JP5072789B2 - Method and apparatus for measuring longitudinal and transverse sound velocities in materials by laser ultrasonic method - Google Patents

Description

  The present invention relates to a laser ultrasonic measurement apparatus and method for detecting ultrasonic waves generated by irradiating a material to be inspected with a laser, and in particular, to reduce damage to the surface of an object to be inspected and to increase the speed of sound of longitudinal and transverse waves. The present invention relates to a laser ultrasonic measurement apparatus and method for measuring.

The laser ultrasonic method irradiates a material to be inspected with pulsed laser light, and transmits ultrasonic waves using the generated thermal stress or the vaporization reaction force generated by the vaporization of the material to be inspected in the vicinity of the surface, This technology irradiates the receiving point with another laser beam that oscillates continuously, receives the displacement induced by the ultrasonic wave using its straightness and coherence, and detects the ultrasonic wave that has propagated through the material to be inspected. .
The laser ultrasonic method is capable of non-contact detection of material cracks and internal defects or evaluation of material properties using ultrasonic waves, and is expected to be applied in various material evaluation fields. Yes.

The principle of the laser ultrasonic method will be described with reference to FIG.
When a laser for generating ultrasonic waves, which is a high-energy pulse laser, is irradiated onto the surface of a steel material, for example, a material to be inspected, distortion occurs due to thermal expansion and contraction that occurs on the metal surface due to the impact. The generated distortion propagates inside the steel as ultrasonic waves. Next, when the metal surface is irradiated with a continuous laser beam having a single frequency for ultrasonic detection, the reflected light undergoes a frequency change (Doppler shift) corresponding to the vibration of the surface due to the propagated ultrasonic wave. The following shows an expression indicating the Doppler shift amount (Δf).
Δf = 2V / λ
Where V = surface displacement speed, λ = laser wavelength

  A measuring instrument using the laser ultrasonic method includes a laser interferometer such as a Fabry-Perot interferometer. The Fabry-Perot interferometer operates as a filter that resonates and transmits only a specific frequency. For example, when there is a defective portion inside the steel material, the surface vibration is different from that of a normal steel material, so the Doppler shift amount shows a value different from that of the normal steel material. Therefore, the transmitted light material that passes through the Fabry-Perot interferometer is changed, so that the inspection material can be inspected for cracks and defects, or can be evaluated.

  In the laser ultrasonic method, the state of the inspection object can be observed without bringing anything into contact with the inspection object as long as the laser beam reaches. By using an optical fiber or a mirror, the laser beam can be subjected to ultrasonic inspection relatively easily even for a material to be inspected that is difficult to contact or approach.

  The principle of generating ultrasonic waves by laser light irradiation will be described with reference to FIG. (A) is a case in which ultrasonic waves are generated by ablation, which is a phenomenon of irradiating a part to be inspected with laser light and evaporating a part of the object to be inspected. This is a case where an ultrasonic wave is generated by causing a thermoelastic change in the object to be inspected.

  In the ablation generation case, an elastic wave is generated by the evaporation reaction force, so that irradiation marks are generated in the material. On the other hand, in the thermoelastic case, the generation of elastic waves accompanying thermal expansion / contraction due to rapid heating by the laser is generated, but no irradiation trace is generated in the material. The thermoelastic case does not damage the material, but since the fluence level is nearly two orders of magnitude lower in the thermoelastic case than in the ablation condition, the generated ultrasonic sound pressure is reduced at a higher rate.

  The conventional laser ultrasonic method is a method used in an ablation region where ultrasonic waves are generated by damaging a target material. In this method, the intensity of the generated ultrasonic wave is strong, and the sound velocity of the longitudinal wave and the transverse wave of the ultrasonic wave is detected. In addition, there are a method for utilizing the detected longitudinal wave and shear wave velocity and utilizing the phase transformation rate for measurement (the following Non-Patent Documents 1 and 2) and a method for measuring the Poisson's ratio (the following Non-Patent Document 3). Proposed.

M.M. Dubois, A.D. Moreau, M.M. Militzer, and J.M. F. Bussiere, “Laser-Ultrasonic Monitoring of Phase Transformations in Steels”, Scripting Materialia, Vol. 39, no. 6, p. 735-741, 1998 M.M. Ericsson, E.M. Lindh-Ulmgren, D.M. Artymowicz and B.M. Hutchison, “'Laser-ultrasonics (LUS) for microstructure charactarization”. Research report IM-2003-113, Swedish institute for metals research, 2003 B. Hutchinson, B.H. Moss, A.M. Smith, A.M. Astill, C.I. Scruby, G.M. Engberg, and J.M. Bjorklund, “Online charactarization of steel structures in hot strip milling laser ultrasonic measurements”, Ironacking and Steelmaking, Vol. 29, no. 1, p. 77-80, 2002 J. et al. D. Achenbach, “Wave propagation in elastic solids”, Amsterdam, North-Holland, 1973 J. Huang, Y. Nagata, S. Krishnaswamy, and J. D. Achenbach, “Laser-based ultrasonics for flaw detection”, Proc. Of IEEE Ultrasonics Symp., P1205-1209, 1994

  In the thermoelastic region where ultrasonic waves are generated without damaging the surface of the object to be inspected, the magnitude of longitudinal and transverse waves differs depending on the direction of ultrasonic wave generation and propagation. It is difficult to measure the speed of sound of waves and shear waves. Therefore, in the conventionally proposed method, the ablation region is used for measuring the longitudinal wave and the shear wave sound velocity, and the damage to the object to be inspected is large.

  In view of the above-described problems, an object of the present invention is to provide a laser ultrasonic measurement device and a laser ultrasonic measurement method that reduce the damage given to the surface of a target material and measure the velocity of longitudinal and transverse waves. .

The laser ultrasonic measurement apparatus and laser ultrasonic measurement method of the present invention provided to solve the above problems are as described below.
The laser ultrasonic measurement apparatus according to the present invention generates ultrasonic waves by irradiating an object to be inspected with pulsed laser light from a laser light source for generating ultrasonic waves, and the continuous wave laser light is generated by the laser light source for detecting ultrasonic waves. This is a laser ultrasonic measurement device that detects the ultrasonic wave propagated through the body to be inspected by detecting the change in light intensity caused by the Doppler shift caused by the ultrasonic wave by irradiating the laser beam with the interferometer. A first irradiation optical system configured to generate and emit interference fringes of a plurality of linear spots with a diffraction grating on the inspection object after the pulse laser beam is converted into a beam having a cross-sectional shape; The pulsed laser beam is generated by a second irradiation optical system for condensing and irradiating the pulse laser beam on the object to be inspected on a single linear spot, and a plurality of linear spots generated by the first irradiation optical system. In the body to be examined In order to separately detect the plate wave ultrasonic wave propagated in the array direction of the spot-like spots and the longitudinal wave ultrasonic wave generated by the single linear spot and propagated in the thickness direction of the inspected object, the continuous wave A detection optical system that focuses and irradiates laser light onto a predetermined inspection point on the object to be inspected, makes the laser reflected light incident on the interferometer and interferes, and receives the laser light transmitted through the interferometer. A light detection unit that outputs a change in light intensity as an electrical signal; and a signal processing unit that receives the electrical signal output from the light detection unit and derives the sound velocity of the object to be inspected. An asymmetric wave plate wave frequency calculation unit that calculates a frequency of an asymmetric wave plate wave by performing frequency analysis based on an electrical signal of a light intensity change caused by a plate wave ultrasonic wave generated by the first irradiation optical system; Longitudinal waves generated by the irradiation optical system 2 Based on the electrical signal of the light intensity change caused by the wave, the longitudinal wave sound speed calculation unit for calculating the longitudinal wave sound speed by performing frequency analysis, the frequency of the asymmetric wave plate wave, and the predetermined sound wave speed based on the longitudinal wave sound speed And a shear wave sound velocity calculation unit for calculating the shear wave sound velocity.

  In the above laser ultrasonic measurement device, the asymmetric wave plate wave frequency calculation unit analyzes the frequency based on the electrical signal of the light intensity change due to the plate wave ultrasonic wave generated by the first irradiation optical system, and performs A0. A frequency of the asymmetric wave plate wave mode is derived, and a phase velocity of the A0 asymmetric plate wave mode is calculated from the frequency of the A0 asymmetric wave plate wave mode and the pitch of the plurality of linear spots. The calculation unit derives the resonance frequency of the ultrasonic wave in the plate thickness direction of the object to be inspected based on the frequency analysis based on the electrical signal of the light intensity change caused by the longitudinal ultrasonic wave generated by the second irradiation optical system. The longitudinal wave sound speed is calculated from the resonance frequency and the plate thickness of the object to be inspected, and the transverse wave sound speed calculation unit includes the frequency and phase velocity of the A0 asymmetric wave plate wave mode, and the longitudinal wave ultrasonic wave. Resonance frequency and Using longitudinal wave sonic speed, characterized by deriving a shear wave velocity by carrying out repetitive operations on the basis of the theoretical formula of the ultrasound wave propagation.

  In the laser ultrasonic measurement method of the present invention, an ultrasonic wave is generated by irradiating an object to be inspected with a laser light source for generating ultrasonic waves, and the continuous wave laser light is generated by the laser light source for detecting ultrasonic waves. A laser ultrasonic measurement device is used to detect the ultrasonic wave propagated through the body to be inspected by detecting the change in light intensity caused by the Doppler shift caused by the ultrasonic wave. In the conventional laser ultrasonic measurement method, the pulse laser beam is converted into a beam having a cross-sectional shape, and then interference fringes of a plurality of linear spots are generated and irradiated on the inspection object by a diffraction grating. A first laser light irradiation step, a second laser light irradiation step of condensing and irradiating the pulsed laser light onto a single linear spot on the object to be inspected, and the first laser light irradiation step. Generated by The plate wave ultrasonic wave generated by the linear spot and propagated in the inspected body in the arrangement direction of the linear spot, and the longitudinal wave generated by the single linear spot and propagated in the thickness direction of the inspected object In order to detect each ultrasonic wave separately, the continuous wave laser beam is focused and irradiated on a predetermined inspection point on the object to be inspected, and the reflected laser beam is incident on the interferometer to cause interference. A step of receiving a laser beam transmitted through the interferometer and outputting a change in light intensity as an electric signal; and an electric signal output from the photodetecting unit is input, and the sound speed of the object to be inspected is adjusted. A signal processing step for deriving, wherein the signal processing step performs frequency analysis on the basis of the electrical signal of the light intensity change caused by the plate wave ultrasonic wave generated by the first laser light irradiation step to perform asymmetric wave plate wave. Asymmetry to calculate the frequency of A longitudinal wave sound speed calculating step of calculating a longitudinal wave sound speed by performing a frequency analysis based on a plate wave frequency calculating step and an electrical signal of a light intensity change caused by a longitudinal wave ultrasonic wave generated by the second laser light irradiation step; A transverse wave sound velocity calculating step of calculating a transverse wave sound velocity using a predetermined formula based on the frequency of the asymmetric wave plate wave and the longitudinal wave sound velocity.

  In the laser ultrasonic measurement method, the asymmetric wave plate wave frequency calculating step performs frequency analysis based on the electrical signal of the light intensity change caused by the plate wave ultrasonic wave generated by the first laser light irradiation step. A frequency of the A0 asymmetric plate wave mode is derived, a phase velocity of the A0 asymmetric plate wave mode is calculated from the frequency of the A0 asymmetric plate wave mode and the pitch of the plurality of linear spots, and the longitudinal wave sound velocity calculating step Is based on the electrical signal of the light intensity change due to the longitudinal wave ultrasonic wave generated by the second laser light irradiation step to derive the resonance frequency of the ultrasonic wave in the plate thickness direction of the object to be inspected, The longitudinal wave sound velocity is calculated from the resonance frequency and the plate thickness of the object to be inspected, and the transverse wave sound velocity calculating step includes the frequency and phase velocity of the A0 asymmetric wave plate wave mode, the resonance frequency of the longitudinal wave ultrasonic wave, longitudinal wave Using fast, characterized by deriving the shear wave velocity is repeatedly calculated based on the theoretical formula of the ultrasound wave propagation.

  The above-mentioned laser ultrasonic measurement apparatus and laser ultrasonic measurement method can perform longitudinal wave and transverse wave sound velocity measurements with high accuracy using laser light having a low laser pulse energy density that reduces damage to the surface of the material to be inspected. it can. Therefore, damage to the object to be inspected can be reduced by using a laser in the thermoelastic change region. As a result, it is possible to perform non-destructive and non-contact inspection of cracks and defects or material evaluation of inspection materials with higher accuracy.

  DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments for carrying out a laser ultrasonic measurement apparatus and a laser ultrasonic measurement method of the present invention will be described with reference to the drawings.

According to the theory of plate waves described in Non-Patent Document 4, the plate waves include symmetric wave plate waves and asymmetric wave plate waves each having a plurality of modes.
Here, the plate wave is a well-known ultrasonic propagation phenomenon in the field of ultrasonic applications such as ultrasonic flaw detection, and is a wave that appears to vibrate the entire plate by propagating the longitudinal wave and the transverse wave together. is there. An asymmetric plate wave is a plate wave having an asymmetric waveform, and a symmetric plate wave plate wave is a plate wave having a symmetric wave waveform (note that symmetric wave plate waves and asymmetric wave plate waves are composed of them). The relationship between the longitudinal wave and the transverse wave is shown by FIG.
In the present embodiment, the longitudinal ultrasonic wave velocity and the transverse wave acoustic velocity of the plate wave are detected by using a laser ultrasonic measurement device suitable for each of the asymmetric wave plate wave measurement and the longitudinal wave sound velocity measurement.

<First Arrangement of Laser Ultrasonic Measuring Device for Asymmetric Wave Plate Detection>
FIG. 1A is a diagram showing a schematic configuration of the first configuration 10a of the laser ultrasonic measurement apparatus according to the present embodiment. The first arrangement 10a of the laser ultrasonic measurement device is intended to generate an ultrasonic wave of an asymmetric wave plate wave on the inspection object 5 and detect the asymmetric wave plate wave.
FIG. 1B is a plan view seen from above the object to be inspected 5. As shown to (a), the ultrasonic wave generation laser light source 11 is a laser for generating the ultrasonic wave in the to-be-inspected object 5, and emits a pulse laser beam with a predetermined repetition period. The pulsed laser light emitted from the ultrasonic wave generation laser light source 11 is converted into a linear beam (linear light) in cross section by the cylindrical lens 43 and then diffracted by the diffraction grating 14 to be inspected 5 Are irradiated as a plurality of linear spots. The cylindrical lens 43 and the diffraction grating constitute a first irradiation optical system.
The diffraction grating 14 is, for example, a transparent glass provided with a large number of thin linear patterns in parallel, and the laser beam applied to the diffraction grating 14 is diffracted by the respective linear patterns, and is on the surface of the inspection object 5. Interference fringes composed of a plurality of linear spots are formed, and the surface of the inspection object 5 is heated by the plurality of linear spots to generate ultrasonic waves as plate wave ultrasonic waves (hereinafter also referred to as plate waves) W. .

  The wavelength λ of the generated plate wave W is equal to the pitch p of the interference fringes. The plate wave W propagates to the detection point 6 by propagating the inspection object 5 in the in-plane direction of the inspection object 5 in the direction orthogonal to the direction of the linear spots of the interference fringes, that is, in the arrangement direction of the plurality of linear spots. Upon arrival, when the detection laser light emitted from the ultrasonic detection laser light source 12 as continuous wave laser light is condensed and irradiated to the detection point 6 by an optical system (not shown), it passes through the detection point 6. Due to the vibration of the plate wave W, a Doppler shift occurs in the wavelength of the laser beam for detection. In this way, the detection point 6 is installed at a position at a predetermined distance from the spot in the arrangement direction of the plurality of linear spots. Then, the laser reflected light of the detection laser in which the Doppler shift has occurred is guided to a laser interferometer 12 such as a Fabry-Perot interferometer by an optical system (not shown), and is caused to interfere in the laser interferometer 12. At this time, the resonance condition of the laser interferometer 12 is slightly shifted from the resonance condition of the reflected light of the detection laser. When the reflected laser beam for detection that has been temporarily Doppler shifted is transmitted through the laser interferometer 12, a change in wavelength due to the Doppler shift is converted into a change in light intensity in the transmitted laser light. An optical system that irradiates the detection point 6 with the detection laser light emitted from the ultrasonic detection laser light source 12 and introduces the laser reflected light from the inspection object 5 to the laser interferometer 12 and the laser interferometer 12. And a detection optical system.

  Then, the vibration due to the plate wave W is detected by detecting the transmitted laser beam with an optical detector (not shown) composed of a known photodetector having a good sensitivity to the wavelength of the laser beam and converting it to an electrical signal. A waveform (hereinafter also referred to as a plate wave waveform) can be detected as an electric signal (voltage waveform). Such ultrasonic detection in the laser ultrasonic method is described in Non-Patent Document 5, for example. The signal processing unit 60 can derive desired measurement information by performing signal processing on the electrical signal as described below.

  The signal processing unit 60 takes the detected electrical signal of the plate wave W as a digital signal by an A / D converter, and then performs Fourier transform using, for example, FFT as a frequency analysis, thereby distributing the size distribution of each frequency component. Is derived. The peak frequency in the distribution is obtained, and the frequency of the asymmetric wave plate wave generated in the inspection object is detected. The plate wave velocity c of the plate wave is calculated from the frequency thus detected and the wavelength λ determined from the interference fringe pitch p.

FIG. 2A is a diagram showing an example of a time-varying waveform of a plate wave voltage signal detected by the first configuration of the laser ultrasonic measurement apparatus. The measurement conditions in this example are as follows, and the laser beam for generating ultrasonic waves is sufficient for generating ultrasonic waves in an intermediate region between ablation and thermoelasticity, which does not cause significant melting or evaporation of the object to be inspected. Strength.
Inspected object 5: thickness (d) 0.5 mm × length (l) 50 mm × width (w) 50 mm, material of the object to be inspected (steel type, SS400)
Laser light source for ultrasonic generation: YAG laser (maximum pulse energy 450 mJ / pulse (attenuated by ND filter), pulse width, 10 ns
Laser pulse energy density: 2.5 mJ / mm2 (between ablation and thermoelasticity)
Interference fringes: pitch p 1.0 mm, number of interference fringes: 5 Distance from generation point (third interference fringe) to detection point 6: 15 mm
Distance from detection point 6 to test object edge 10mm
A peak due to the plate wave appears in a region 71 of the waveform of the electrical signal in which the plate wave illustrated in FIG. The plate-wave voltage signal waveform shown in FIG. 2A is a diagram in which an analog signal is A / D converted into a digital signal, captured in a computer, and displayed on a display screen. The wavelength of the detected plate wave is 1.0 mm, which is the same as the pitch of the interference fringes. The region 72 is a reflected wave of a plate wave reflected from the edge of the inspection object 5 due to the shape of the inspection object 5 used in the measurement.

FIG. 2B is a diagram showing a frequency component distribution (frequency spectrum) of a plate wave voltage signal detected by the first configuration of the laser ultrasonic measurement apparatus. A frequency spectrum constituting the time waveform can be calculated by subjecting the time waveform shown in FIG. 2A to Fourier transform using the FFT operation in the signal processing unit 60. Since it appears as a peak in the region 73 of FIG. 2B, a plate wave frequency of 2.5 MHz can be detected. 2A and 2B, since the wavelength λ = 1.0 mm and the frequency f is 2.5 MHz, the plate wave velocity is: wavelength λ × frequency f = 2.5 mm / μs, That is, 2.5 × 10 ³ m / sec.
Thus, the frequency of the plate wave and the plate wave velocity can be detected using the first configuration 10a of the laser ultrasonic measurement device. As will be described later, the frequency and the plate wave velocity are the frequency and plate wave velocity of the asymmetric plate wave.

<Description of symmetric wave wave and asymmetric wave wave>
With reference to FIG. 3, an example of a calculated value of a dispersion relation of a plurality of modes of a symmetric wave plate wave and an asymmetric wave plate wave is shown as frequency × thickness of a test object, and vertical axis as plate wave velocity. Here, it is shown that the plate wave detected by the apparatus of FIG. 2 is an asymmetric wave plate wave using the dispersion relation of the plate wave mode shown in FIG.
Non-Patent Document 4 describes a calculation formula for calculating the longitudinal wave velocity and the transverse wave velocity constituting the plate wave. In the non-patent document 4, the plate wave An (n is an integer) in the asymmetric wave mode is expressed by the following formula 1 in the relationship of “speed” and “frequency × thickness”. The wave Sn (n is an integer) is expressed by the relationship of “speed” and “frequency × thickness” by the following formula 2. Hereinafter, the asymmetric wave (symmetric wave) plate wave mode is also simply referred to as an asymmetric wave (symmetric wave) plate wave.
Further, in the non-patent document 4, the Poisson's ratio ν determined by the longitudinal wave velocity and the transverse wave velocity of the plate wave is expressed as Equation 3.

Here, VL represents the longitudinal wave sound velocity, and VS represents the transverse wave sound velocity. Also, ξ = 2π / λ is shown.
The solid line shown in FIG. 3 indicates the phase velocity of each plate wave mode, the broken line indicates the group velocity of each plate wave mode, and the solid line and the wave line in each plate wave mode are obtained by solving Equations 1 and 2.

In the embodiment of the laser ultrasonic measuring apparatus described with reference to FIG. 2, the thickness d of the object to be inspected 5 = 0.5 mm, the wavelength λ of the detected waveform = 1.0 mm, the frequency f = 2.5 MHz, the plate wave The speed c = 2.5 mm / μs. Since c = λf = (λ / d) × (fd) and λ / d = 2.0, the detected waveform has a relationship of c = 2.0fd.
Therefore, c = 2.0fd is shown as a chain line 74 in FIG. The chain line 74 (λ / d = 2.0) intersects the A0 mode indicated by the solid line in the asymmetric wave plate wave (An).
On the other hand, the frequency of the spectrum indicated by the arrow 73 in FIG. 2B is 2.5 MHz. In the above embodiment, since the thickness d of the object to be inspected 5 is 0.5 mm, the plate wave frequency of this detection spectrum is shown by a straight line 75 in FIG. Since the intersection point 76 between the straight line 75 and the chain line 74 is near the A0 mode relation line, it can be seen that the plate wave shown in FIG. 2 is an A0 mode asymmetric wave plate wave.
However, the intersection 76 between the straight line 75 and the chain line 74 is not on the symmetric wave plate wave mode Sn. Therefore, it turns out that a symmetrical wave plate wave cannot be detected in the above embodiment.
As described above, in the plate wave theory described in Non-Patent Document 4, a symmetric wave plate wave that is predicted to be generated cannot be detected by the laser ultrasonic measurement device according to the above embodiment.

  As described above, the phase velocity c of the asymmetric wave plate wave A0 is determined by the above-described embodiment of the laser ultrasonic measurement apparatus. However, since the detection value is insufficient only with the detection result according to the above embodiment, the longitudinal wave velocities VS and the transverse wave velocities VL in Equation 1 cannot be calculated. Therefore, the longitudinal wave velocity VS and the transverse wave velocity VL cannot be calculated only by the frequency and phase velocity of the asymmetric wave plate wave detected by the laser ultrasonic measurement device 10a.

<Second Configuration of Laser Ultrasonic Measuring Apparatus for Longitudinal Sound Velocity Detection>
FIG. 4A is a diagram showing a schematic configuration of the second configuration arrangement 10b of the laser ultrasonic measurement apparatus according to the present embodiment as viewed from the side surface of the object 5 to be inspected. The second arrangement 10b of the laser ultrasonic measurement device is an arrangement for generating longitudinal wave ultrasonic waves on the inspected object 5 and detecting the longitudinal waves, and is intended to measure the acoustic velocity of the longitudinal waves. It is what. FIG. 4B is a plan view of the inspection object 5 as viewed from above. As shown in FIG. 4 (a), the second configuration 10b of the laser ultrasonic measurement device is other than the diffraction grating 14 in the first configuration 10a of the laser ultrasonic measurement device. It has almost the same configuration. However, the second arrangement 10b of the laser ultrasonic measurement device converts the laser light for ultrasonic generation by the cylindrical lens 44 into a single linear beam (linear light) having a linear cross-sectional shape, Irradiation as a linear spot in the vicinity of the detection point 6 is performed. As described above, the cylindrical lens 44 forms a second irradiation optical system. Although not shown, a laser that can be moved or detached so as to bypass the diffraction grating 14 is provided in the optical path of the laser beam for generating ultrasonic waves in the first arrangement 10a of the laser ultrasonic measurement device, thereby providing a laser. It is possible to share the first configuration and the second configuration of the ultrasonic measurement apparatus 10a.

FIG. 5 shows that a longitudinal ultrasonic wave is generated in the plate thickness direction of the inspection object 5 by the second configuration 10b of the laser ultrasonic measurement device, reflected by the bottom surface of the inspection object 5, and returned to the upper surface again. It is a figure which shows the waveform of an electric signal when an ultrasonic wave is detected. The measurement conditions in the measurement example are as follows.
Inspected object 5: thickness (d) 0.5 mm × length (l) 50 mm × width (w) 50 mm, inspected object (steel type, SS400), physical property value longitudinal wave sound velocity = 5,820 m / s
Laser light source for ultrasonic generation: YAG laser (maximum pulse energy 450 mJ / pulse (attenuated by ND filter), pulse width, 10 ns
Laser pulse energy density: 2.0 mJ / mm2 (thermoelastic region)
Linear beam size: 1mm x 0.5mm
Detection laser beam spot: 1mm in diameter

FIG. 5A shows a time waveform of an electric signal detected when the pulse laser beam of a linear spot is irradiated, and FIG. 5B shows a Fourier transform of the time waveform shown in FIG. 5A by FFT. The distribution waveform of the frequency component is shown.
As shown in FIG. 5B, the peak 75 of the frequency spectrum can be detected at 5.82 MHz by irradiation with the linear pulse laser beam.

The detected spectrum 5.82 MHz is a frequency that resonates in the height direction of the inspection object 5 (that is, the resonance frequency), and the wavelength λ of the resonance wave is 2d, where d is the thickness of the inspection object 5. At this time, the sound velocity of the resonance wave is CL = λ × f, and thus CL = 2d × f = 2 × 0.5 mm × 5.82 MHz = 5,820 m / s.
The calculated 5,820 m / s coincides with the value of the longitudinal wave speed obtained by another known ultrasonic measurement method as the physical property value of the inspection object 5. From this result, it can be seen that the peak 75 of the frequency spectrum is a peak due to the resonance wave.

  As described above, the phase velocity c of the asymmetric wave plate wave can be detected from the plate wave measurement by the first configuration 10a of the laser ultrasonic measurement device. Further, the longitudinal wave sound velocity VL can be detected from the longitudinal wave measurement by the second configuration 10b of the laser ultrasonic measurement device. Using these measured numerical values, the numerical values of all the variables other than the transverse wave velocity VS and the numerical value can be specified by detection in the above-described numerical formula 1. It is possible to calculate the shear wave velocity VS by solving Equation 1 by numerical analysis as described below.

<Numerical analysis example of shear wave velocity VS>
An example of numerical analysis of the transverse wave speed of sound VS will be described with reference to FIG.
FIG. 6 shows the result of simulating the relationship between the frequency and the phase velocity of the A0 mode wave calculated using Equation 1 of the asymmetric wave plate wave. The following four cases are simulated.
Case 1 (solid line): VL = 5200 m / s, ν = 0.29
Case 2 (chain line): VL = 5000 m / s, ν = 0.29
Case 3 (one-dot chain line): VL = 4800 m / s, ν = 0.29
Case 4 (dotted line): VL = 5000 m / s, ν = 0.324
As shown in the figure, the calculated value of the longitudinal wave sound speed is obtained from Equation 1 using the shear wave sound speed as an operating variable, and the shear wave is again measured so as to minimize the error between the calculated value and the detected value 5,820 m / s of the longitudinal wave speed VL. By correcting the sound velocity, the shear wave velocity can be obtained.
As described above, as a numerical analysis example of the longitudinal wave velocity VL and the transverse wave velocity VS, a plurality of cases for each longitudinal wave velocity VL and transverse wave velocity VS according to theoretical values are prepared in advance, and the A0 mode plate wave obtained from each case is prepared. By identifying the case where the error between the velocity, the detected longitudinal wave velocity, and the plate wave velocity due to the transverse wave velocity as a variable of numerical analysis is less than a certain value, the transverse wave velocity can be identified.

FIG. 7 is a flowchart showing the calculation process of the shear wave sound velocity.
First, in the first configuration 10a of the laser ultrasonic measurement device, the signal processing unit 60 performs Fourier transform on the time waveform obtained from the detection light by frequency analysis (for example, FFT) to obtain a plate wave in the plate wave A0 mode. The plate wave phase velocity A0-v (r) (m / s) of the plate wave A0 mode is calculated from the frequency A0-f (r) (MHz) and the frequency (step S101).

In the second configuration 10b of the laser ultrasonic measurement device, the signal processing unit 60 performs Fourier transform on the time waveform obtained from the detection light by frequency analysis (for example, FFT) to obtain the resonance frequency. Using this resonance frequency, the thickness d (mm) of the inspected object 5 input before or after measurement is used to calculate the longitudinal thickness sound velocity VL (m / s) of the inspected object 5 ( Step S102).
The signal processor 60 receives the calculated frequency A0-f (MHz), phase velocity A0-v (m / s), and frequency S1-f (MHz) as input parameters (step S103).
Furthermore, the calculation start speed, the end speed, and the calculation step (ΔVS) of the transverse wave speed VS are input to the signal processing unit 60 as optional calculation parameters (step S104).

The signal processing unit 60 follows the longitudinal wave sound velocity VL calculated in step S102 and the theoretical equation shown in Equation 1, and the A0 mode plate wave frequency A0-f (p) (MHz) calculated from the transverse wave sound velocity VS determined by the calculation parameters. ) And the plate wave phase velocity A0-v (p) (m / s) are calculated (step S105).
Next, the plate wave frequency A0-f (p) (MHz) and the plate wave phase velocity A0-v (p) (m / s) obtained by the theoretical formula using the calculation parameters and calculated in steps S101 and S102. The error between the plate wave frequency A0-f (r) (MHz) and the plate wave phase velocity A0-v (r) (m / s) is calculated (step S106).
If the error is not within the specified threshold (step S107), the transverse wave velocity VS is increased by the calculation step (ΔVS) (step S108), and steps S105 and S106 are repeated again.
If the error is within the specified threshold (step S107), the transverse wave velocity VS at that time is set as the analysis value, and the calculation is terminated.

FIG. 8 is an overall view of an example of a laser ultrasonic measurement apparatus 10c according to the present invention.
The ultrasonic wave generation laser light source 11 irradiates the inspection object 5 with the high-power pulse laser light EL via the mirrors 31a and 31b in order to generate an ultrasonic wave in the inspection object 5. The pulse laser beam EL is converted into a linear beam (linear beam) by the cylindrical lens 43, and then diffracted by the diffraction grating 14 and irradiated onto the object 5 as a plurality of linear spots.

  Further, the pulse laser beam EL is retracted from the mirror 31b by the mirror drive unit 15, and the pulse laser beam EL is converted into a linear beam by the cylindrical lens 44, and then a single beam is applied to the object 5 to be inspected via the mirror 31c. Irradiate as a linear spot.

  The portion of the inspection object 5 irradiated with the pulsed laser light EL is thermally expanded and then contracted to generate distortion, and the ultrasonic wave propagates to the inspection object 5. The ultrasonic waves generated by the former plurality of linear spots are propagated as plate waves by the plate-shaped inspection material.

From the ultrasonic detection laser light source 12, the continuous wave laser beam DL is transmitted through the lens 42 and the mirrors 41a and 41b by the pulse laser beam EL as shown in FIG. 1 (a) and FIG. 4 (b). The detection part 6 of the to-be-inspected object 5 which has a predetermined positional relationship with the linear spot is irradiated.
The continuous wave laser beam DL is irradiated to a measurement point on the inspection object 5. The frequency of the laser reflected light RL of the laser light DL from the object to be inspected 5 undergoes a Doppler shift due to surface vibration. As the ultrasonic detection laser light source 12, the laser reflected light RL is converged by the lens 43 via the mirrors 41 c and 41 d and enters the FP interferometer 13.

  The FP interferometer 13 causes the incident laser beam to reciprocate between the mirrors 13a and 13b to cause interference (resonance), and allows a part of the light amount to pass through the rear mirror 13b. The spectrum of the laser light transmitted through the FP interferometer 13 is composed of a steep peak in an extremely narrow wavelength region and slopes on both sides. The distance between the mirrors 13a and 13b is slightly shifted from the resonance condition of the wavelength of the laser beam, and is set so that the wavelength of the laser beam is located in the wavelength region of the slope. At this time, the Doppler shift of the laser reflected light is converted into a light intensity change when passing through the FP interferometer 13. Then, the light transmitted through the FP interferometer 13 is incident on a light-sensitive detector 20 made of an avalanche photodiode or the like that can respond at high speed. In the photodetector 20, the light intensity of the transmitted light is converted into an electric signal S <b> 1 and input to the signal processing unit 60.

In the signal processing unit 60, the asymmetric plate wave frequency calculation unit 61 can execute the processing described in step S101 of FIG. The longitudinal wave sound speed calculation unit 62 can execute the processing described in step S102 of FIG. The shear wave sound speed calculation unit 63 can execute the processing described in steps S103 to S108 in FIG.
The signal processing unit 60 includes a processor, and the calculation units 61 to 63 described above may be implemented by the processor executing a program stored in a memory (not shown). The calculation result of the calculation unit can be output to the display device 19.

  Upon receiving an instruction from the signal processing unit 60, the control unit 50 controls the mirror driving unit 15 in order to change the configuration from the laser ultrasonic measurement device 10a to the laser ultrasonic measurement device 10b or vice versa. . Further, the laser irradiation timing of the ultrasonic wave generation laser light source 11 can be controlled by controlling the oscillation control unit 18. The controller 50 can be implemented with an electronic circuit or the like.

As described above, the laser ultrasonic measurement apparatus and method thereof according to the present embodiment measure the acoustic velocity of longitudinal waves and shear waves with laser light having a low laser pulse energy density that reduces damage to the surface of the material to be inspected. Non-destructive, non-contact, and highly accurate. Therefore, damage to the object to be inspected can be reduced by using a laser in the thermoelastic change region.
In addition, the present invention is not limited to the use of such a weak pulse laser, but can be applied to a coexistence region of ablation and a thermoelastic change region, or an ablation region.

FIG. 1 is a diagram for explaining the outline of the first arrangement of the laser ultrasonic measurement apparatus. FIG. 2 is a diagram showing an asymmetric wave plate wave detected by the first arrangement of the laser ultrasonic measurement device. FIG. 3 is a diagram for explaining a dispersion relationship of a plurality of modes of a symmetric wave plate wave and an asymmetric wave plate wave. FIG. 4 is a diagram for explaining the outline of the second arrangement of the laser ultrasonic measurement device. FIG. 5 is a diagram showing waveforms detected by the second configuration of the laser ultrasonic measurement apparatus. FIG. 6 is a diagram showing the relationship between the frequency and phase velocity of the A0 mode wave calculated using the asymmetric wave plate wave equation. FIG. 7 is a flowchart showing the calculation process of the shear wave sound velocity. FIG. 8 is an overall view of a laser ultrasonic measurement apparatus according to the present invention. FIG. 9 is a diagram for explaining the measurement principle of the laser ultrasonic method. FIG. 10 is a diagram for explaining two types of generation principles of ultrasonic waves by laser light irradiation.

Explanation of symbols

DESCRIPTION OF SYMBOLS 5 Test object 6 Detection point 10a Laser ultrasonic measurement apparatus 10b Laser ultrasonic measurement apparatus 10c Laser ultrasonic measurement apparatus 11 Laser light source for ultrasonic generation 12 Laser light source for ultrasonic detection 13 Fabry-Perot interferometer 15 Mirror drive unit 18 Oscillation Control unit 19 Display device 20 Photo detector 50 Control unit 60 Signal processing unit 61 Asymmetric wave plate wave frequency calculation unit 62 Longitudinal wave sound velocity calculation unit 63 Transverse wave sound velocity calculation unit

Claims (4)

The object to be inspected is irradiated with pulsed laser light from an ultrasonic wave generating laser light source to generate ultrasonic waves, and the object to be inspected is irradiated with continuous wave laser light by an ultrasonic detecting laser light source to interfere with the reflected laser light. A laser ultrasonic measurement device that detects an ultrasonic wave propagated through the inspected body by detecting a change in light intensity caused by Doppler shift caused by the ultrasonic wave by causing interference with a meter;
A first irradiation optical system that generates and emits interference fringes of a plurality of linear spots with a diffraction grating on the inspection object after the pulse laser beam is converted into a beam having a cross-sectional shape;
A second irradiation optical system for condensing and irradiating the pulsed laser light on a single linear spot on the inspection object;
It is generated by a plurality of linear spots generated by the first irradiation optical system, and is generated by a plate wave ultrasonic wave propagated in the inspected body in the arrangement direction of the linear spots and the single linear spot. In order to separately detect each longitudinal wave ultrasonic wave propagated in the thickness direction of the inspection object, the continuous wave laser light is condensed and irradiated on a predetermined inspection point on the inspection object, and the laser reflected light is reflected on the inspection object. A detection optical system that enters and interferes with the interferometer,
A light detector that receives the laser light transmitted through the interferometer and outputs a change in light intensity as an electrical signal;
An electrical signal output from the light detection unit is input, and a signal processing unit that derives the sound speed of the object to be inspected is provided.
The signal processing unit
An asymmetric wave plate wave frequency calculating unit that calculates a frequency of an asymmetric wave plate wave by performing frequency analysis based on an electrical signal of a light intensity change caused by a plate wave ultrasonic wave generated by the first irradiation optical system;
A longitudinal wave sound velocity calculating unit that calculates a longitudinal wave sound velocity by performing frequency analysis based on an electrical signal of a light intensity change caused by a longitudinal wave ultrasonic wave generated by the second irradiation optical system;
A laser ultrasonic wave measuring apparatus comprising: a transverse wave sound velocity calculating unit that calculates a transverse wave sound velocity using a predetermined formula based on the frequency of the asymmetric wave plate wave and the longitudinal wave sound velocity. The asymmetric wave plate wave frequency calculation unit derives the frequency of the A0 asymmetric wave plate wave mode by performing frequency analysis based on the electrical signal of the light intensity change by the plate wave ultrasonic wave generated by the first irradiation optical system. The phase velocity of the A0 asymmetric plate wave mode is calculated from the frequency of the A0 asymmetric plate wave mode and the pitch of the plurality of linear spots,
The longitudinal wave sound velocity calculation unit performs frequency analysis on the basis of an electrical signal of a light intensity change caused by longitudinal wave ultrasonic waves generated by the second irradiation optical system, and resonates ultrasonic waves in the plate thickness direction of the inspection object. Deriving the frequency, calculating the longitudinal wave sound velocity from the resonance frequency and the thickness of the object to be inspected,
The transverse wave velocity calculation unit repeatedly calculates based on a predetermined ultrasonic wave propagation theoretical formula using the frequency and phase velocity of the A0 asymmetric wave plate wave mode, and the resonance frequency and longitudinal wave velocity of the longitudinal ultrasonic wave. The laser ultrasonic measurement apparatus according to claim 1, wherein the transverse sound velocity is derived. The object to be inspected is irradiated with pulsed laser light from an ultrasonic wave generating laser light source to generate ultrasonic waves, and the object to be inspected is irradiated with continuous wave laser light by an ultrasonic detecting laser light source to interfere with the reflected laser light. A laser ultrasonic measurement method using a laser ultrasonic measurement device for detecting an ultrasonic wave propagating through the inspected body by detecting a change in light intensity caused by Doppler shift caused by the ultrasonic wave by causing interference with a meter,
A first laser light irradiation step of generating and irradiating interference fringes of a plurality of linear spots with a diffraction grating on the object after the pulse laser light is converted into a beam having a cross-sectional shape;
A second laser light irradiation step of condensing and irradiating the pulsed laser light on a single linear spot on the inspection object;
Generated by a plurality of linear spots generated by the first laser light irradiation step, and generated by a plate wave ultrasonic wave propagated in the direction of arrangement of the linear spots in the inspected body, and the single linear spot. In order to separately detect each longitudinal wave ultrasonic wave propagated in the thickness direction of the object to be inspected, the continuous wave laser light is condensed and irradiated to a predetermined inspection point on the object to be inspected, and the laser reflected light is An ultrasonic detection step of making the interferometer enter and interfere;
A light detection step of receiving laser light transmitted through the interferometer and outputting a change in light intensity as an electrical signal;
An electric signal output from the light detection unit is input, and a signal processing step for deriving the sound speed of the object to be inspected, and
The signal processing step includes
An asymmetric wave plate wave frequency calculating step for calculating a frequency of an asymmetric wave plate wave by performing frequency analysis based on an electrical signal of a light intensity change caused by a plate wave ultrasonic wave generated by the first laser light irradiation step;
A longitudinal wave sound velocity calculating step of calculating a longitudinal wave sound velocity by performing frequency analysis based on an electrical signal of a light intensity change caused by a longitudinal wave ultrasonic wave generated by the second laser light irradiation step;
A transverse ultrasonic velocity calculation step of calculating a transverse wave velocity using a predetermined formula based on the frequency of the asymmetric wave plate wave and the longitudinal wave velocity. The asymmetric wave plate wave frequency calculation step derives the frequency of the A0 asymmetric wave plate wave mode by performing frequency analysis based on the electrical signal of the light intensity change caused by the plate wave ultrasonic wave generated by the first laser light irradiation step. And calculating the phase velocity of the A0 asymmetric plate wave mode from the frequency of the A0 asymmetric plate wave mode and the pitch of the plurality of linear spots,
The longitudinal wave sound velocity calculating step performs frequency analysis based on the electrical signal of the light intensity change due to the longitudinal wave ultrasonic wave generated by the second laser light irradiation step, and performs ultrasonic analysis in the plate thickness direction of the inspection object. Deriving the resonance frequency and calculating the longitudinal wave sound velocity from the resonance frequency and the thickness of the object to be inspected,
The transverse wave velocity calculation step is repeatedly performed based on a predetermined theoretical equation of ultrasonic propagation using the frequency and phase velocity of the A0 asymmetric wave plate wave mode and the resonance frequency and longitudinal wave velocity of the longitudinal ultrasonic wave. The ultrasonic wave measuring method according to claim 3, wherein a shear wave velocity is derived.

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