JP2020153866A - Program, method and device for evaluating internal structure - Google Patents

Abstract

To provide a program, a method and a device for evaluating an internal structure of an analyte with high accuracy by taking into consideration a velocity distribution of an acoustic wave.SOLUTION: Provided is a program for evaluating an internal structure of an analyte S1 by using scattered waves according to an optoacoustic method. The program makes a computer execute: processing for preparing reference data assuming a velocity distribution of an acoustic wave in the analyte S1; processing for calculating a position of a scattering source where an optoacoustic wave is scattered from a specific position, on the basis of measurement data expressing an amplitude of the acoustic wave measured per time at the specific position among plural irradiation positions on the analyte S1 irradiated with pulse laser, and on the basis of the reference data; processing for mapping the measurement data to the position of the scattering source; and processing for repeating the calculation and mapping of the position of the scattering source, and superimposing the mappings for each of the plurality of the irradiation positions.SELECTED DRAWING: Figure 1

Description

The present invention relates to a program, method and apparatus for evaluating the internal structure of a subject using a photoacoustic method.

Carbon Fiber Reinforced Plastic (CFRP) has a weight specific strength about 10 times that of steel, and is used in a wide range of fields including the aircraft industry, taking advantage of its advantages such as corrosion resistance and fatigue resistance. There is. The market for FRP is expanding more and more, including not only CFRP but also Glass FRP using glass fiber. In order for Japan to maintain a high level of international competitiveness in such a market, it is urgent to establish maintenance technology that not only provides high-quality products but also ensures safety during use. The flaws that are difficult to detect and evaluate with the current non-destructive inspection technology for CFRP members are damage such as delamination and fiber-resin interface peeling (debonding), fiber swell and resin rich. Poorly formed parts such as parts, poor coating of coating agent, etc.

Ultrasonic testing (UT) and radiation transmission tests are mainly used for inspection of CFRP members of aircraft. In particular, UT is said to be effective in estimating the depth of planar flaws inside CFRP. However, in a general pulse echo UT, the profile of the incident wave is determined by the band characteristics of the ultrasonic probe for transmission, so if you want to evaluate the flaw near the surface layer, it will be confused with the tail of the surface reflected wave. It may be difficult to detect. This difficult-to-detect area is called the dead zone of ultrasonic waves.

As a technique for applying a laser to such an ultrasonic echo, a method of irradiating the surface of a subject with a laser and utilizing the reflected light undergoing Doppler shift by the ultrasonic echo is also being studied (see Patent Document 1). ). In this method, the aperture synthesis method is applied based on the observation timing data of the laser subjected to the Doppler shift to identify the position of the defect.

In recent years, non-contact UTs using lasers have also been attempted to be applied. Under certain conditions, such as when the laser intensity is low and the pulse width is small, adiabatic expansion occurs due to light absorption, and thermoelastic waves are generated. This thermoelastic wave is called a photoacoustic wave. Photoacoustic waves are being actively used especially in the medical field, and in the case of the human body, application to skin diagnosis and blood vessel imaging is being studied by utilizing the penetration of light into the inside.

On the other hand, when the laser intensity becomes high, the substance at the irradiation site is removed (ablation), and a pressure wave is generated as a reaction force of the substance scattering. When the laser intensity is further increased, the substance is ionized to generate plasma, and a strong pressure wave is generated accordingly. This is called a laser-induced stress wave or a photomechanical wave.

JP-A-11-271281

As described above, the photoacoustic wave generated by the non-contact UT using a laser propagates as an elastic wave inside a subject having acoustic anisotropy such as CFRP. For example, the strength of CFRP changes depending on the orientation of fibers and the number of layers, and the direction dependence of material strength appears as acoustic anisotropy. As a result, the speed of ultrasonic energy propagation (wave packet) changes depending on the direction. In particular, in the case of CFRP reinforced in one direction, a difference in group velocity of nearly three times occurs between the fastest direction and the slowest direction even in the same longitudinal wave propagation. Therefore, if the visualization is performed without considering these, the non-scratched parts are erroneously reconstructed, the scratches cannot be detected, and the accuracy of the inspection is greatly impaired.

The present invention has been made in view of such circumstances, and provides a program, a method and an apparatus capable of evaluating the internal structure of a subject with high accuracy in consideration of the velocity distribution of acoustic waves. The purpose.

(1) In order to achieve the above object, the program of the present invention is a program for evaluating the internal structure of a subject using scattered waves by a photoacoustic method, and determines the velocity distribution of acoustic waves in the subject. Based on the process of preparing the assumed reference data, the measurement data representing the amplitude of the acoustic wave measured at each time at a specific position among the plurality of irradiation positions on the subject irradiated with the pulse laser, and the reference data. The process of calculating the position of the scattering source in which the photoacoustic wave is scattered from the specific position, the process of mapping the measurement data to the position of the scattering source, and the above-mentioned processing for each of the plurality of irradiation positions. It is characterized in that the calculation of the position of the scattering source and the process of repeating the mapping and superimposing the mapping are performed by a computer.

In this way, in order to calculate the position of the scattering source using the reference data that assumes the velocity distribution of the acoustic wave in the subject, the internal structure of the subject is evaluated with high accuracy in consideration of the velocity distribution of the acoustic wave. be able to. Further, since the laser beam is used as an input source, a pulsed photoacoustic wave having a wide frequency band can be generated depending on the method of incident light, and the influence of the dead zone can be reduced.

(2) Further, the program of the present invention is characterized in that the reference data is set so that the velocity distribution of the acoustic wave has anisotropy in the subject. As a result, the internal structure can be evaluated with high accuracy even for a subject having acoustic anisotropy and the velocity of ultrasonic energy propagation (wave packet) changing depending on the direction.

(3) Further, the program of the present invention is characterized in that the velocity distribution of the acoustic wave is set according to the shape or structure of the subject grasped in advance in the reference data. As a result, it is possible to improve the consistency between the actual subject and the reference data and evaluate the internal structure with higher accuracy.

(4) Further, the program of the present invention assumes that the photoacoustic wave is scattered by the scattering source from the specific position at the speed of the photoacoustic wave at the position of the scattering source and returns to the specific position. It is characterized in that the position of the scattering source is calculated. This makes it possible to efficiently calculate the position of the scattering source for a subject having a velocity distribution symmetrical with respect to the axis of the irradiation direction.

(5) Further, the program of the present invention is characterized in that the measurement data is acquired by the aperture synthesis method. As a result, it can be assumed that the photoacoustic wave is generated predominantly at the irradiation position, and the position of the scattering source can be calculated efficiently.

(6) Further, the method of the present invention is a method for evaluating the internal structure of a subject using scattered waves by a photoacoustic method, and prepares reference data assuming a velocity distribution of acoustic waves in the subject. From the specific position based on the step to be performed and the measurement data representing the amplitude of the acoustic wave measured at each time at a specific position among the plurality of irradiation positions on the subject irradiated with the pulse laser and the reference data. The step of calculating the position of the scattering source in which the photoacoustic wave is scattered, the step of mapping the measurement data to the position of the scattering source, and the calculation of the position of the scattering source for each of the plurality of irradiation positions. It is characterized by including a step of repeating the mapping and superimposing the mapping. This makes it possible to evaluate the internal structure of the subject with high accuracy in consideration of the velocity distribution of the acoustic wave. Moreover, the influence of the dead zone can be reduced.

(7) Further, the apparatus of the present invention is an apparatus for evaluating the internal structure of a subject using scattered waves by a photoacoustic method, and manages reference data assuming a velocity distribution of acoustic waves in the subject. Based on the reference data management unit, measurement data representing the amplitude of the acoustic wave measured at each time at a specific position among the plurality of irradiation positions on the subject irradiated with the pulse laser, and the reference data. The position calculation unit that calculates the position of the scattering source in which the photoacoustic wave is scattered from the specific position, the mapping unit that maps the measurement data to the position of the scattering source, and the plurality of irradiation positions are described. It is characterized by including a mapping superimposing portion in which the calculation of the position of the scattering source and the mapping are repeated and the mappings are superimposed. This makes it possible to evaluate the internal structure of the subject with high accuracy in consideration of the velocity distribution of the acoustic wave. Moreover, the influence of the dead zone can be reduced.

According to the present invention, the internal structure of the subject can be evaluated with high accuracy in consideration of the velocity distribution of the acoustic wave.

(A) and (b) are schematic views showing the photoacoustic measurement system and the photoacoustic microscope of the present invention, respectively. It is a block diagram which shows the structure of the apparatus (PC) of this invention. It is a flowchart which shows the whole method of this invention. It is a flowchart which shows the imaging process. It is sectional drawing which shows the periphery of the irradiation position of a photoacoustic microscope. It is a table which shows the characteristic value of CFRP and water. (A) to (c) are photographs showing the appearance of the CFRP specimen, schematic views showing the structure, and micrographs, respectively. (A) to (c) are perspective views, xy cross-sectional views, and zx cross-sectional views of the velocity distribution in the CFRP specimen. (A) and (b) are perspective views and photographs showing the positions of peeling in the specimen. (A) and (b) are graphs showing photoacoustic waves at points A and B, respectively. It is a graph which shows the Fourier transform spectrum of a photoacoustic wave. (A) to (c) are a perspective view showing the result of imaging with a filter, an xz diagram, and an xz diagram showing the result of imaging without a filter, respectively.

[Configuration of photoacoustic measurement system]
1 (a) and 1 (b) are schematic views showing a photoacoustic measurement system 100 and a photoacoustic microscope 130, respectively. As shown in FIG. 1A, the photoacoustic measurement system 100 includes a PC 110, a scanner controller 122, a switching circuit 125, a pulse laser light source 128, a photoacoustic microscope 130, a stage 140, a photodetector 150, a DAC, an amplifier 170, and a receiver 180. have.

Further, the photoacoustic microscope 130 includes a fiber 132, a lens 134, a beam splitter 135, an axicon lens 136, a reflection prism 137, and a transducer 138, and the pulse laser is focused on the focal region C1 and the generated light is generated. Detects reflected waves of acoustic waves.

The photoacoustic measurement system 100 irradiates a subject S1 immersed in water with a laser, receives a photoacoustic wave in an ultrasonic region generated by the subject S1, and images an internal image based on the received ultrasonic wave. .. The photoacoustic measurement system 100 is particularly suitable for non-destructive evaluation of flaws just below the surface of CFRP.

The PC 110 controls each part in the system and also functions as a device that performs processing for evaluating the internal structure. Specifically, the PC 110 receives the input of the information of the subject S1 and the measurement conditions from the user, controls the irradiation of the pulse laser, the data collection of the acoustic wave, and processes the collected data.

The scanner controller 122 controls the stage movement for raster scan according to the measurement conditions. As a result, the reflection prism 137 and the transducer 138 can be scanned with respect to the subject S1. Then, the position of the pulsed light focused on the subject S1 and the position of the sound wave focal point of the transducer 138 are moved by this two-dimensional scanning, and the photoacoustic wave is detected at each measurement position to obtain the photoacoustic signal data of the two-dimensional surface. Can be obtained. In this way, 2D raster scan can be performed.

The switching circuit 125 determines the Q value based on the applied voltage and produces a tunable pulsed laser including a tunable laser excited by the Q-switched laser. The Q-switch method is a method in which the Q value is lowered until a very large number of atoms are in an excited state to suppress oscillation, and after the number of atoms in the excited state is sufficiently large, the Q value is raised again to oscillate.

The start condition of laser oscillation strongly depends on the performance of the optical resonator, that is, the Q value representing the sharpness of resonance. If the Q value is kept small while the laser medium is being excited and the Q value is increased within a short time when the population inversion is sufficiently large, the energy stored in the medium is released at once. To.

The pulsed laser light source 128 emits pulsed light under the control of the PC 110. Since the photoacoustic microscope 130 uses laser light as an input source, a pulsed photoacoustic wave having a wide frequency band can be generated depending on the method of incident light, and the influence of the dead zone can be reduced.

The photoacoustic microscope 130 is a reflection type photoacoustic microscope 130 that detects scattered photoacoustic waves. It is preferable that the aperture synthesis method is applied to the photoacoustic microscope 130. As a result, it can be assumed that the photoacoustic wave is generated predominantly at the irradiation position, and the position of the scattering source can be calculated efficiently. The fiber 132 guides the pulsed light to the optical system for irradiating the subject S1. The generated pulsed laser beam is introduced into the optical system via the fiber 132. The lens 134 collimates the pulsed laser beam emitted from the fiber 132. The beam splitter 135 reflects some pulsed laser light in the optical system.

The light emitted from the fiber 132 is collimated, and the axicon lens 136 spreads the pulsed laser beam transmitted through the beam splitter 135 in an annular shape. Then, the pulsed laser focused by the condenser lens is irradiated to the target position. When measuring the photoacoustic wave, the alignment is performed so that the optical focus is located on the subject S1. Water is present between the transducer 138 and the subject S1. In this way, the pulsed light is absorbed by the subject S1 and a photoacoustic wave is generated.

The transducer 138 is installed near the center of the reflection prism 137. The transducer 138 has an acoustic lens and can detect sound waves generated from its focal position with high sensitivity.

Further, the transducer 138 has a focusing type ultrasonic probe inside the condenser lens, detects the generated photoacoustic wave, and converts the change in sound pressure intensity into an electric signal. The focal position of the acoustic lens coincides with the focal position of the focused light. The signal obtained by the probe is amplified by the amplifier 170 and then received by the receiver 180.

The stage 140 can carry the subject S1 and can scan on a two-dimensional surface by moving the xy axis. Further, the stage 140 is installed on a mechanism for adjusting the direction perpendicular to the scannable two-dimensional surface.

The photodetector 150 detects a part of the pulsed light reflected by the beam splitter 135 and focused by the lens as reference light. The pulsed light signal detected by the photodetector 150 is used, for example, as a trigger signal for correcting an error caused by a fluctuation in the amount of light or measuring timing.

The DATA 160 quantizes the detected photoacoustic signal and converts it into a digital signal. The data stored in the DATA 160 is transmitted to the PC 110 and stored in the memory of the PC 110. The PC 110 performs signal processing and image processing. The PC 110 outputs the image-processed data to the display. The amplifier 170 amplifies the signal strength of the photoacoustic signal converted into an electrical signal. The receiver 180 receives the amplified photoacoustic signal and transmits it to the PC 110 via the DAQ 160.

[Configuration of PC as processing device]
FIG. 2 is a block diagram showing the configuration of the PC 110. The PC 110 is an example of a device for evaluating an internal structure having a processor and a memory, and evaluates the internal structure of the subject S1 using measurement data by a photoacoustic method. The PC 110 includes a pulse laser control unit 111, a monitor data management unit 112, a reference data management unit 113, a measurement data management unit 114, a position calculation unit 115, a mapping unit 116, a mapping superimposition unit 117, and a reconstruction unit 118. ..

The pulsed laser control unit 111 controls the irradiation of the subject S1 with the pulsed laser according to the measurement conditions, and also controls the movement of the stage 140 to which the pulsed laser is irradiated, enabling raster scanning.

The monitor data management unit 112 manages a signal of a part of pulsed light (reference light) reflected by the beam splitter as monitor data, corrects an error caused by a fluctuation in the amount of light of the photoacoustic signal, and corrects an error caused by a fluctuation in the amount of light of the photoacoustic signal. Determine the measurement timing.

The reference data management unit 113 manages reference data that assumes the velocity distribution of the acoustic wave in the subject S1. The reference data is effective when the velocity distribution of the acoustic wave having anisotropy is set at the position in the subject S1. As a result, the internal structure can be evaluated with high accuracy even for a member having acoustic anisotropy and the velocity of ultrasonic energy propagation (wave packet) changing depending on the direction.

As the reference data, it is preferable that the velocity distribution of the acoustic wave is set according to the shape or structure of the subject S1 grasped in advance. For example, when the anisotropy is large in the longitudinal direction of the rectangular plate, the velocity distribution of the acoustic wave inside can be set from the shape. The shape or structure of the subject S1 includes not only the outer shape of the subject that can be observed from the outside but also the internal structure of the subject. For example, there is a case where the subject S1 has a two-layer structure and the velocity distributions of the two layers are different. Then, the consistency between the actual subject S1 and the reference data can be improved, and the internal structure can be evaluated with higher accuracy. The measurement data management unit 114 manages the amplitude data associated with the measured position and time at the irradiation position of the pulse laser as the measurement data.

The position calculation unit 115 calculates the position of the scattering source in which the photoacoustic wave is scattered from the specific position based on the measurement data and the reference data. It represents the amplitude of the acoustic wave measured for each time at a specific position among a plurality of irradiation positions on the subject S1 irradiated with the pulse laser. By using the reference data assuming the velocity distribution in this way, the position of the scattering source can be specified by the distance and the direction. In this way, the internal structure of the subject S1 can be evaluated with high accuracy in consideration of the velocity distribution of the acoustic wave. Details of the calculation of the position of the scattering source will be described later.

The mapping unit 116 maps the amplitude of the acoustic wave from the measurement data to the position of the scattering source. The mapping superimposition unit 117 repeats the calculation and mapping of the position of the scattering source for each of the plurality of irradiation positions, and superimposes the mapping. The reconstruction unit 118 can output the result of superimposing the mapping on the colored three-dimensional image on a display or the like.

[Procedure for internal structure evaluation]
Next, a method for evaluating the internal structure using the photoacoustic measurement system 100 configured as described above will be described. FIG. 3 is a flowchart showing the entire method for evaluating the internal structure of the subject S1.

First, the subject is placed on the stage 140 and immersed in water (step S101). Then, the user inputs the subject information and the measurement conditions into the device 110 (step S102). The subject information includes information such as the size and shape of the installed subject and information on the velocity of the acoustic wave at each position. The velocity is a vector quantity consisting of magnitude and direction, and is usually distributed symmetrically with respect to the irradiation axis in the subject. Further, the measurement conditions include information on the intensity of the pulsed laser and the measurement range in the subject.

For example, when the CFPR prepreg is a subject, the fibers of each layer parallel to the irradiation surface and perpendicular to each other have x and y directions, and the velocity of the acoustic wave toward the x and y directions is in the other direction. The speed can be set larger than that. Further, in the z direction, which is the thickness direction of the subject, the velocity of the acoustic wave may be set uniformly, or the velocity distribution of each layer may be set finely.

Next, the subject is irradiated with a pulse laser at a specific position according to the measurement conditions (step S103). In the photoacoustic measurement system 100, the movement of the stage 140 and the irradiation of the laser are linked, and the laser oscillated in the z direction at each position irradiates the surface of the subject S1. Then, the photoacoustic wave generated by the pulse laser is detected together with the laser irradiation (step S104).

It is determined whether or not the irradiation has been completed at all the irradiation positions by the raster scan (step S105), and if not, the imaging process is performed (step S106), and the irradiation destination is moved to the next specific position. (Step S107), the process returns to step S103. When the irradiation to all the irradiation positions is completed, the measurement is terminated. Details of the imaging process will be described below.

In the above example, the imaging process is performed every time the irradiation position changes, and the imaging process is performed in parallel with the measurement of the acoustic wave. However, the data may be processed after the data collection is completely completed. ..

FIG. 4 is a flowchart showing the imaging process. It is assumed that the reference data is prepared in advance. First, the PC 110 accepts the input of measurement data (step S201). Then, based on the measurement data and the reference data, the position of the scattering source of the photoacoustic wave that brings the amplitude to the irradiation position of the pulse laser at each time is calculated (step S202). The details of calculating the position of the scattering source will be described later.

The amplitude is mapped to the position of the obtained scattering source (step S203), and the mapping of each time is superimposed (step S204). It is determined whether or not the mapping at all the irradiation positions is completed (step S205), and if not, the process returns to step S203, the irradiation positions are changed, and the mapping is repeated. On the other hand, when the superposition of the mapping at all the irradiation positions is completed, the mapping result is reconstructed into a three-dimensional image and output (step S206), and the mapping data processing is completed.

[Calculation method of scattering source]
(Principle of photoacoustic wave generation)
When a substance is irradiated with laser light, it is scattered or absorbed according to the optical properties of the substance. The absorbed energy causes photothermal and photochemical actions. Irradiating a substance with light with a very short pulse width causes thermal expansion in a short time, which causes stress. Using this principle, a photoacoustic wave in the ultrasonic region is generated. The profile of the generated photoacoustic wave depends on the wavelength and pulse width of the laser beam used. Further, the pressure p of the photoacoustic wave distributed in the depth + z direction when the laser beam is uniformly irradiated at z = 0 is proportional to the light absorption coefficient and the Grüneisen coefficient Γ (= K / Cp / ρ). ..

Here, Cp is the constant pressure specific heat of the substance, is the coefficient of thermal expansion, K is the volume elasticity, ρ is the density, and F is the fluence (energy of incident light per unit area). This pressure distribution serves as a wave source, and the photoacoustic wave propagates in the −z and + z directions.

(Group velocity of photoacoustic waves)
The group velocity g can be calculated by the equation (2). ρ is the density, v is the phase velocity, c is the elastic stiffness, d is the wave deflection direction, n is the phase propagation direction, and the summation rule is applied to i, l, and k.

The group velocity in the subject is obtained as a three-dimensional distribution g (= (g _x , _gy , g _z )). Therefore, when the subject has acoustic anisotropy, different group velocities can be obtained depending on the direction. By using such a group velocity, it is possible to realize a visualization algorithm considering the direction dependence of the speed of sound.

(Aperture synthesis method)
Here, three-dimensional visualization of the inside of the subject is performed by the aperture synthesis method (SAFT). FIG. 5 is a cross-sectional view showing the periphery of the irradiation position of the photoacoustic microscope 130. In SAFT considering the direction dependence of the sound velocity distribution, assuming that the coordinates of the irradiation point of the laser beam are o ⁱ as shown in FIG. 5, the photoacoustic waveform received at the same position after the laser beam is irradiated at the i point is V. I expressed ^(o i, t) and.

At this time, the source of the wave at time t is estimated. From equation (1) and FIG. 6, it can be assumed that in a subject such as CFRP, the photoacoustic wave is predominantly generated on the contact surface with water (subject surface). That is, the wave motion at time t can be regarded as the time until the photoacoustic wave generated on the surface of the subject at the irradiation point of the laser beam returns from the internal scatterer. Considering the velocity distribution g of the photoacoustic wave, the distance r (= | r−o ⁱ |) between the irradiation point o ⁱ and the scattering source r is given by the equation (3).

Using equation (3), the amplitude value V of the time t ^(o i, t) is regarded as it waves generated at the position r, _h i ^(o i, r) in the amplitude value location r mapped as. The amplitude values are superimposed while moving the irradiation point of the laser beam.

In the equation (4), the reachable range of the photoacoustic wave (opening d ₀ ) is assumed, and the correction w of the amplitude intensity due to this is taken into consideration.

Here, d represents the distance between the normal direction of the irradiation point of the laser beam and the position r, as shown in FIG. Then, H is calculated at all positions in the target region while changing the irradiation position of the laser beam, and the H is colored and displayed.

As shown in equation (3), when calculating the position of the scattering source, the photoacoustic wave is scattered from the specific position by the scattering source at the speed of the photoacoustic wave at the position of the scattering source and returns to the specific position. It is preferable to calculate the position of the scattering source on the assumption that. This makes it possible to efficiently calculate the position of the scattering source for a subject having a velocity distribution symmetrical with respect to the axis of the irradiation direction.

[Example]
(Overview)
The optical characteristics of CFRP were investigated, and the group velocity of photoacoustic waves propagating inside CFRP was confirmed. Specimens of CFRP (cross-ply CFRP) whose stacking directions are orthogonal to each other were prepared, and artificial peeling having different depths was created just below the surface. Light was focused on the surface of the specimen by utilizing the fact that photoacoustic waves are mainly generated on the surface layer of CFRP. Aperture synthesis was applied to detect photoacoustic waves.

(Water and CFRP)
In the photoacoustic measurement system 100, the subject is submerged in water for measurement. FIG. 6 is a table showing the characteristic values of CFRP and water. FIG. 6 shows the Grüneisen coefficient of CFRP and water. The CFRP parameters can be calculated from the carbon fiber and resin parameters using the composite law. The algorithm can be constructed assuming that the photoacoustic wave starts propagating from the surface of the subject.

When the wavelength of the laser beam is 532 nm, the light absorption coefficient of CFRP is 200 cm ^-1 , and that of water is about 0.45 × 10 ^-3 cm ^-1 . When these values are applied to Eq. (1), it can be seen that the pressure of the photoacoustic wave generated by CFRP is much higher than that in water. In addition, it can be seen that a large amplitude photoacoustic wave is generated on the CFRP surface due to the large μ, and the intensity sharply decreases inside.

(Group velocity of photoacoustic waves in CFRP)
The strength of CFRP changes depending on the orientation direction of carbon fibers and the number of layers. 7 (a) to 7 (c) are photographs showing the appearance of the CFRP specimen, schematic views showing the structure, and micrographs, respectively. As shown in FIGS. 7A and 7B, the CFRP specimen is a cross-ply (CP) CFRP in which carbon fibers are alternately oriented in the x-direction and the y-direction.

The matrix was an epoxy resin (Toray Industries, Inc. # 2592), and the carbon fiber used was Toray Industries, Inc. T700S. Eight prepregs were laminated ([(0 ° = 90 °) 2] s) and autoclaved at 0.5 MPa while maintained at 130 ° C. After molding, the specimen was cut so that the lengths in the x, y, and z directions of the specimen were 20.0 mm, 20.0 mm, and 1.15 mm, respectively.

FIG. 7 (c) is an enlarged photograph taken by a scanning electron microscope. The diameter of the carbon fiber is about 7 [mu] m, the content of the fibers is about 60%, the density of CFRP after molding are respectively 1.6 g / cm ^3. If the elastic stiffness c of CFRP is known, the group velocity can be obtained from the phase velocity v by the equation (2).

8 (a) to 8 (c) are perspective views, xy cross-sectional views, and zx cross-sectional views of the velocity distribution in the CFRP specimen. Shown in FIGS. 8A to 8C are group velocity distributions of longitudinal waves of CFRP. FIG. 8A shows a three-dimensional distribution g (= (gx, gy, gz)) of the group velocity, and FIGS. 8B and 8C show the in-plane distribution. The group velocity differs depending on the direction, and the longitudinal wave velocity in the x and y directions is about 6 km / s, but the z direction is 3.1 km / s. Therefore, when the CFRP surface is irradiated with a laser beam, it does not spread in a spherical shape unlike an isotropic material, so a visualization algorithm considering the direction dependence of the speed of sound was applied.

(Device used)
As the pulsed laser light source, NanoL90-100 manufactured by Litron was used. A small Q-switched Nd: YAG laser was used. The wavelength of the light was 532 nm, the pulse width was 4 ns, the frequency was 100 Hz, and the intensity of the laser light emitted from the light source was 0.6 mJ.

The spot diameter when the light was introduced into the fiber was 170 μm. A wideband piezoelectric probe (Olympus, V214-BB-RM) centered at 50 MHz was used as the focused ultrasonic probe for receiving the generated photoacoustic wave. It was designed so that the focusing point of the laser beam and the focusing point of the probe coincided with each other, and the focal point of the light was about 6.5 mm from the bottom surface of the capacitor. The reference light was received by a photodetector (Thorlabs, DET10a / M).

(Aperture synthesis method)
From equation (1) and FIG. 6, it is assumed that the photoacoustic wave in CFRP is predominantly generated on the contact surface with water (CFRP surface). That is, the wave motion at time t can be regarded as the time until the photoacoustic wave generated on the CFRP surface at the irradiation point of the laser beam returns from the internal scatterer. Considering the velocity distribution g of the photoacoustic wave, the distance r (= | r−o ⁱ |) between the irradiation point o ⁱ and the scattering source r is given by the equation (3).

Using equation (3), the amplitude value V ^(o i, t) at time t is considered that it is a wave generated at the position r, and mapped as _h i ^(o i, r) in the amplitude value location r. Then, the amplitude values were superposed while moving the irradiation point of the laser beam.

(CFRP specimen)
Based on the waveform obtained by the photoacoustic measurement system 100, the visualization of artificial peeling in CFRP using SAFT will be verified. 9 (a) and 9 (b) are perspective views and photographs showing the positions of peeling in the specimen. The specimens shown in FIGS. 9 (a) and 9 (b) were prepared for a visualization experiment.

The material was the same as that used in FIGS. 7 (a) to 7 (c), and artificial peeling was created inside. A nitoflon sheet having a width of 5 mm and a length of 20 mm was embedded between the prepregs, and after the resin was cured, the sheet was pulled out to prepare an artificial peeling. Artificial peeling was provided between the 3rd and 4th layers and between the 4th and 5th layers.

(Example of measurement waveform)
10 (a) and 10 (b) are graphs showing photoacoustic waves at points A and B, respectively. 10 (a) and 10 (b) show examples of waveforms acquired by the photoacoustic measurement system 100. 10 (a) and 10 (b) are waveforms acquired at points A and B shown in FIG. 9 (b), respectively.

In both FIGS. 10A and 10B, the photoacoustic signal generated on the surface of the CFRP appeared in a waveform in the vicinity of 0.9 μs. In FIG. 10A, the wave measured about 0.74 μs after the photoacoustic signal from the front surface is considered to be the reflected wave from the back surface of the CFRP. Considering that the speed of sound in the z direction is 3.1 km / s, the path along which the photoacoustic wave travels is about 2.3 mm. This route is about twice the thickness of the CFRP specimen (1.15 mm). That is, the photoacoustic wave is predominantly generated on the surface of CFRP, and the reflection on the bottom surface is measured.

In FIG. 10B, the wave measured about 0.37 μs after the photoacoustic signal from the surface of the CFRP specimen is the reflected wave from the artificial peeling prepared in the 4th and 5th layers. I understand.

FIG. 11 is a graph showing a Fourier transform spectrum of a photoacoustic wave. FIG. 11 shows the Fourier spectrum of the waveform from point B. From FIG. 11, a very wide band waveform is obtained around 30 MHz. It can be seen that the generated photoacoustic wave has a center frequency lower than the nominal center frequency (50 MHz) of the probe, but has a wide frequency characteristic from a low frequency to 100 MHz.

(Imaging result)
A raster scan of pulsed laser irradiation was performed to image a three-dimensional image of the CFRP specimen. The scan pitch of the xy stage was 50 μm and the sampling rate was 500 MHz. The opening used in SAFT was d ₀ = 0.5 mm. In addition, a 100 kHz high-pass filter was applied to the received waveform.

12 (a) to 12 (c) are a perspective view showing the result of imaging with a filter, an xz diagram, and an xz diagram showing the result of imaging without a filter, respectively. 12 (a) to 12 (c) show the results of imaging. Here, using KURUMI, which is a 3D high-speed viewer, H exceeding the threshold value is plotted while changing the color in the depth direction (z direction).

FIG. 12 (a) shows the inside of the specimen from a bird's-eye view, and FIG. 12 (b) shows the xx cross section of the specimen. From these results, it can be seen that the positions and shapes of the two artificial delaminations (Delamination 1, 2) in CFRP are clearly reconstructed. Further, the pattern of artificial peeling shown in FIG. 12A reflects the carbon fiber direction of the prepreg immediately above the pattern.

FIG. 12 (c) shows the results of plotting smaller than the threshold value of H in the case shown in FIG. 12 (b). At this time, not only the artificial peeling but also the boundary of the laminated prepreg appeared. In the general ultrasonic pulse echo method, when the position of the flaw is shallow, the problem is that the reflected wave from the surface trails and overlaps with the scattered wave from the flaw just below the surface. However, in this method, the boundary between the prepregs of the first layer and the second layer can also be confirmed separately from the surface, demonstrating that it has high spatial resolution.

Incidentally, SAFT imaging using photoacoustic waves, determines an extent d ₀ of the interior of the photoacoustic wave in accordance with the spot diameter of the light, change the frequency band of the ultrasonic probe in accordance with the depth of the object to be inspected You need to do it. Since SAFT superimposes the acquired waveforms in consideration of the spread of the beam, it has an advantage that the scan pitch can be made rough to some extent. This will also lead to high-speed scanning, which is useful when the target of non-destructive inspection is a large cross section.

In the above imaging result, the position of the scattering source is calculated using the equation (3), but it is also possible to perform imaging on the assumption that the group velocity is uniform inside the specimen. In this case, since the position different from the actual position of the scattering source is calculated as the position of the scattering source, when the same CFPR specimen is imaged, the accuracy becomes low and the position of the artificial peeling becomes more unclear.

(Conclusion)
The non-destructive inspection method using the photoacoustic wave of the present invention has made it possible to image the peeling just below the surface in the evaluation of the peeling just below the surface of CFRP. The photoacoustic measurement system 100 that generates photoacoustic waves irradiates a laser beam of 532 nm from the upper part of the CFRP, and receives the photoacoustic waves in the MHz band generated on the surface of the CFRP with a focused probe.

As a result, it was shown that the generation intensity of the photoacoustic wave is proportional to the light absorption coefficient and the Grüneisen coefficient, and that when the CFRP is irradiated with the laser beam, the photoacoustic wave is predominantly generated from the surface. It was confirmed that this is also valid from the appearance time of the waveform measured by the photoacoustic microscope.

The sound velocity (group velocity) of the photoacoustic wave generated inside the CFRP differs depending on the direction due to the material anisotropy of the CFRP. Therefore, in order to image the internal image of CFRP with high accuracy, the waveform was synthesized while considering the spherical spread of the photoacoustic wave from the CFRP surface and the orientation dependence of the sound velocity distribution.

Waveforms obtained with a photoacoustic microscope were synthesized using the aperture synthesis method (SAFT), and three-dimensional imaging inside the CFRP was performed. It was shown that the wide band of the generated photoacoustic wave makes it possible to image a flaw just below the surface of CFRP, which is a dead zone in a normal UT.

From these results, similar visualization is expected for fiber swells and resin-rich areas (fiber-deficient areas). In addition, from the viewpoint of practical application of CFRP inspection, it is expected that an air couple ultrasonic probe will be used as a photoacoustic wave receiving probe to develop a completely non-contact method.

100 Photoacoustic measurement system 110 Device 111 Pulse laser control unit 112 Monitor data management unit 113 Reference data management unit 114 Measurement data management unit 115 Position calculation unit 116 Mapping unit 117 Mapping superimposition unit 118 Reconstruction unit 122 Scanner controller 125 Switching circuit 128 Pulse Laser light source 130 Photoacoustic microscope 132 Fiber 134 Lens 135 Beam splitter 136 Axicon lens 137 Reflection prism 138 Transducer 140 Stage 150 Photodetector 160 DAQ
170 Amplifier 180 Receiver C1 Focus area S1 Subject

Claims (7)

A program for evaluating the internal structure of a subject using scattered waves by the photoacoustic method.
The process of preparing reference data that assumes the velocity distribution of acoustic waves in the subject, and
Based on the measurement data representing the amplitude of the acoustic wave measured at each time at a specific position among the plurality of irradiation positions on the subject irradiated with the pulse laser and the reference data, the photoacoustic wave is generated from the specific position. The process of calculating the position of the scattered scattering source and
The process of mapping the measurement data to the position of the scattering source, and
A program characterized in that the calculation of the position of the scattering source and the mapping are repeated for each of the plurality of irradiation positions, and the processing of superimposing the mappings is executed by a computer. The program according to claim 1, wherein the reference data is set so that the velocity distribution of the acoustic wave has anisotropy in the subject. The program according to claim 1 or 2, wherein the velocity distribution of the acoustic wave is set according to the shape or structure of the subject grasped in advance in the reference data. It is characterized in that the position of the scattering source is calculated on the assumption that the photoacoustic wave is scattered by the scattering source from the specific position and returns to the specific position at the velocity of the photoacoustic wave at the position of the scattering source. The program according to any one of claims 1 to 3. The program according to any one of claims 1 to 4, wherein the measurement data is acquired by an aperture synthesis method. It is a method for evaluating the internal structure of a subject using scattered waves by the photoacoustic method.
Steps to prepare reference data that assumes the velocity distribution of acoustic waves in the subject,
Based on the measurement data representing the amplitude of the acoustic wave measured at each time at a specific position among the plurality of irradiation positions on the subject irradiated with the pulse laser and the reference data, the photoacoustic wave is generated from the specific position. Steps to calculate the location of the scattered scattering source,
The step of mapping the measurement data to the position of the scattering source,
A method comprising, for each of the plurality of irradiation positions, a step of repeating the calculation of the position of the scattering source and the mapping, and superimposing the mapping. A device for evaluating the internal structure of a subject using scattered waves by the photoacoustic method.
A reference data management unit that manages reference data that assumes the velocity distribution of acoustic waves in a subject,
Based on the measurement data representing the amplitude of the acoustic wave measured at each time at a specific position among the plurality of irradiation positions on the subject irradiated with the pulse laser and the reference data, the photoacoustic wave is generated from the specific position. A position calculation unit that calculates the position of the scattered scattering source,
A mapping unit that maps the measurement data to the position of the scattering source,
An apparatus comprising, for each of the plurality of irradiation positions, a mapping superimposing unit for repeating the calculation of the position of the scattering source and the mapping and superimposing the mapping.