object to be inspected or AE generated in the object to be inspected with high detection accuracy using an optical fiber. It is an object of the present invention to provide a method and an apparatus for detecting an elastic wave which can be performed.
Another object of the present invention is to provide an inspection method for inspecting the state of an object to be inspected using the above-described elastic wave detection device.
[0006]
[Means for Solving the Problems]
The inspection method according to the first aspect of the present invention provides a method in which at least one of the first and second optical fibers installed with respect to the inspection target is guided from one end to the other end by a single light source. Based on the light intensity of the superimposed light of the split light, an elastic wave generated in the object to be inspected is monitored, and when an elastic wave generated in the object to be inspected is detected, the object to be inspected is detected. An object is excited with an elastic wave, the excited elastic wave is detected based on the light intensity of the superimposed light, and the state of the inspection object is inspected based on the detected elastic wave.
[0007]
Preferably, time and frequency information is extracted from the light intensity information, and based on the time and frequency information, a source of the elastic wave is located, and directivity is directed toward the located source. Excites the elastic wave having.
[0008]
An inspection method according to a second aspect of the present invention includes the steps of: exciting an elastic wave to an object to be inspected; at least one of the first and second optical fibers installed on the object to be inspected; The elastic wave is detected based on the light intensity of the superimposed light of the light dispersed from the single light source guided toward the other end, and the state of the inspection object is determined based on the detected elastic wave. Inspect
[0009]
Preferably, the inspection object is irradiated with a laser beam to excite an elastic wave.
[0010]
According to the elastic wave detection method of the present invention, at least one of the first and second optical fibers is set on the object to be inspected, and guided from one end of the first and second optical fibers to the other end. An elastic wave generated from the inspection object is detected based on the light intensity of the superimposed light of the light separated from the single light source.
[0011]
Preferably, time and frequency information is extracted from the light intensity information, and the generation source of the elastic wave is located based on the time and frequency information.
[0012]
More preferably, the frequency characteristics of the first and second optical fibers are made different from each other, both the first and second optical fibers are installed on the object to be inspected, and time information is obtained from the information on the light intensity. And information on the frequency is extracted, and the generation source of the elastic wave is located based on the information on the time and the frequency.
[0013]
The elastic wave detection device of the present invention is configured such that a single light source, a splitter that splits light from the single light source, and at least one of the splitters is installed with respect to the inspection object and guides the light split by the splitter. Coupling means for superimposing first and second optical fibers, light guided from one end of the first and second optical fibers to the other end, and light for detecting the light intensity of the superimposed light A detector that extracts time and frequency information from a detection signal of the photodetector and detects an elastic wave generated in the inspection object from the information;
[0014]
According to the present invention, when an elastic wave is generated in an object to be inspected or an elastic wave is excited in the object to be inspected, the elastic wave is incident on an optical fiber provided for the object to be inspected. The incidence of this elastic wave changes the refractive index of the optical fiber, changes the state of the light split from a single light source guided from one end to the other end, and the superimposed light has a lower intensity than the elastic wave. Modulated accordingly. An elastic wave is detected by a predetermined process of the light intensity signal of the superimposed light including the information of the elastic wave.
[0015]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
First embodiment
FIG. 1 is a configuration diagram of an elastic wave detection device according to an embodiment of the present invention.
In FIG. 1, the elastic wave detecting device 1 includes a light source 2, a polarizer 3, a splitter 4, an optical fiber for reference light 5, an optical fiber for sensor 6, a coupler 7, a photo detector 8, , A processing device 9.
The elastic wave detection device 1 has a configuration based on a so-called Mach-Zehnder interferometer.
[0016]
The light source 2 outputs a laser beam having a specific wavelength. As the light source 2, for example, a laser diode is used.
The polarizer 3 controls the polarization of the laser light output from the light source 2 to be in a fixed direction, and outputs the laser light to the splitter 4.
[0017]
The splitter 4 splits the laser light input from the polarizer 3, outputs one laser light to the reference light optical fiber 5, and outputs the other laser light to the sensor optical fiber 6. As the splitter 4, a polarization preserving splitter that preserves the polarization direction controlled by the polarizer 3 is used.
[0018]
One of the laser beams split by the splitter 4 is input to the optical fiber for reference light 6. As the reference light optical fiber 6, for example, a single mode type bare fiber coated with acrylic or PVC (Polyvinyl Chloride) or the like is used. The optical fiber for reference light 6 is a polarization maintaining optical fiber.
The other of the laser beams split by the splitter 4 is input to the sensor optical fiber 5. The optical fiber for sensor 5 has the same configuration (same frequency characteristics) as the optical fiber for reference light 6, and a polarization maintaining optical fiber is used.
[0019]
The coupler 7 superimposes the laser light guided by the reference light optical fiber 6 and the sensor optical fiber 5 and outputs the laser light to the photodetector 8. The light from the optical fiber for reference light 6 and the optical fiber for sensor 5 are superposed by the coupler 7, and interference light is generated by the optical path difference between the optical fiber for reference light 6 and the optical fiber for sensor 5.
[0020]
The light detector 8 detects the light intensity of the interference light output from the coupler 7. Specifically, an electric signal corresponding to the light intensity of the incident interference light is generated. As the photodetector 8, for example, a photodiode is used.
[0021]
The processing device 9 converts the detection signal consisting of the analog signal of the photodetector 8 into a digital signal and takes it in. This processing device 9 is constituted by, for example, a personal computer. The processing device 9 performs a filtering process on the captured detection signal, and analyzes a group velocity dispersion and a temporal change of a specific frequency component by a wavelet transform or the like.
[0022]
Detection characteristics of elastic wave detector 1
Next, the elastic wave detection sensitivity characteristics of the elastic wave detection device 1 having the above configuration will be described.
FIG. 2 is a perspective view showing a state in which the elastic wave detection device 1 having the above-described configuration is installed on an object to be inspected.
In FIG. 2, the inspection object W is made of, for example, a thin steel plate.
The sensor optical fiber 5 is bonded to the surface of the inspection object W so as to cross the inspection object W.
[0023]
A position at a distance L from the sensor optical fiber 5 on the surface of the inspection object W is defined as a sound source position (elastic wave generation source) S, a steel ball B is dropped at this position, and an elastic wave (AE) is generated by the impact. generate. Here, an AE generated by damage such as a crack or breakage of a material is referred to as a primary AE, and an AE generated by an external force such as an impact is referred to as a secondary AE. When the steel ball B is dropped on the inspection object W, a secondary AE is generated.
[0024]
The secondary AE generated at the sound source position S propagates through the inspection target object W and enters the sensor optical fiber 5.
When the secondary AE enters the sensor optical fiber 5, the refractive index of the sensor optical fiber 5 changes. For this reason, the state of the laser light guided through the sensor optical fiber 5 changes.
On the other hand, since the reference light optical fiber 6 is separated from the inspection object W, the generated secondary AE does not enter the reference light optical fiber 6.
[0025]
In the coupler 7, the secondary AE in the sensor optical fiber 5 is superposed on the incident laser light and the laser light conveyed through the reference optical fiber 6 to generate interference light. The light intensity of the interference light changes according to the secondary AE incident on the sensor optical fiber 5.
The photodetector 8 detects the light intensity of the interference light that changes according to the secondary AE incident on the sensor optical fiber 5 and outputs the detected light intensity to the processing device 9.
[0026]
FIG. 3 is a graph showing a waveform of the secondary AE detected by actually dropping the steel ball B shown in FIG. 2 on the surface of the inspection object W.
In FIG. 3, the waveforms of Comparative Example 1 and Comparative Example 2 are also displayed in an overlapping manner.
[0027]
The embodiment shows an AE detection waveform detected by the elastic wave detection device 1 having the above configuration.
Comparative Example 1 is different from the elastic wave detecting device 1 having the above-described configuration in that the polarizer 3 is not used and the splitter 4 is not a polarization maintaining splitter but a normal splitter, and the sensor optical fiber 5 and the reference optical fiber are used. 6 shows an AE detection waveform obtained by the elastic wave detector when a normal single mode type optical fiber is used instead of the polarization maintaining optical fiber.
[0028]
Comparative Example 2 shows an AE detection waveform by an elastic wave detector based on a so-called Michelson interferometer.
FIG. 6 is a diagram illustrating an example of a configuration of an elastic wave detection device based on a Michelson interferometer.
The elastic wave detection device 100 shown in FIG. 6 includes a light source 104, a processing device 106, a photodetector 105, a splitter 102, a sensor optical fiber 110, and a reference light optical fiber 111.
In the elastic wave detection device 100, a laser beam is output from the light source 104, and the laser beam is branched by the splitter 102 and guided to the optical fiber for sensor 110 and the optical fiber for reference light 111. The laser light guided to the sensor optical fiber 110 and the reference light optical fiber 111 is reflected by the end face 110a of the sensor optical fiber 110 and the end face 111a of the reference light optical fiber 111, respectively, and this reflected light is split by the splitter 102. The light beams are superimposed and interference light is generated. The light intensity of the interference light is detected by the photodetector 105 and taken into the processing device 106.
[0029]
Comparative Example 2 described above shows the waveform of the AE detected by the sensor optical fiber 110 when the sensor optical fiber 110 is installed at the same position as the sensor optical fiber 5 installed on the inspection object W.
[0030]
The measurement conditions shown in FIG. 3 are such that the inspection object W is a thin steel plate having dimensions of 914 × 914 mm and a thickness of 0.9 mm, and the distance L between the sound source position S and the sensor optical fiber 5 is 200 mm. . The optical fiber for sensor 5 and the optical fiber for reference light 6 were coated fibers coated with PVC, and the optical fiber for sensor 110 and the optical fiber for reference light 111 were bare fibers. In addition, between the coated fiber and the bare fiber, the bare fiber has higher detection sensitivity.
[0031]
In FIG. 3, comparing the example with the comparative example 2, the detection sensitivity of the elastic wave detection device 1 according to the present embodiment is about 150 times lower than the detection sensitivity of the elastic wave detection device 100 based on the Michelson interferometer. It has about twice the sensitivity. That is, the elastic wave detection device 1 according to the present embodiment has a sensitivity that is approximately 150 times the detection sensitivity of the elastic wave detection device 100 based on the Michelson interferometer even when coated fiber is used.
Further, comparing the example with the comparative example 1, the detection sensitivity of the elastic wave detection device 1 according to the present embodiment is about five times as high. That is, by using the polarizer 3, the polarization maintaining splitter, and the polarization maintaining optical fiber, the sensitivity is greatly improved. That is, by preserving the plane of polarization of the laser light, it is possible to prevent the S / N ratio from being reduced when the optical fiber is bent or the optical fiber has a temperature change.
[0032]
The results described above are the results regarding the detection characteristics of AE of large amplitude and low frequency of the elastic wave detection device 1 according to the present embodiment.
Next, the detection characteristics of the weak AE of the elastic wave detection device 1 according to the present embodiment will be described.
The weak AE is generated, for example, by damage such as cracking or destruction of the material, but can be generated by bending the lead of the mechanical pencil. Core bending is used as a standard sound source for AE sensors and measuring instruments. Specifically, when the mechanical pencil is bent, the AE of a weak plate wave (Lamb wave) is generated due to the peeling of the core. In most structures, AE can be detected as the above-mentioned Lamb wave. Various dispersive mode waves exist in the Lamb wave.
[0033]
FIG. 4 is a graph showing an AE waveform detected by bending the lead of the mechanical pencil at the sound source position S of the inspection object W, and FIG. It is an obtained wavelet group velocity diagram.
The wavelet transform changes the form of a window function depending on time and frequency, and extracts time and frequency information from the entire region of the detected waveform. That is, time-frequency conversion is performed at each time of the detected waveform, and the signal strength for each frequency band is obtained in time series. Thereby, information on the arrival time of the wave in the specific mode at the specific frequency is obtained, and for example, the sound source position can be located.
[0034]
The bending condition of the mechanical pencil at the sound source position S was such that a mechanical force of 4 N was released to the mechanical pencil core having a hardness of 2H and a diameter of 0.5 mm in 0.9 μs. In addition, a DC voltage of 1.5 V was applied between the lead of the mechanical pencil and the inspection object W, and an open voltage when the lead was pressed against was used as a trigger signal.
By pressing the mechanical pencil core, S₀A mode initial wave is generated.₀A mode Lamb wave is generated. These S₀Mode initial wave and A₀The mode Lamb wave can be obtained by calculation under predetermined conditions.
S₀The mode first wave is A₀This is a low-frequency zero-order symmetric mode wave whose amplitude is smaller than that of the mode Lamb wave.
A₀A mode Lamb wave is a low-frequency, zero-order asymmetric mode wave having a large amplitude. Lamb waves have a dispersive property whose speed depends on frequency.
In FIG. 4, S₀Although the 動 mode initial wave was not reliably detected, it is considered that this was caused by using a coated fiber as the optical fiber.
On the other hand, A₀The mode Lamb wave is detected reliably.
[0035]
In FIG. 5, what is indicated by shading is a wavelet coefficient variance obtained by performing a wavelet transform process on the detected wave. Also, the solid line shows the A obtained by calculation.₀Mode Lamb wave group velocity dispersion.
The black ridge (high wavelet coefficient band) of the wavelet coefficient variance in FIG.₀Mode Lamb wave group velocity dispersion is well matched up to about 60 kHz. That is, A₀わ か る You can see that the mode Lamb wave has been extracted by the wavelet transform.
[0036]
As described above, the elastic wave detection device 1 according to the present embodiment includes, for example, a primary AE that is a weak ultrasonic wave that causes corrosion (self-destruction of atmospheric rust) and minute cracks of a structural member, an external impact, and the like. Has the sensitivity to detect both secondary AEs, which are elastic waves generated by the application of the AE.
[0037]
Inspection methods
Next, an inspection method using the elastic wave detection device 1 having the above configuration will be described.
FIG. 7 is a flowchart illustrating the procedure of the inspection method according to the present embodiment.
First, the elastic wave detection device 1 having the above-described configuration is installed on an object to be inspected such as a steel structure, a pipe, a tank, or the like, and a primary AE in which damage such as corrosion, corrosion cracking, and fatigue fracture occurs in the steel structure. Is detected (step S1).
The elastic wave detecting device 1 is installed in a structure in advance, and the primary AE is constantly monitored (monitored) for the occurrence of the primary AE, and by detecting the primary AE, corrosion, minute cracks, and fatigue fracture occur in the structure. Can be detected.
[0038]
Even if it is detected that a structure such as a corrosion, a micro-crack, or a fatigue fracture has occurred, it is difficult to evaluate the state unless the location of the occurrence can be specified. In particular, in the case of a large structure, it is important to specify a damaged portion.
For this reason, the sound source position at which the primary AE has occurred is located from the primary AE detected by the elastic wave detection device 1 (step S2).
This sound source position performs the above-described wavelet transform on the detected primary AE, extracts a wave of a specific frequency, and locates the sound source position based on the extracted information. The method for locating the sound source position will be further described later.
[0039]
After detecting that damage such as corrosion, minute cracks, and fatigue fracture has occurred in the structure, it is necessary to evaluate the state. That is, it is necessary to determine a countermeasure according to the degree of damage.
For this reason, the directional ultrasonic wave is excited toward the located sound source position (damage position) on the inspected object, and is the ultrasonic wave transmitted through the inspected object or the ultrasonic wave reflected by the inspected object. The secondary AE is detected by the above-described elastic wave detection device 1 (step S3).
Specifically, for example, a steel plate of a structure such as a tank loses its thickness when corroded. In order to evaluate the state of the thinned portion, a directional ultrasonic wave (plate) is applied to the thinned portion of the steel plate. (Wave) is excited, and the ultrasonic wave reflected by the thinned portion is detected by the elastic wave detecting device 1 described above.
[0040]
Next, the state near the sound source position is analyzed based on the detected secondary AE (step S4). By analyzing the state near the sound source position, the damage state near the sound source position can be evaluated. Since the secondary AE, which is an ultrasonic wave transmitted through the inspection object or an ultrasonic wave reflected by the inspection object, includes information on the state of the damaged portion, the information is extracted by a predetermined process.
[0041]
For example, when ultrasonic waves (plate waves) are excited along a steel sheet in a place where the thinning due to corrosion of the steel sheet occurs, the ultrasonic waves, which are the secondary AEs, are reflected at the thinned part, so that this reflected wave is detected. By doing so, the state of thinning can be estimated.
[0042]
Here, a description will be given of the result of an experiment performed to determine whether the acoustic wave detection device 1 can excite ultrasonic waves along a steel plate and detect reflected waves of the ultrasonic waves.
FIG. 8 is a perspective view showing a schematic configuration of an experimental system that excites directional ultrasonic waves and generates reflected waves.
In FIG. 8, the sensor optical fiber 5 was installed on the surface of the steel plate PL so as to cross in the longitudinal direction, and one end face Ef1 in the longitudinal direction of the steel plate PL was hit with a Schmitt hammer to excite Lamb waves.
Note that the steel plate PL has a size of 460 mm × 1860 mm × 6 mm, and a coated fiber was used as the sensor optical fiber 5, and this was bonded to the position 300 mm from the end face Ef with silicon grease. For reference, an AE sensor 200 (PICO sensor manufactured by PAC, 540 kHz resonance type) was installed at a position 30 mm from the end face Ef to detect a Lamb wave.
[0043]
9 is a diagram for explaining a propagation path of a Lamb wave excited in the steel plate PL, FIG. 10 is a graph showing a detection waveform detected by the elastic wave detection device 1, and FIG. 6 is a graph showing a detection waveform.
In FIG. 9, the Lamb wave excited on the steel plate PL is represented by S₀Then S₀R0 is Lamb wave S₀Is a reflected wave reflected by the other end surface Ef2 of the steel plate PL and incident on the optical fiber for sensor 5.
S₀R1 is the reflected wave S₀R0 is a reflected wave further reflected on one end face Ef1 of the steel plate PL and incident on the sensor optical fiber 5.
S₀R2 is the reflected wave S₀R1 is a reflected wave further reflected by the other end surface Ef2 of the steel plate PL and incident on the sensor optical fiber 5.
S₀R3 is the reflected wave S₀R2 is a reflected wave further reflected on one end face Ef1 of the steel plate PL and incident on the sensor optical fiber 5.
[0044]
As can be seen from FIG. 10, the elastic wave detecting device 1₀R0-S₀It can be seen that detection can be reliably made up to R3.
On the other hand, as can be seen from FIG.₀R0-S₀R3 cannot be clearly detected.
[0045]
As described above, according to the elastic wave detection device 1, a reflected wave of the secondary AE can be detected with high sensitivity.
[0046]
Next, a method of locating the sound source position by the elastic wave detection device 1 will be described.
FIG. 12 is a diagram showing a method for installing the sensor optical fiber 5 for the object to be inspected for locating the sound source position.
When the inspection object W is a rectangular steel plate, a series of sensor optical fibers 5 are bent at a plurality of positions as shown in FIG. The fiber 5 is traversed at a plurality of positions (three places), and the traversed portion (opposed portion) of the sensor optical fiber 5 is fixed to the inspection object W by a fixing means such as bonding.
Each transverse portion of the sensor optical fiber 5 with respect to the inspection object W is defined as sensor units Sa, Sb, Sc for detecting AE.
The sensor portions Sa, Sb, Sc are opposed to each other, and the distance Gab between the sensor portions Sa and Sb is different from the distance Gbc between the sensor portions Sb and Sa.
[0047]
Since the sensor units Sa, Sb, Sc are located at different positions, and the interval Gab and the interval Gbc are different from each other, the AE of the specific frequency of the specific mode generated from the same sound source position is determined by the position of the sound source position. , The sensors arrive at the sensor units Sa, Sb, Sc at different times.
Therefore, the sound source position can be specified by detecting the arrival time difference of the AE of the specific frequency in the specific mode among the sensor units Sa, Sb, Sc.
[0048]
FIG. 13 is a diagram illustrating a sound source position on the inspection target object W.
It is assumed that AE is generated from the sound source position A as shown in FIG. 13 in a state where the sensor optical fiber 5 is installed on the inspection object W.
The sound source position A is located at a distance La from the sensor unit Sa, a distance Lb from the sensor unit Sb, and a distance Lc from the sensor unit Sc.
[0049]
For example, assuming that the sound source position A is between the sensor units Sa and Sb and is located closer to the sensor unit Sa, the AE from the sound source position A is in the order of the sensor unit Sa, the sensor unit Sb, and the sensor unit Sc. To reach. That is, the arrival order is determined according to the propagation distance from the sound source position A.
[0050]
Assuming that the arrival time difference of the AE to each of the sensor units Sa, Sb, Sc is ΔT1, ΔT2 (absolute value),
ΔT1: ΔT2 = (Lb−La): (Lc−Lb) (1)
Holds.
[0051]
Further, since there is a relationship of Gab = La + Lb, Gbc = Lc−Lb, and Gab and Gbc are known values, if the arrival time differences ΔT1 and ΔT2 are obtained, the distances La, Lb, and Lc can be specified, and the sound source can be specified. Position A can be located.
[0052]
Next, a description will be given of the result of actually generating the sound source position A when Gab is set to 500 mm, Gbc is set to 300 mm, La is set to 100 mm, Lb is set to 400 mm, and Lc is set to 700 mm.
FIG. 14A is a graph showing a detection waveform of the elastic wave detection device 1 when AE is generated at the sound source position A, and FIG. 14B is a graph showing a time-lapse of a wavelet when the detection waveform of FIG. It is a graph which shows a change.
From FIG. 14B, the arrival time of the AE is 0.71 ms, the second arrival time is 2.33 ms, and the third arrival time is 3.85 ms.
[0053]
Under the above conditions, (Lb-La) :( Lc-Lb) is 300: 300. ΔT1 = 1.62 ms and ΔT2 = 1.52 ms.
Therefore, ΔT1: ΔT2 = 1.62: 1.52, which is approximately 1: 1.
That is, the above equation (1) substantially holds. From this, if the arrival time differences ΔT1 and ΔT2 can be detected, the sound source position can be located.
[0054]
The method for locating a sound source described above can be used, for example, for locating a sound source due to damage to a pipe. One sensor optical fiber 5 is wound along the longitudinal direction at different intervals around the pipe to form a plurality of sensor sections, and the arrival time of the AE generated by damage to the pipe to each sensor section is determined. By measuring the time difference, the damaged position of the pipe can be located.
[0055]
Next, an applicable object of the elastic wave detection method using the elastic wave detection device 1 according to the present embodiment will be described.
For example, the present invention can be applied to detection of AE generated by corrosion of a bottom plate of a tank. As described above, after detecting the primary AE generated by corrosion from the bottom plate of the tank, the sound source position is located, the secondary AE having directivity toward the sound source position is actively excited, and the reflected wave is detected. To evaluate the state of bottom wall thinning.
Inspection of thinning due to corrosion of the bottom plate of the tank is usually performed by removing the contents of the tank and then measuring the fixed point with an ultrasonic thickness gauge.To measure, remove the contents, clean the tank, Much time and money are required for fixed point measurement.
For this reason, by installing the sensor optical fiber 5 of the elastic wave detection device 1 according to the present embodiment on the bottom plate, it is not necessary to remove the contents or clean the tank, and to evaluate the state of the thinning of the bottom plate. Can be.
[0056]
Also, for example, a structure using steel materials such as bridges and pipes, ships and marine structures, buildings and tunnels, ships and marine structures (for example, oil drilling platforms), and the like, where the steel materials may deteriorate due to corrosion. If so, it is applicable.
[0057]
In addition, if the sensor optical fiber 5 of the elastic wave detection device 1 is installed on the body, wings, pressure bulkheads, and the like during the flight of the aircraft, the fatigue state of the aircraft can be constantly monitored.
[0058]
Further, for example, if the sensor optical fiber 5 of the elastic wave detection device 1 is installed in a structure that is easily degraded not only by corrosion but also by radioactivity, such as a nuclear power generation facility or a high radioactivity level waste liquid tank, the destruction by radioactivity can occur. Can be monitored.
[0059]
In the present embodiment, a case has been described where the primary AE generated from the inspected object is detected, and then the secondary AE is excited to evaluate the damage state. However, the configuration may be such that the primary AE is simply detected. Alternatively, the secondary AE may be excited from the beginning to detect the secondary AE.
[0060]
Second embodiment
FIG. 15 is a diagram showing a configuration of an elastic wave detection device 300 according to another embodiment of the present invention. Note that the same components as those of the elastic wave detection device 1 according to the first embodiment are denoted by the same reference numerals.
The elastic wave detection device 300 according to the present embodiment is different from the elastic wave detection device 1 according to the first embodiment in that the elastic wave detection device 1 does not use the optical fiber 6 for reference light, This is the point that the optical fibers for sensors 5A and 5B are used. That is, in this embodiment, two optical fibers are used for AE detection.
[0061]
The sensor optical fibers 5A and 5B have different frequency characteristics.
FIGS. 16A and 16B are diagrams showing a cross-sectional structure of the sensor optical fibers 5A and 5B. FIG. 16A shows the structure of the sensor optical fiber 5B, and FIG. 16B shows the structure of the sensor optical fiber 5A. ing.
The sensor optical fibers 5A and 5B have the same bare fiber BF, and each bare fiber BF is coated with a different coating CTa and CTb, respectively.
The coating CTa is made of, for example, PVC, and the coating CTb is made of, for example, PMMA. The coatings CTa and CTb have different thicknesses.
The sensor optical fibers 5A and 5B have different frequency characteristics due to different coating materials and film thicknesses.
[0062]
In the present embodiment, as shown in FIG. 15, the sensor optical fibers 5A and 5B are installed on the inspection object W, respectively.
First, the results of examining the detection sensitivity and frequency characteristics of the sensor optical fibers 5A and 5B will be described.
The measurement conditions were as follows. Single-mode fiber was used for the bare fiber BF having a diameter of 125 μm of the sensor optical fibers 5A and 5B, PVC having an outer diameter of 0.9 mm was used for the coating CTa, and PMMA having an outer diameter of 0.25 mm was used for the coating CTb.
A 1200 mm × 1200 mm × 0.9 mm steel plate was used as the inspection object W, and the sensor optical fibers 5 </ b> A and 5 </ b> B were installed on the surface of the steel plate at a distance of 700 mm.
[0063]
In FIG. 17, (a) is a graph showing an AE detection waveform of the optical fiber for sensor coated with PVC, (b) is a graph showing a power spectrum of the detection wave of (a), and (c) is a graph. It is a graph which shows the detection waveform of AE of the optical fiber for sensors coated with PMMA, and (d) is a graph which shows the power spectrum of the detection wave of (c).
As can be seen from FIG. 17, in the frequency band of 10 kHz or less, the sensitivity of the sensor coated optical fiber 5B coated with PVC is higher, and above 10 kHz, the sensitivity of the sensor optical fiber 5A coated with PMMA is higher.
[0064]
Next, the sound source position SA was located by utilizing the difference in sensitivity between the above-mentioned optical fiber for sensor 5B coated with PVC and the optical fiber for sensor 5A coated with PMMA.
The primary AE was generated by changing the distance L1 between the sound source position SA and the sensor optical fiber 5A at intervals of 100 mm, and bending the mechanical pencil core at the sound source position SA.
Wavelet transform was performed on the detection wave composed of two wave packets, and the sound source position SA was located from the time difference between the maximum peaks of the frequency component of 10 kHz (wave in the specific mode). Since the frequency sensitivity at 10 kHz is higher in the sensor optical fiber 5A coated with PMMA, the peak of the wavelet intensity is larger in the sensor optical fiber 5A coated with PMMA.
[0065]
FIG. 18 is a graph showing the orientation results.
As can be seen from FIG. 18, the location where the sound source position SA was located was substantially accurate, and the error in the location was 5.5% at the maximum.
[0066]
As described above, according to the present embodiment, the position of the sound source can be located by using the two optical fibers for detecting the AE and detecting the arrival time difference of the wave of the specific frequency from the sound source. More measurements can be made.
By making the frequency characteristics of the two sensor optical fibers 5A and 5B different, the sound source position SA can be uniquely located. That is, if the frequency characteristics of the two sensor optical fibers 5A and 5B are the same, there is no information as to which of the sensor optical fibers 5A and 5B is closer to the sound source position SA. When locating, two locating positions are obtained. However, since the difference in frequency characteristics is information as to which of the sensor optical fibers 5A and 5B is closer to the sound source position SA, the sound source position SA is uniquely located. It becomes possible to do.
In order to determine the position in a two-dimensional plane in an actual structure, two sets of sensor optical fibers 5A and 5B are installed in directions orthogonal to each other, and a sound source is placed between the opposing sensor optical fibers 5A and 5B. It is sufficient to locate each position.
[0067]
Third embodiment
In the above-described first embodiment, a case has been described in which a Lamb wave as a secondary AE having directivity to a test object is excited in a steel plate using a Schmitt hammer.
In the present embodiment, a method will be described in which an ultrasonic wave having directivity is excited on an object to be inspected by a pulsed YAG laser instead of a mechanical shock such as a Schmitt hammer, and the ultrasonic wave is detected.
[0068]
FIG. 19 is a diagram for explaining an ultrasonic excitation method and a detection method according to the present embodiment.
As shown in FIG. 19, first, the object to be inspected is a state in which the sensor optical fiber 5 of the elastic wave detection device 1 according to the above-described first embodiment is curved in a circular shape (with a radius of 50 mm) with one turn. Install on W
As the inspection object W, for example, an aluminum plate having a thickness of 50 mm is used.
Silicon oil is applied to the surface of the inspection object W.
[0069]
The center position of a circle formed by the sensor optical fiber 5 on the surface of the inspection object W in the above state is defined as a sound source position SB, and the sound source position SB is, for example, 5 mJ via a multi-mode optical fiber (not shown). Of the YAG pulse laser LY.
When the YAG pulse laser LY is irradiated to the sound source position SB, the Rayleigh wave is excited by the breakdown of the silicon oil.
[0070]
20 and 21 show results of detection of Rayleigh waves by the elastic wave detection device 1.
FIG. 20 is a graph showing the waveform of the detected wave, and FIG. 21 is a graph showing the power spectrum of the detected wave.
As can be seen from FIGS. 20 and 21, since the elastic wave detection device 1 has extremely high sensitivity, it can be understood that a Rayleigh wave in a frequency band of 100 kHz to 800 kHz can be detected.
[0071]
Since the Rayleigh wave excited by the point-like sound source propagates in a cylindrical shape, the Rayleigh wave in a high frequency band can be detected by making the sensor optical fiber circular.
[0072]
【The invention's effect】
ADVANTAGE OF THE INVENTION According to this invention, AE which generate | occur | produces in a structure using the optical fiber or AE which excited the structure can be detected with high detection accuracy.
[Brief description of the drawings]
FIG. 1 is a configuration diagram of an elastic wave detection device according to an embodiment of the present invention.
FIG. 2 is a perspective view showing a state in which the elastic wave detection device is installed on an object to be inspected.
FIG. 3 is a graph showing a waveform of a secondary AE detected by actually dropping the steel ball B shown in FIG. 2 on the surface of the inspection object W;
FIG. 4 is a graph showing an AE waveform detected by bending the lead of a mechanical pencil at a sound source position S of a test object W;
FIG. 5 is a wavelet group velocity diagram obtained by performing a wavelet transform process on a detected wave in a processing device.
FIG. 6 is a diagram illustrating an example of a configuration of an elastic wave detection device based on a Michelson interferometer.
FIG. 7 is a flowchart illustrating a procedure of an elastic wave detection method according to the first embodiment of the present invention.
FIG. 8 is a perspective view showing a schematic configuration of an experimental system for exciting a directional ultrasonic wave and generating a reflected wave.
FIG. 9 is a diagram for explaining a propagation path of a Lamb wave excited in the steel plate PL.
FIG. 10 is a graph showing a detection waveform detected by the elastic wave detection device.
FIG. 11 is a graph showing a detection waveform detected by the AE sensor.
FIG. 12 is a diagram showing a method for installing a sensor optical fiber for an object to be inspected for locating a sound source position.
FIG. 13 is a diagram showing a sound source position on the inspection object.
14A is a graph showing a detected waveform when an AE is generated at a sound source position A, and FIG. 14B is a graph showing a temporal change of a wavelet when the detected waveform of FIG. It is.
FIG. 15 is a diagram illustrating a configuration of an elastic wave detection device according to a second embodiment of the present invention.
FIG. 16 is a diagram showing a cross-sectional structure of two optical fibers for sensors.
17A is a graph showing a detected waveform of AE of an optical fiber for a sensor coated with PVC, FIG. 17B is a graph showing a power spectrum of the detected wave of FIG. 17A, and FIG. It is a graph which shows the detection waveform of AE of the optical fiber for sensors coated with PMMA, and (d) is a graph which shows the power spectrum of the detection wave of (c).
FIG. 18 is a graph showing the orientation result.
FIG. 19 is a diagram for explaining an ultrasonic wave excitation method and a detection method according to a third embodiment of the present invention.
FIG. 20 is a graph showing a waveform of a Rayleigh wave detected by the elastic wave detection device.
FIG. 21 is a graph showing a power spectrum of a detection wave detected by the elastic wave detection device.
[Explanation of symbols]
1,300 ... elastic wave detector
2. Light source
3. Polarizer
4: Splitter
5. Optical fiber for reference light
6 Optical fiber for sensor
7 ... Coupling device
8. Photodetector
9 ... Processing device

Claims (13)

At least one is based on the light intensity of superimposed light of light split from a single light source guided from one end of the first and second optical fibers installed to the inspection object to the other end. Monitoring the elastic waves generated in the inspection object,
When detecting an elastic wave generated in the object to be inspected, the elastic wave is excited in the object to be inspected,
Based on the light intensity of the superimposed light, the excited elastic wave is detected,
An inspection method for inspecting a state of the inspection object based on the detected elastic wave. Extracting time and frequency information from the light intensity information,
The inspection method according to claim 1, wherein a source of the elastic wave is located based on the time and frequency information. The inspection method according to claim 2, wherein an elastic wave having directivity is excited toward the specified generation source. Excitation of elastic waves to the inspected object,
At least one of the first and second optical fibers installed with respect to the inspection object has a light intensity of a superimposed light of light split from a single light source guided from one end to the other end. Based on the elastic wave,
An inspection method for inspecting a state of the inspection object based on the detected elastic wave. The inspection method according to claim 4, wherein the inspection object is irradiated with a laser beam to excite an elastic wave. Apply silicone oil to the surface of the inspection object,
Irradiating the surface of the object to be inspected with laser light,
The inspection method according to claim 5, wherein a Rayleigh wave is excited by the breakdown of the silicon oil. The inspection method according to claim 6, wherein one of the first and second optical fibers is disposed on a surface of the inspection object so as to surround a periphery of the irradiation position of the laser light. At least one of the first and second optical fibers is installed on the inspection object,
An elastic wave generated from the object to be inspected based on the light intensity of a superimposed light of light split from a single light source guided from one end of the first and second optical fibers to the other end. Elastic wave detection method for detecting a wave. Extracting time and frequency information from the light intensity information,
9. The elastic wave detection method according to claim 8, wherein a generation source of the elastic wave is located based on the time and frequency information. Making the frequency characteristics of the first and second optical fibers different,
Setting both the first and second optical fibers with respect to the inspection object;
Extracting time and frequency information from the light intensity information,
10. The elastic wave detection method according to claim 9, wherein a generation source of the elastic wave is located based on the time and frequency information. At least one of the first and second optical fibers is curved such that at least three opposing portions oppose each other, and the distance between the opposing portions is different from each other. Installed in
9. The elastic wave according to claim 8, wherein a source of the elastic wave is located based on a difference in arrival time of the elastic wave which is generated on the inspection object and reaches each of the facing portions, which is obtained from the time and frequency information. Detection method. A single light source,
A splitter for dispersing light from a single light source,
At least one of the first and second optical fibers is provided for the object to be inspected, and guides light split by the splitter.
Coupling means for superimposing light guided from one end of the first and second optical fibers toward the other end;
A photodetector for detecting the light intensity of the superimposed light;
An elastic wave detection device comprising: a processing unit that extracts time and frequency information from a detection signal of the photodetector and detects an elastic wave generated in the inspection object from the information. Further comprising a polarizer for controlling the polarization direction of the light from the single light source,
The splitter stores a polarization direction of light from the polarizer,
13. The elastic wave detection device according to claim 12, wherein the first and second optical fibers are polarization maintaining optical fibers that preserve a polarization direction of light split by the splitter.

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