All-fiber ultrasonic metal plate defect detection device and method based on photoinduced ultrasound

Abstract

The present invention discloses an all-fiber ultrasonic metal plate defect detection device based on photoultrasound, which includes a first laser, an ultrasonic excitation fiber, a light trap, a second laser, a fiber Bragg grating, a photodetector and a signal processing device. The ultrasonic excitation fiber is fixed on the metal plate to be tested, and the fiber Bragg grating is mounted on the metal plate to be tested. The ultrasonic pulse energy generated by the ultrasonic excitation fiber of the present invention is stronger, so the fiber Bragg grating can be used to directly detect the ultrasonic pulse, realizing all-fiber photoultrasonic detection, and expanding the application scenarios of all-fiber photoultrasonic detection; the present invention is a solution for all-fiber photoultrasonic excitation and detection, which can be surface-mounted or internally embedded in the object to be tested to realize online detection, thereby improving the flexibility of ultrasonic detection.

Description

Technical Field

[0001] The present invention relates to the technical field of photo-induced ultrasonic non-destructive testing, and in particular to an all-fiber ultrasonic metal plate defect detection device and method based on photo-induced ultrasonic testing. Background Art

[0002] Ultrasonic testing devices are widely used in industrial flaw detection applications such as steel and aviation due to their safety, excellent penetration, and high detection speed. Traditional ultrasonic flaw detection relies on ultrasonic probes, which are often made of piezoelectric materials. However, piezoelectric materials have poor electromagnetic interference resistance, narrow bandwidth, and difficulty balancing sensitivity. Furthermore, their low level of automation makes online flaw detection difficult.

[0003] All-fiber ultrasonic detection devices based on photo-induced ultrasound have excellent resistance to electromagnetic interference, and the demodulation method of fiber Bragg gratings combined with edge filtering can ensure high sensitivity under wide bandwidth conditions. So far, existing photo-induced ultrasound detection devices have only been based on the principle of photo-induced ultrasound to excite ultrasound or use free-space light to achieve ultrasound excitation and detection, but they cannot achieve easy-to-deploy all-fiber ultrasonic detection. Ultrasonic detection devices using free-space light have difficulty in detecting and locating multi-structure damage, and the difficulty in achieving buried deployment limits their application scenarios.

[0004] Currently, existing photo-induced ultrasound detection devices rely solely on the principle of photo-induced ultrasound to excite ultrasound or utilize free-space light for both excitation and detection. However, these devices lack the ability to easily deploy all-fiber ultrasonic testing. Ultrasonic detection devices utilizing free-space light struggle to detect and locate damage in multiple structures, and the difficulty of achieving embedded deployment limits their application scenarios. Summary of the Invention

[0005] The purpose of the present invention is to provide an all-fiber ultrasonic detection device and method based on photoultrasound. The ultrasonic pulse energy generated by the ultrasonic excitation optical fiber of the present invention is stronger, so the ultrasonic pulse can be directly detected using a fiber Bragg grating, realizing all-fiber photoultrasound detection and expanding the application scenarios of all-fiber photoultrasound detection. The present invention is a solution for all-fiber photoultrasound excitation and detection, which can be surface-mounted or internally embedded in the object to be tested to realize online detection and improve the flexibility of ultrasonic detection.

[0006] To achieve this purpose, the present invention is designed to implement an all-fiber ultrasonic metal plate defect detection device based on photo-induced ultrasound, which includes a first laser, an ultrasonic excitation fiber, a light trap, a second laser, a fiber Bragg grating, a photodetector, and a signal processing device. The ultrasonic excitation fiber is fixed to the metal plate to be tested, and the fiber Bragg grating is mounted on the metal plate to be tested. The first laser is used to output pulsed laser light. A portion of the pulsed laser light in the ultrasonic excitation fiber passes through the ultrasonic excitation fiber and is absorbed by the light trap. Another portion of the pulsed laser light in the ultrasonic excitation fiber is coupled into the ultrasonic excitation fiber cladding. A portion of the light coupled into the ultrasonic excitation fiber cladding is absorbed by the light trap, and another portion of the light coupled into the ultrasonic excitation fiber cladding is converted into heat energy, generating ultrasonic pulses through the thermoelastic effect. The ultrasonic pulses generated by the ultrasonic excitation fiber are scattered when they encounter defects in the metal plate to be tested.

[0007] The second laser is used to output continuous light to the fiber Bragg grating. The vibration caused by the ultrasonic pulse propagating in the metal plate under test causes the wavelength of the continuous light propagating and reflected in the fiber Bragg grating to change, forming a metal plate detection optical signal.

[0008] The photoelectric detector converts the metal plate detection light signal into a corresponding metal plate detection electrical signal, and the signal processing device is used to determine whether the metal plate under test has defects by judging whether the metal plate detection electrical signal contains the scattered signal.

[0009] Beneficial effects of the present invention:

[0010] The present invention adheres an ultrasonic excitation optical fiber to the metal plate to be tested. The ultrasonic excitation optical fiber and pulsed laser generate ultrasonic pulses for detection. Compared with the ultrasonic pulses generated by piezoelectric materials in the prior art, the pulse time is shorter; compared with the ultrasound generated by the free-space photo-ultrasound scheme in the prior art, the peak-to-peak value is larger and the energy is stronger, thereby improving the sensitivity and spatial resolution of ultrasonic detection. Compared with the scheme of optical fiber photo-ultrasound excitation and piezoelectric material combined detection in the prior art, the present invention improves the efficiency of converting light energy into heat energy and generates more powerful ultrasonic pulses. Therefore, the ultrasonic pulse can be directly detected using a fiber Bragg grating, realizing all-fiber photo-ultrasound detection and expanding the application scenarios of all-fiber photo-ultrasound detection. The present invention is a scheme for all-fiber photo-ultrasound excitation and detection, which can be surface-mounted or internally embedded in the object to be tested to achieve online detection and improve the flexibility of ultrasonic detection. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 It is a structural schematic diagram of the present invention;

[0012] Figure 2 Schematic diagram of the preparation process of the ultrasonic excitation optical fiber in the present invention;

[0013] Figure 3The time domain waveform of the high-frequency mode ultrasonic pulse signal after signal processing of the present invention;

[0014] Figure 4 is the group velocity dispersion curve of the object to be measured in the present invention;

[0015] Figure 5 Schematic diagram of the defect location algorithm in the present invention;

[0016] Among them, 1 is the first laser, 2 is the fiber jumper, 3 is the fiber adapter, 4 is the bare fiber, 5 is the ultrasonic excitation fiber, 5.1 is the bare fiber, 5.2 is the bare fiber with a spherical end face, 6 is the light trap, 7 is the second laser, 8 is the single-mode fiber, 9 is the fiber circulator, 10 is the fiber Bragg grating, 11 is the photodetector, 12 is the power amplifier, 13 is the signal collector, 14 is the signal processing equipment, and 15 is the metal plate to be measured. DETAILED DESCRIPTION

[0017] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments:

[0018] like Figures 1 to 5 The all-fiber ultrasonic metal plate defect detection device shown in the figure based on photo-induced ultrasound includes a first laser 1, an ultrasonic excitation fiber 5, a light trap 6, a second laser 7, a fiber Bragg grating 10, a photodetector 11, and a signal processing device 14. The ultrasonic excitation fiber 5 is fixed to a metal plate 15 (aluminum plate) to be tested, and the fiber Bragg grating 10 is mounted on the metal plate 15 to be tested. The first laser 1 is used to output pulsed laser light. A portion of the pulsed laser light in the ultrasonic excitation fiber 5 passes through the ultrasonic excitation fiber 5 and is absorbed by the light trap 6. Another portion of the pulsed laser light in the ultrasonic excitation fiber 5 is coupled into the cladding of the ultrasonic excitation fiber 5. A portion of the light coupled into the cladding of the ultrasonic excitation fiber 5 is absorbed by the light trap 6. Another portion of the light coupled into the cladding of the ultrasonic excitation fiber 5 is converted into thermal energy, and an ultrasonic pulse is generated through the thermoelastic effect (the laser pulse is absorbed by the thermoelastic effect and converted into thermal energy, which is converted into mechanical energy to generate ultrasound). The ultrasonic pulse generated by the ultrasonic excitation fiber 5 will scatter when it encounters defects (loss defects) in the metal plate 15 to be tested. This design can achieve structural health detection of the plate.

[0019] The second laser 7 is used to output continuous light to the fiber Bragg grating 10. The vibration caused by the ultrasonic pulse propagating in the metal plate 15 to be tested causes the wavelength of the continuous light propagating and reflected in the fiber Bragg grating 10 to change, forming a metal plate detection optical signal;

[0020] The photodetector 11 converts the metal plate detection optical signal into a corresponding metal plate detection electrical signal. The signal processing device 14 is used to determine whether the metal plate 15 under test has a defect by determining whether the metal plate detection electrical signal contains the scattered signal. The collected signal is subtracted from the metal plate detection reference signal. If the difference does not contain a wave packet with a peak-to-peak value greater than 10mV, it indicates that there is no defect. Otherwise, it indicates that there is a defect.

[0021] In the above technical solution, after the signal processing device 14 determines that the metal plate 15 under test has a defect, it locates the defect position in the following manner:

[0022] The signal processing device 14 performs high-pass filtering on the metal plate detection electrical signal to obtain a high-frequency mode signal (100kHz to 1MHz), multiplies the frequency of the high-frequency mode signal by the thickness of the metal plate 15 to obtain a frequency-thickness product, and determines the group velocity of the high-frequency mode signal by the frequency-thickness product. The group velocity is obtained by applying the calculated frequency-thickness product to the group velocity dispersion curve of the object to be measured ( Figure 4 ) find the corresponding group velocity value of the S0 mode in the ultrasonic excitation optical fiber 5, the group velocity dispersion curve of the object to be measured is calculated by the material parameters of the aluminum plate through the Matlab program; the ultrasonic pulse generated by the ultrasonic excitation optical fiber 5 will be scattered when encountering the defects of the metal plate 15 to be measured, and the metal plate detection electrical signal received by the signal processing device 14 through the fiber Bragg grating 10 contains this scattered signal. The metal plate detection electrical signal containing the scattered signal is compared with the metal plate detection reference signal to extract the scattered signal. The time period from the generation of the scattered signal to the peak of the scattered signal is the scattering signal flight time. The product of the group velocity and the scattering signal flight time is the flight distance of the scattered signal from the excitation node of the ultrasonic excitation optical fiber 5 through the defect to the fiber Bragg grating 10. The excitation node position of the ultrasonic excitation optical fiber 5 and the fiber Bragg grating 10 at this time are used as the focus, and the scattering signal flight distance is used as the major axis to form a first ellipse;

[0023] The metal plate detection reference signal is obtained by using the device to pre-detect a defect-free metal plate; the pre-detection process is that the vibration caused by the ultrasonic pulse propagating in the defect-free metal plate causes the wavelength of the continuous light propagating and reflected in the fiber Bragg grating 10 to change, thereby forming the metal plate detection reference signal;

[0024] The position where the fiber Bragg grating 10 is mounted on the metal plate 15 to be tested is changed, and the flight distance of the scattered signal from the excitation node of the ultrasonic excitation fiber 5 through the defect to the fiber Bragg grating 10 after the mounting position of the fiber Bragg grating 10 is changed is obtained. A second ellipse is formed with the excitation node position of the ultrasonic excitation fiber 5 and the position of the fiber Bragg grating 10 at this time as the focus and the flight distance of the scattered signal at this time as the major axis;

[0025] The position where the fiber Bragg grating 10 is mounted on the metal plate 15 to be tested is changed again, and the flight distance of the scattered signal from the excitation node of the ultrasonic excitation fiber 5 through the defect to the fiber Bragg grating 10 after the mounting position of the fiber Bragg grating 10 is obtained. A third ellipse is formed with the excitation node position of the ultrasonic excitation fiber 5 and the position of the fiber Bragg grating 10 at this time as the focus and the flight distance of the scattered signal at this time as the major axis;

[0026] The intersection area of ​​the first ellipse, the second ellipse and the third ellipse is the defect position of the metal plate 15 to be tested.

[0027] The time from the excitation node of the ultrasonic excitation fiber 5 through the node (x, y) to the fiber Bragg grating 10 is expressed as follows:

[0028]

[0029] in, is the coordinate of the excitation node of the ultrasonic excitation fiber 5, is the coordinate of the position where the kth fiber Bragg grating 10 is mounted on the metal plate 15 to be measured, c g is the Lamb wave group velocity. The group velocity indicates the propagation speed of the wave packet, that is, the speed at which the Lamb wave propagates in the test piece. If the node (x, y) is close to the defect position, the envelope amplitude of the corresponding difference signal is large. The difference signal is the scattered signal mentioned above. If the node (x, y) is far from the defect position, the envelope amplitude of the corresponding difference signal is small.

[0030] The weighted amplitude is calculated using full amplitude addition, and the expression is as follows:

[0031]

[0032] Where I(x, y) represents the weighted amplitude at the node (x, y), and the scattering signal data collected corresponding to the positions where N fiber Bragg gratings 10 are mounted on the metal plate 15 to be tested are calculated, r k (t k (x, y)) represents the envelope amplitude of the difference signal (scattering signal) corresponding to the position of the k-th fiber Bragg grating 10 mounted on the metal plate 15 to be tested, and the defect position is located according to the size of the envelope amplitude.

[0033] The weighted amplitudes corresponding to the positions where all fiber Bragg gratings 10 are mounted on the metal plate 15 to be tested are normalized, and the located defect positions are finally presented in the form of an amplitude map. The area with the maximum amplitude in the image is determined to be the defect position. If there is no area with an amplitude exceeding 0.9, it is determined that there is no defect.

[0034] The above technical solution also includes a fiber optic patch cord 2 (large-core fiber optic patch cord), a fiber optic adapter 3, and a bare fiber 4 (large-core bare fiber). The pulsed laser light output by the first laser 1 sequentially passes through the fiber optic patch cord 2, the fiber optic adapter 3, and the bare fiber 4 into the ultrasonic excitation fiber 5. The core diameters of the large-core fiber optic patch cord and the large-core bare fiber range from 400 to 1000 μm.

[0035] In the above technical solution, the ultrasonic excitation optical fiber 5 is prepared on the outgoing light path of the bare optical fiber 4.

[0036] In the above technical solution, the ultrasonic excitation fiber 5 is fixed to the metal plate 15 being tested using a mixed epoxy resin and graphite adhesive. Epoxy resin has a certain mechanical strength and provides good adhesion properties, while graphite helps cure the epoxy resin and has high-temperature resistance. The combination of the ultrasonic excitation fiber 5 and the mixed epoxy resin and graphite adhesive improves the efficiency of converting light energy into heat energy, and the generated ultrasonic pulse energy is stronger.

[0037] The above technical solution also includes a single-mode optical fiber 8 (core 8-9 μm, cladding 125 μm, wavelength 1550 nm), an optical fiber circulator 9, a power amplifier 12 and a signal collector 13. The continuous light output by the second laser 7 passes through the single-mode optical fiber 8 and the optical fiber circulator 9 to the fiber Bragg grating 10. The vibration caused by the ultrasonic pulse propagating in the metal plate 15 to be tested causes the wavelength of the continuous light propagated and reflected in the fiber Bragg grating 10 to change, forming a metal plate detection light signal. The metal plate detection light signal is transmitted to the photodetector 11 through the optical fiber circulator 9. The photodetector 11 converts the metal plate detection light signal into a corresponding metal plate detection electrical signal. The power amplifier 12 amplifies the metal plate detection electrical signal to achieve 0-20 dB amplification. The signal collector 13 collects the power-amplified metal plate detection electrical signal and transmits it to the signal processing device 14. The core diameters of the optical fiber jumper 2 and the bare optical fiber 4 are 400-1000 μm, and the core diameter of the single-mode optical fiber 8 is 8-9 μm. In the above technical solution, the energy of the pulsed laser absorbed by the light trap 6 is less than 20% of the total energy of the pulsed laser.

[0038] In the above technical solution, the preparation method of the ultrasonic excitation optical fiber 5 is as follows: prepare two sections of bare optical fibers 5.1 with flat end faces, use arc melting for 9 to 10 seconds to obtain two sections of bare optical fibers 5.2 with spherical end faces, use an optical fiber processing platform to align the two sections of bare optical fibers 5.2 with spherical end faces in the horizontal and vertical directions, so that the two spherical surfaces are just connected, use arc pre-fusion for 7 to 8 seconds, and then arc fusion for 5 to 6 seconds. During the fusion process, the optical fibers at both ends are simultaneously pushed toward the middle by 20 to 80 μm to obtain the ultrasonic excitation optical fiber 5.

[0039] In the above technical solution, the diameter of the sphere in the spherical bare optical fiber 5.2 is 1 to 1.5 times the core diameter of the bare optical fiber 5.1, and the distance between the two spheres is 0.1 to 0.5 times the diameter of the sphere.

[0040] In the above technical solution, the ultrasonic pulse width is 10 to 400 μs, and the ultrasonic pulse frequency range is 30 to 2000 kHz;

[0041] In the above technical solution, when the pulse energy of the first laser 1 is 10-15 mJ, the pulse width is 5-20 ns, and the optical power of the second laser 7 is 5-10 dBm, the peak-to-peak value range of the ultrasonic pulse not amplified by the power amplifier is 20-60 mV, the continuous light wavelength range generated by the second laser 7 is 1260-1675 nm, and the optical power is greater than -20 dBm;

[0042] In the above technical solution, the coupling efficiency between the first laser 1 and the optical fiber jumper 2 is greater than 90%;

[0043] In the above technical solution, the central wavelength range of the fiber Bragg grating 10 is 1260-1675 nm, and the bandwidth is 0.2-0.4 nm;

[0044] In the above technical solution, the bandwidth of the photodetector 11 is 30 kHz to 1.6 GHz, and the wavelength range is 1260 to 1675 nm.

[0045] In the above technical solution, the sampling rate of the signal collector 13 is 100-200 MSa / s, and the bandwidth is 100-300 MHz.

[0046] As a specific embodiment, the wavelength of the pulsed laser generated by the first laser 1 of the present invention is 1064nm, the pulse width is 10ns, and the repetition frequency is 10Hz. The wavelength of the continuous laser generated by the second laser 7 is 1550.3nm; the center wavelength of the fiber Bragg grating 10 is 1550nm, the bandwidth is 0.3nm, the wavelength of the continuous laser is at the right edge of the fiber Bragg grating 10, the fiber Bragg grating 10 detects the vibration generated by the ultrasonic pulse propagating on the surface of the object, and the center wavelength drifts. The ultrasonic pulse signal is obtained through the edge filtering and demodulation technology of the fiber Bragg grating. At this time, the width of the ultrasonic pulse signal is 10 to 400μs, and the frequency range is 30-2000kHz.

[0047] As a specific embodiment, the core diameter of the large-core fiber optic jumper of the present invention is 800 μm, the pulsed laser spot diameter output by the first laser is 3 mm, and the first laser 1 and the large-core fiber optic jumper 2 of the present invention need to couple free-space light to the fiber end face, and the coupling efficiency is greater than 90%.

[0048] As a specific embodiment, the bandwidth of the photodetector 11 is 30kHz-1.6GHz, and the wavelength range is 1200~1600nm, which not only covers the frequency range of ultrasonic pulses, but also eliminates the influence of DC bias, converting the optical signal into an electrical signal. The photodetector is connected to a power amplifier with a gain range of 3~20dB. If necessary, the power amplifier can amplify the signal and collect it for further signal processing.

[0049] An all-fiber ultrasonic metal plate defect detection method based on photoinduced ultrasound comprises the following steps:

[0050] Step 1: Fix the ultrasonic excitation optical fiber 5 on the metal plate 15 to be tested, and attach the fiber Bragg grating 10 to the metal plate 15 to be tested;

[0051] Step 2: The first laser 1 outputs a pulsed laser. Part of the pulsed laser light in the ultrasonic excitation fiber 5 passes through the ultrasonic excitation fiber 5 and is absorbed by the light trap 6. Another part of the pulsed laser light in the ultrasonic excitation fiber 5 is coupled into the cladding of the ultrasonic excitation fiber 5. Part of the light coupled into the cladding of the ultrasonic excitation fiber 5 is absorbed by the light trap 6. Another part of the light coupled into the cladding of the ultrasonic excitation fiber 5 is converted into thermal energy and generates an ultrasonic pulse through the thermoelastic effect. The ultrasonic pulse generated by the ultrasonic excitation fiber 5 is scattered when it encounters defects in the metal plate 15 being tested.

[0052] Step 3: The second laser 7 outputs continuous light to the fiber Bragg grating 10. The vibration caused by the ultrasonic pulse propagating in the metal plate 15 causes the wavelength of the continuous light propagating and reflected in the fiber Bragg grating 10 to change, forming a metal plate detection light signal;

[0053] Step 4: The photodetector 11 converts the metal plate detection light signal into a corresponding metal plate detection electrical signal;

[0054] Step 5: The signal processing device 14 determines whether the metal plate 15 to be tested has defects by judging whether the metal plate detection electrical signal contains the scattered signal.

[0055] The present invention improves the efficiency of converting light energy into heat energy by using an ultrasonically excited optical fiber in conjunction with a mixed adhesive of epoxy resin and graphite, and generates a more powerful ultrasonic pulse. Therefore, the ultrasonic pulse can be directly detected using a fiber Bragg grating, thereby realizing all-fiber photo-ultrasonic detection and expanding the application scenarios of all-fiber photo-ultrasonic detection. The present invention is a solution for all-fiber photo-ultrasonic excitation and detection, which can be surface-mounted or internally embedded in the object to be tested to realize online detection and improve the flexibility of ultrasonic detection.

[0056] The contents not described in detail in this specification belong to the prior art known to professional and technical personnel in this field.

Claims

1. An all-fiber ultrasonic metal plate defect detection device based on photo-induced ultrasound, characterized by: The invention comprises a first laser (1), an ultrasonic excitation optical fiber (5), a light trap (6), a second laser (7), a fiber Bragg grating (10), a photodetector (11) and a signal processing device (14). The ultrasonic excitation optical fiber (5) is fixed on a metal plate (15) to be measured, and the fiber Bragg grating (10) is mounted on the metal plate (15) to be measured. The first laser (1) is used to output pulsed laser light. A portion of the pulsed laser light in the ultrasonic excitation optical fiber (5) passes through the ultrasonic excitation optical fiber (5) and is absorbed by the light trap (6). Another portion of the pulsed laser light in the ultrasonic excitation optical fiber (5) is coupled into the cladding of the ultrasonic excitation optical fiber (5). A portion of the light coupled into the cladding of the ultrasonic excitation optical fiber (5) is absorbed by the light trap (6). Another portion of the light coupled into the cladding of the ultrasonic excitation optical fiber (5) is converted into heat energy and generates ultrasonic pulses through a thermoelastic effect. The ultrasonic pulses generated by the ultrasonic excitation optical fiber (5) are scattered when encountering defects of the metal plate (15) to be measured. The second laser (7) is used to output continuous light to the fiber Bragg grating (10), and the vibration caused by the ultrasonic pulse propagating in the metal plate (15) to be detected causes the wavelength of the continuous light propagating and reflected in the fiber Bragg grating (10) to change, thereby forming a metal plate detection light signal; The photoelectric detector (11) converts the metal plate detection light signal into a corresponding metal plate detection electrical signal, and the signal processing device (14) is used to determine whether the metal plate (15) under test has defects by judging whether the metal plate detection electrical signal contains the scattered signal; The preparation method of the ultrasonic excitation optical fiber (5) is as follows: preparing two sections of bare optical fibers (5.1) with smooth end faces, using arc melting to obtain two sections of bare optical fibers (5.2) with spherical end faces, using an optical fiber processing platform to align the two sections of bare optical fibers (5.2) with spherical end faces in the horizontal direction and the vertical direction so that the two spherical surfaces are just connected, using arc pre-fusion, and then using arc fusion. During the fusion process, the optical fibers at both ends are simultaneously pushed toward the middle to obtain the ultrasonic excitation optical fiber (5).

2. The all-fiber ultrasonic metal plate defect detection device based on photo-induced ultrasound according to claim 1 is characterized in that: After the signal processing device (14) determines that the metal plate (15) under test has a defect, the defect position is located in the following manner: The signal processing device (14) performs high-pass filtering on the metal plate detection electrical signal to obtain a high-frequency mode signal, multiplies the frequency of the high-frequency mode signal by the thickness of the metal plate (15) to be measured to obtain a frequency-thickness product, and determines the group velocity of the high-frequency mode signal by the frequency-thickness product. The ultrasonic pulse generated by the ultrasonic excitation optical fiber (5) will be scattered when encountering the defect of the metal plate (15) to be measured. The metal plate detection electrical signal received by the signal processing device (14) through the fiber Bragg grating (10) contains this scattered signal. The metal plate detection electrical signal containing the scattered signal is compared with the metal plate detection reference signal to extract the scattered signal. The time period from the generation of the scattered signal to the peak of the scattered signal is the scattering signal flight time. The product of the group velocity and the scattering signal flight time is the scattering signal flight distance from the excitation node of the ultrasonic excitation optical fiber (5) through the defect to the fiber Bragg grating (10). The excitation node position of the ultrasonic excitation optical fiber (5) and the fiber Bragg grating (10) at this time are the focus, and the scattering signal flight distance is the major axis to form a first ellipse; The position where the fiber Bragg grating (10) is mounted on the metal plate (15) to be tested is changed, and the flight distance of the scattered signal from the excitation node of the ultrasonic excitation optical fiber (5) through the defect to the fiber Bragg grating (10) after the mounting position of the fiber Bragg grating (10) is changed is obtained, and a second ellipse is formed with the excitation node position of the ultrasonic excitation optical fiber (5) and the position of the fiber Bragg grating (10) at this time as the focus and the flight distance of the scattered signal at this time as the major axis; The position of the fiber Bragg grating (10) mounted on the metal plate (15) to be tested is changed again, and the flight distance of the scattered signal from the excitation node of the ultrasonic excitation optical fiber (5) through the defect to the fiber Bragg grating (10) after the mounting position of the fiber Bragg grating (10) is changed again is obtained, and a third ellipse is formed with the excitation node position of the ultrasonic excitation optical fiber (5) and the position of the fiber Bragg grating (10) at this time as the focus and the flight distance of the scattered signal at this time as the major axis; The intersection area of ​​the first ellipse, the second ellipse and the third ellipse is the defect position of the metal plate (15) to be tested.

3. The all-fiber ultrasonic metal plate defect detection device based on photo-induced ultrasound according to claim 1 is characterized in that: It also includes an optical fiber jumper (2), an optical fiber adapter (3) and an optical fiber bare fiber (4); the pulse laser output by the first laser (1) passes through the optical fiber jumper (2), the optical fiber adapter (3) and the optical fiber bare fiber (4) in sequence and enters the ultrasonic excitation optical fiber (5).

4. The all-fiber ultrasonic metal plate defect detection device based on photo-induced ultrasound according to claim 3 is characterized in that: The ultrasonic excitation optical fiber (5) is prepared on the outgoing light path of the bare optical fiber (4).

5. The all-fiber ultrasonic metal plate defect detection device based on photo-induced ultrasound according to claim 4 is characterized in that: The ultrasonic excitation optical fiber (5) is fixed on the metal plate (15) to be measured using a mixed adhesive of epoxy resin and graphite.

6. The all-fiber ultrasonic metal plate defect detection device based on photo-induced ultrasound according to claim 1, characterized in that: It also includes a single-mode optical fiber (8), an optical fiber circulator (9), a power amplifier (12) and a signal collector (13), wherein the continuous light output by the second laser (7) passes through the single-mode optical fiber (8) and the optical fiber circulator (9) to the optical fiber Bragg grating (10), the vibration caused by the ultrasonic pulse propagating in the metal plate (15) to be tested causes the wavelength of the continuous light propagated and reflected in the optical fiber Bragg grating (10) to change, forming a metal plate detection light signal, the metal plate detection light signal is transmitted to the photodetector (11) through the optical fiber circulator (9), the photodetector (11) converts the metal plate detection light signal into a corresponding metal plate detection electrical signal, the power amplifier (12) amplifies the power of the metal plate detection electrical signal, and the signal collector (13) collects the power-amplified metal plate detection electrical signal and transmits it to the signal processing device (14).

7. The all-fiber ultrasonic metal plate defect detection device based on photo-induced ultrasound according to claim 1, characterized in that: The energy of the pulse laser absorbed by the light trap (6) is less than 20% of the total energy of the pulse laser.

8. The all-fiber ultrasonic metal plate defect detection device based on photo-induced ultrasound according to claim 1 is characterized in that: The diameter of the sphere in the spherical bare optical fiber (5.2) is 1 to 1.5 times the core diameter of the bare optical fiber (5.1), and the distance between two spheres is 0.1 to 0.5 times the diameter of the sphere.

9. An all-fiber ultrasonic metal plate defect detection method based on photo-induced ultrasound, characterized in that: It includes the following steps: Step 1: Fix the ultrasonic excitation optical fiber (5) on the metal plate (15) to be tested, and attach the fiber Bragg grating (10) to the metal plate (15) to be tested; Step 2: The first laser (1) outputs a pulsed laser. A portion of the pulsed laser light in the ultrasonic excitation optical fiber (5) passes through the ultrasonic excitation optical fiber (5) and is absorbed by the light trap (6). Another portion of the pulsed laser light in the ultrasonic excitation optical fiber (5) is coupled into the cladding of the ultrasonic excitation optical fiber (5). A portion of the light coupled into the cladding of the ultrasonic excitation optical fiber (5) is absorbed by the light trap (6). Another portion of the light coupled into the cladding of the ultrasonic excitation optical fiber (5) is converted into thermal energy and generates an ultrasonic pulse through a thermoelastic effect. The ultrasonic pulse generated by the ultrasonic excitation optical fiber (5) is scattered when encountering defects in the metal plate (15) being tested. Step 3: The second laser (7) outputs continuous light to the fiber Bragg grating (10), and the vibration caused by the ultrasonic pulse propagating in the metal plate (15) to be detected causes the wavelength of the continuous light propagating and reflected in the fiber Bragg grating (10) to change, thereby forming a metal plate detection light signal; Step 4: The photodetector (11) converts the metal plate detection light signal into a corresponding metal plate detection electrical signal; Step 5: The signal processing device (14) determines whether the metal plate (15) under test has defects by judging whether the metal plate detection electrical signal contains the scattered signal; The preparation method of the ultrasonic excitation optical fiber (5) is as follows: preparing two sections of bare optical fibers (5.1) with smooth end faces, using arc melting to obtain two sections of bare optical fibers (5.2) with spherical end faces, using an optical fiber processing platform to align the two sections of bare optical fibers (5.2) with spherical end faces in the horizontal direction and the vertical direction so that the two spherical surfaces are just connected, using arc pre-fusion, and then using arc fusion. During the fusion process, the optical fibers at both ends are simultaneously pushed toward the middle to obtain the ultrasonic excitation optical fiber (5).

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