KR20100012758A - Laser ultrasonic measuring device and laser ultrasonic measuring method - Google Patents

Abstract

PURPOSE: A laser ultrasonic measuring device and a laser ultrasonic measuring method are provided to measure the sound speed of transversal wave and longitudinal wave and reduce damage to the surface of a target material. CONSTITUTION: A laser ultrasonic measuring device comprises a laser light source(11) for ultrasonic generation, a diffraction grating(14), a signal processing unit(60) and a laser interferometer(12). The laser light source for the ultrasonic generation generates ultrasonic wave in a test material(5). The laser light source for the ultrasonic generation emits pulse laser light to the laser at specified cycles. The pulse laser light emitted from laser light source for the ultrasonic generation is emit on the test material after being converted into a linear beam in a cylindrical lens(43). A linear pattern installed in parallel to the transparent glass. The signal processing part inputs electric signals of the detected plate-shaped wave into an A/D converter as digital signals. The laser interferometer guides the laser reflected beam for detection.

Description

Laser ultrasonic measuring device and laser ultrasonic measuring method

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser ultrasonic measuring apparatus and method for detecting ultrasonic waves generated by irradiating a laser to an object to be detected, and in particular, laser ultrasonic measuring to measure the sound velocity of longitudinal and transverse waves by reducing damage to the surface of an inspected object. An apparatus and a method thereof are provided.

The laser ultrasound method uses a thermal stress generated by irradiating a pulsed laser beam to a detection target material or a vaporization reaction generated by vaporization of the detection material itself near the surface to transmit an ultrasonic wave and generate a separate laser light that continuously oscillates. It is a technique of detecting an ultrasonic wave propagated in a target to be detected by receiving a displacement induced by ultrasonic waves by irradiating a receiving point and using its linearity or interfering coherence.

The laser ultrasonic method can perform the ultrasonic wave detection without contacting the crack of a material, the detection of an inherent defect, or the evaluation of a material characteristic, and is expected to apply to various field of material evaluation.

The principle of laser ultrasonication will be described with reference to FIG. 9.

When the laser for generating ultrasonic wave, which is a high-energy pulse laser, is irradiated to the surface of a steel, for example, a material to be detected, distortion occurs due to thermal expansion and contraction generated on the metal surface due to the impact. Then, the generated distortion propagates inside the steel by ultrasonic waves. Next, when a continuous laser light of a single frequency for ultrasonic detection is irradiated onto the metal surface, the reflected light receives a frequency change (Doppler shift) according to the surface vibration by the propagated ultrasonic waves. Equation 1 showing the Doppler shift amount Δf is shown below.

Δf = 2V / λ [Hz]

Where V = surface displacement velocity, λ = laser wavelength

The measuring instrument using the laser ultrasonic method is equipped with laser interferometers, such as a Fabry-Perot interferometer. The Fabry Perot interferometer acts as a filter that resonates and transmits only a specific frequency. For example, when there are defects inside the steel, the Doppler shift amount shows a value different from that of the ordinary steel because the surface vibration is different from that of the ordinary steel. For this reason, the amount of transmitted light passing through the Fabry-Perot interferometer changes, and inspection or material evaluation of cracks and defects of the inspection material can be performed.

In the laser ultrasound method, the state of the inspection object can be observed without any contact with the inspection object as long as it is within the range of the laser light. The laser beam can be used to perform ultrasonic inspection relatively easily even on a detection material that is difficult to contact or approach by using an optical fiber or a mirror.

The generation principle of the ultrasonic wave by laser light irradiation is demonstrated using FIG. (a) is a case in which ultrasonic waves are generated by ablation, which is a phenomenon in which a laser beam is irradiated to a test object to evaporate a part of the test object, and (b) is a test subject to which the laser light is irradiated to the test object. It is a case of generating ultrasonic waves by generating a thermoelastic change in the sieve.

In the ablation occurrence case, the elastic wave is generated by the evaporation reaction force, and thus traces irradiated to the material occur. On the other hand, in the thermoelastic case, generation of acoustic waves accompanied with thermal expansion and contraction due to rapid heating by a laser occurs, but no trace is irradiated to the material. The thermoelastic case does not damage the material, but compared to ablation conditions, the acoustic sound pressure level is reduced by more than two because the energy level is nearly two orders of magnitude lower in the thermoelastic case.

Conventional laser ultrasonication is a method used in an ablation area that damages an object and generates ultrasonic waves. This method has a strong intensity of the generated ultrasonic waves and detects the sound velocity of the longitudinal waves or the transverse waves of the ultrasonic waves. Moreover, the method of utilizing a phase transformation rate for measurement using the detected sound velocity of a longitudinal wave or a horizontal wave (following nonpatent literature 1 and 2), and the method of measuring the Poisson's ratio (following nonpatent literature 3) are proposed.

[Non-Patent Document 1] M. Dubois, A. Moreau, M. Militzer, and J. F. Bussiere, "Laser-Ultrasonic Monitoring of Phase Transformations in Steels", Scripta Materialia, Vol. 39, No. 6, p. 735-741, 1998

[Non-Patent Document 2] M. Ericsson, E. Lindh-Ulmgren, D. Artymowicz and B. Hutchison, "Laserultrasonics (LUS) for microstructure characterization", Research report IM-2003-113, Swedish institute for metals research, 2003

<Non-Patent Document 3> B. Hutchinson, B. Moss, A. Smith, A. Astill, C. Scruby, G. Engberg, and J. Bjorklund, "Online characterization of steel structures in hot strip mill using laser ultrasonic measurements" , Ironmaking and Steelmaking, Vol. 29, No. 1, p. 77-80, 2002

In the thermoelastic region that generates ultrasonic waves without damaging the surface of the test object, it is difficult to measure the sound velocity of the longitudinal waves and the transverse waves because the magnitude of the longitudinal and transverse waves differs depending on the direction of propagation of the ultrasonic waves and the directivity of the ultrasonic waves. . For this reason, in the conventionally proposed method, the ablation area is used for the measurement of the longitudinal wave and the transverse wave sound velocity, and the damage to the inspected object is large.

In order to solve the above problems, an object of the present invention is to provide a laser ultrasonic measuring apparatus and a laser ultrasonic measuring method for reducing the damage to the surface of the target material and measuring the sound velocity of the longitudinal wave and transverse wave.

Laser ultrasonic measuring device and laser ultrasonic measuring method of the present invention provided to solve the above problems is as described below.

The laser ultrasonic measuring apparatus of the present invention irradiates a test object with a pulsed laser light by means of a laser light source for generating an ultrasonic wave to generate ultrasonic waves, and irradiates a continuous wave laser light with the object under test by means of an ultrasonic detection laser light source. Is an ultrasonic wave measuring device which detects the ultrasonic wave propagated in the test object by detecting the change in the light intensity caused by the Doppler shift caused by the ultrasonic wave by interferometer, and the pulsed laser light as a beam having a linear cross section A first irradiating optical system for generating and irradiating a plurality of linear spots with an diffraction grating on the inspected object after conversion, and concentrating and irradiating the pulsed laser light to the inspected object in a single linear spot Generated by a second irradiation optical system and a plurality of linear spots generated by the first irradiation optical system The continuous wave laser beam for detecting the respective wave wave ultrasonic waves propagated in the specimen to the array direction of the linear spot and each longitudinal wave generated by the single linear spot and propagated in the thickness direction of the specimen. A detection optical system that collects and irradiates a predetermined inspection point on the object under test, causes the laser reflected light to enter the interferometer, causes interference, and receives a laser beam that has passed through the interferometer, and outputs a change in light intensity as an electrical signal. A detection unit and a signal processing unit for inputting an electrical signal output from the light detection unit to derive a sound velocity of the object under test, and the signal processing unit having an electrical signal of light intensity change by the plate wave ultrasonic wave generated by the first irradiation optical system. An asymmetric wave wave frequency calculating unit configured to calculate a frequency of the asymmetric wave wave by performing a frequency analysis on the basis of 2 A longitudinal wave velocity calculation unit for calculating a longitudinal wave velocity by performing a frequency analysis on the basis of an electric signal of the change in light intensity by longitudinal wave ultrasonic waves generated by the irradiation optical system, and a predetermined equation based on the frequency of the asymmetric wave plate wave and the longitudinal wave velocity. It characterized by consisting of a shear wave sound speed calculation unit for calculating the shear wave sound speed by using.

In addition, in the laser ultrasonic measuring apparatus, the asymmetric wave platen frequency calculating unit derives a frequency of the asymmetric wave plate wave by performing a frequency analysis on the basis of the electrical signal of the change in the light intensity by the plate wave ultrasonic wave generated by the first irradiation optical system, Calculating the phase velocity of the asymmetric plate wave from the frequency of the wave plate wave and the pitches of the plurality of linear spots, wherein the longitudinal wave sound velocity calculating unit is based on an electrical signal of light intensity change by the longitudinal wave ultrasonic wave generated by the second irradiation optical system. A frequency analysis is performed to derive the resonant frequency of the ultrasonic wave in the plate thickness direction of the inspected object, and calculate the longitudinal sound velocity from the resonant frequency and the plate thickness of the inspected object, wherein the transverse sound speed calculator calculates the frequency of the asymmetric wave plate wave. And phase velocity and resonant frequency and longitudinal sound velocity of the longitudinal ultrasonic wave. It is characterized by deriving the shear wave sound velocity by performing an iterative calculation based on the theoretical formula of the predetermined ultrasonic wave.

In the laser ultrasonic measuring method of the present invention, an ultrasonic wave is generated by irradiating a pulsed laser light to an inspected object by a laser light source for generating an ultrasonic wave, and the continuous wave laser light is irradiated to the inspected object by the ultrasonic light source for detecting the laser beam. A laser ultrasonic measuring method using a laser ultrasonic measuring device which detects an ultrasonic wave propagated in the subject by detecting a change in light intensity due to the Doppler shift caused by the ultrasonic wave by causing interference in an interferometer, and cross-sections the pulsed laser light. A first laser light irradiation step of converting a shape into a linear beam and generating and irradiating a plurality of linear spots with a diffraction grating on the inspected object, and irradiating the pulsed laser light onto the inspected object A second laser light irradiation step of collecting and irradiating a single linear spot; and the first ray Generated by a plurality of linear spots generated by the light irradiation process and propagated in an array direction of the linear spot by the wave wave ultrasonic wave and the single linear spot and in the thickness direction of the inspected object. Ultrasonic detection step of concentrating and irradiating the continuous wave laser light to a predetermined inspection point on the object to detect each longitudinal wave ultrasonic wave propagated, and causing the laser reflected light to enter the interferometer to cause interference, and passing through the interferometer And a signal processing step of receiving a laser beam and outputting a change in light intensity as an electric signal, and a signal processing step of inputting an electric signal output from the light detection step to derive a sound velocity of the inspected object. The processing step is performed based on the frequency signal based on the electrical signal of the light intensity change by the wave wave ultrasonic wave generated by the first laser light irradiation step. Calculating the frequency of the asymmetric wave plate wave and calculating the longitudinal wave sound velocity by performing frequency analysis on the basis of an electrical signal of light intensity change caused by the longitudinal wave ultrasonic wave generated by the second laser light irradiation step. And a transverse wave sound speed calculation step of calculating a transverse sound speed using a predetermined equation based on the longitudinal wave sound speed calculation step and the frequency of the asymmetric wave plate wave and the longitudinal wave sound speed.

In the above-described laser ultrasonic measuring method, the asymmetric wave wave frequency calculation step may be performed based on an electrical signal of light intensity change caused by the plate wave ultrasonic wave generated by the first laser light irradiation step, thereby analyzing the frequency of the asymmetric wave wave wave. And calculating the phase velocity of the asymmetric plate wave from the frequency of the asymmetric wave plate wave and the pitches of the plurality of linear spots, and the longitudinal wave sound velocity calculating step is based on the change of the light intensity by the longitudinal wave ultrasonic wave generated by the second laser light irradiation step. The frequency analysis is performed based on the electrical signal to derive the resonant frequency of the ultrasonic wave in the plate thickness direction of the inspected object, and the longitudinal wave speed is calculated from the resonant frequency and the plate thickness of the inspected object. Frequency and phase velocity of plate wave and resonance frequency and longitudinal of longitudinal wave ultrasonic wave It is characterized by deriving the shear wave sound velocity by repeating the calculation based on the theoretical formula of the ultrasonic wave propagation using the wave sound velocity.

In the above-mentioned laser ultrasonic measuring apparatus and laser ultrasonic measuring method, the sound velocity measurement of longitudinal waves and transverse waves can be performed with high precision by the laser beam of low laser pulse energy density which reduces the damage of the surface of a to-be-detected material. Therefore, the inspected object can be reduced by the use of a laser in the thermoelastic change region. As a result, inspection or material evaluation of cracks and defects of inspection materials with higher precision can be made non-destructive and non-connected.

EMBODIMENT OF THE INVENTION Hereinafter, the form for implementing the laser ultrasonic measuring apparatus and laser ultrasonic measuring method of this invention is demonstrated with reference to drawings.

In the theory of the plate wave described in Non-Patent Document 4 (J.D. Achenbach, "Wave propagation in elastic solids", Amsterdam, North-Holland, 1973), there are a symmetric wave and asymmetric wave wave each having a plurality of modes.

Here, the plate wave is an ultrasonic wave propagation phenomenon that is well known in the field of ultrasonic application such as ultrasonic flaw detection, and is a wave that appears to vibrate the whole plate by propagating the longitudinal wave and the transverse wave as one unit. An asymmetric wave is a wave having an asymmetric wave, and a symmetric wave is a wave having a wave of a symmetric wave (also, the relationship between the symmetric wave and the asymmetric wave wave and the longitudinal and transverse waves constituting them are described below with reference to FIG. 2 to 4).

In this embodiment, the longitudinal wave sound velocity and the transverse sound velocity of a plate wave are detected using the laser ultrasonic measuring apparatus suitable for asymmetric wave wave measurement and longitudinal wave velocity measurement.

<First Configuration of Laser Ultrasonic Measuring Device for Asymmetric Wave Fan Wave Detection>

FIG. 1A is a diagram showing a schematic configuration of a first configuration arrangement 10a of the laser ultrasonic measuring apparatus according to the present embodiment. The first configuration arrangement 10a of the laser ultrasonic measuring apparatus aims to generate the ultrasonic wave of the asymmetric wave plate wave to the inspected object 7 to detect the asymmetric wave plate wave.

FIG. 1B is a plan view of the inspected object 5 viewed from above. As shown in (a), the ultrasonic wave laser light source 11 emits pulsed laser light at a predetermined repetition cycle with a laser for generating ultrasonic waves on the inspected object 5. The pulsed laser light emitted from the ultrasonic wave generation source 11 is converted into a linear beam (linear light) in the cylindrical lens 43, and then diffracted in the diffraction grating 14 to be inspected. 5) is irradiated with a plurality of linear spots. The first irradiation optical system is configured by the cylindrical lens 43 and the diffraction grating.

The diffraction grating 14 is, for example, provided with a plurality of linear patterns in parallel to the transparent glass. The laser beam irradiated thereon is diffracted by the respective linear patterns, and a plurality of linear patterns on the surface of the inspected object 5 are provided. An interference fringe formed of a linear spot is formed, and the surfaces of the inspected object 5 are heated by the plurality of linear spots to generate ultrasonic waves by the plate wave ultrasonic wave (hereinafter also referred to as pan wave).

The wavelength? Of the generated wave wave W is equal to the pitch p of the interference fringe. The wave wave W propagates the inspected object 5 in the direction orthogonal to the linear spot direction of the interference fringe in the in-plane direction of the inspected object 5, that is, in the arrangement direction of the plurality of linear spots. When 6) is reached, the detection point 6 is collected by concentrating and irradiating the detection laser light emitted from the ultrasonic laser light source 12 as the continuous wave laser light to the detection point 6 with an optical system (not shown). The Doppler shift occurs in the wavelength of the laser beam for detection due to the vibration of the plate wave W passing therethrough. Thus, the detection point 6 is provided in the position of the distance preset in the said spot in the arrangement direction of several linear spot. Then, the laser reflected light of the detection laser in which the Doppler shift has occurred is guided to an optical laser interferometer 12 such as a Fabry-Perot interferometer by an optical system (not shown), and causes interference in the laser interferometer 12. At this time, the resonance condition of the laser interferometer 12 is slightly out of the resonance condition of the reflected light of the detection laser. When the Doppler shifted detection laser reflected light is transmitted through the laser interferometer 12, the wavelength change caused by the Doppler shift is converted into the change in the light intensity in the transmitted laser light. The optical system and laser interferometer which irradiate the detection laser light emitted from the laser light source 12 for ultrasonic detection above to the detection point 6, and guide the laser reflected light from the inspected object 5 to the laser interferometer 12 for incident. At 12, a detection optical system is configured.

Then, the transmitted laser light is detected by a light detector (not shown) composed of a known photodetector having a good sensitivity to the wavelength of the laser light and converted into an electrical signal. Can also be detected as an electrical signal (voltage waveform). The signal processing unit 60 can derive the desired measurement information by performing signal processing on the electric signal as described below.

The signal processing unit 60 inputs the detected electric signal of the wave of the wave wave W as a digital signal to the A / D converter, and then, for example, Fourier transforms using an FFT by frequency analysis to distribute the magnitude of each frequency component. To derive The frequency of the peak is obtained from the distribution to detect the frequency of the asymmetric wave wave generated in the subject. In this way, the plate wave speed c of a plate wave is calculated by the wavelength (lambda) determined by the detected frequency and the pitch p of an interference fringe.

FIG. 2A is a diagram showing an example of a time-varying waveform of a voltage signal of a plate wave detected by the first configuration arrangement of the laser ultrasonic measuring apparatus. The measurement conditions of this example were as follows, and the laser beam for ultrasonic generation was made into the intensity sufficient for the ultrasonic generation in the intermediate region of ablation and thermal elasticity which does not generate remarkable dissolution or evaporation of a test subject.

Test object (5): thickness (d) 0.5mm × length (l) 50mm × width (w) 50mm, material of the test object (steel grade, SS400)

Ultrasonic-generating laser light source: YAG laser (maximum pulse energy 450 mJ / pulse (damped by ND filter), pulse width 10 ns)

Laser pulse energy density: 2.5mJ / mm ² (intermediate area between ablation and thermoelasticity)

Interference Pattern: Pitch 1.0mm

Number of interference fringes: 5

Distance of detection point (6) from occurrence point (interference pattern third): 15 mm

Distance of the object edge from the detection point 6: 10 mm

The peak due to the plate wave appears in the region 71 of the waveform of the electric signal which detects the plate wave shown in Fig. 2A. The waveform of the voltage signal of the plate wave of FIG. 2A is an A / D conversion of an analog signal, digitalized, input into a computer, and displayed on a display screen. The wavelength of this detected plate wave is 1.0 mm equal to the pitch of the interference fringe. In addition, the region 72 is a reflected wave of the plate wave reflected at the edge of the inspected object 5 rather than the shape of the inspected object 5 used in the measurement.

FIG. 2B is a diagram showing the distribution (frequency spectrum) of frequency components of the voltage signal of the plate wave detected by the first configuration arrangement of the laser ultrasonic measuring apparatus. In the signal processing unit 60, the frequency waveform constituting the time waveform can be calculated by Fourier transforming the time waveform shown in FIG. 2A using FFT calculation. Since peaks are shown in the region 73 in Fig. 2B, the frequency of the 2.5 MHz wave wave can be detected. As a result of (a) and (b) of FIG. 2, the wavelength λ is 1.0 mm and the frequency f is 2.5 MHz, so the plate wave speed is wavelength λ × frequency == 2.5 mm / μs, 2.5 x 10 ³ m / sec.

As described above, the frequency and plate wave speed of the plate wave can be detected using the first configuration arrangement 10a of the laser ultrasonic measuring apparatus. In addition, as will be described later, the frequency and plate wave speed are the frequency and plate wave speed of the asymmetric wave plate wave.

(Explanation of symmetric wave and asymmetric wave wave)

An example of the calculated value of the dispersion relationship between the plural modes of the symmetric wave and the asymmetric wave wave using FIG. Here, it is shown that the plate wave detected by the apparatus of FIG. 2 is an asymmetric wave plate wave using the dispersion relationship of the plate wave mode shown in FIG.

The said nonpatent literature 4 describes the calculation formula for calculating the longitudinal wave speed and transverse wave speed which comprise a plate wave. In said non-patent document 4, the plate wave An (n is an integer) of an asymmetric wave mode is represented by the relationship of "speed" and "vibration x thickness" according to Formula (2) shown below, and the plate wave (Sn) of a symmetric wave mode (n is an integer) is represented by the following equation (3) in the relationship between "speed" and "vibration x thickness". In addition, the asymmetric wave (symmetric wave) wave mode is hereinafter also referred to simply as an asymmetric wave (symmetric wave) wave wave.

In addition, in the said nonpatent literature 4, the Poisson's ratio v determined by the longitudinal wave speed of a wave and the transverse wave speed is shown like Formula (4).

Here, V _L represents the longitudinal wave speed V _S represents the transverse wave speed. In addition, ζ = 2π / λ.

The solid line shown in Fig. 3 represents the phase velocity of each mode of the wave, the broken line represents the group speed of the mode of each of the wave, and the solid line and the broken line of the mode of each of the wave can be obtained by solving the equations (2) and (3).

In the embodiment of the laser ultrasonic measuring apparatus described with reference to FIG. 2, the thickness 5 of the inspected object 5 is 0.5 mm, the wavelength of the detection waveform is 1.0 mm, the frequency is 2.5 MHz, and the wave velocity c is 2.5 mm. / ㎲ Further, since c = λf = (λ / d) x (fd) and λ / d = 2.0, the detection waveform has a relationship of c = 2.0fd.

Here, c = 2.0fd is shown by the dashed line 74 in FIG. The dashed line 74 (λ / d = 2.0) overlaps the A ₀ mode indicated by the solid line in the asymmetric wave plate wave An.

On the other hand, the spectral frequency shown by the arrow 73 in Fig. 2B is 2.5 MHz. In the above embodiment, since the thickness d of the test object 5 is 0.5 mm, the wave frequency of the detection spectrum is shown by a straight line 75 in FIG. Intersection 76 of the straight line 75 and dashed line 74 can be seen that the asymmetric fine-wave plate in mode A ₀ wave plate as shown in the 2, because in the vicinity of the linear relationship of A ₀ mode.

However, the intersection 76 of the straight line 75 and the dashed line 74 is not on the symmetric wave plate wave mode Sn. For this reason, it can be seen that the symmetric wave plate wave cannot be detected in the above embodiment.

Thus, in the theory of the plate wave described in the said nonpatent literature 4, the symmetric wave plate wave which generate | occur | produces cannot be detected by the laser ultrasonic measuring apparatus which concerns on the said Example.

In this way, the phase velocity c of the asymmetric wave plate wave A ₀ is found by the above embodiment of the laser ultrasonic measuring apparatus. However, since the detection value is insufficient only by the detection result according to the above embodiment, the longitudinal wave speed V _S and the transverse wave speed V _L in Equation 2 cannot be calculated. Therefore, it can be seen that the longitudinal wave speed V _S and the transverse wave speed V _L cannot be calculated only by the frequency and phase speed of the asymmetric wave plate wave detected by the laser ultrasonic measuring apparatus 10a.

<Second arrangement of laser ultrasonic measuring device for longitudinal wave speed detection>

4: (a) is the figure which looked at the general structure of the 2nd structural arrangement 10b of the laser ultrasonic measuring apparatus in this embodiment from the side of the to-be-tested object 5. In FIG. The second configuration arrangement 10b of the laser ultrasonic measuring apparatus is an arrangement for generating longitudinal ultrasonic waves in the inspected object 5 to detect the longitudinal waves, and for the purpose of measuring the sound velocity of the longitudinal waves. 4B is a plan view of the inspected object 5 from above. As shown in FIG. 4 (a), the second configuration arrangement 10b of the laser ultrasonic measurement apparatus is other than the diffraction grating 14 in the first configuration arrangement 10a of the laser ultrasound measurement apparatus. The optical system has almost the same configuration. However, the second configuration arrangement 10b of the laser ultrasound measuring apparatus converts the laser beam for ultrasonic generation from the cylindrical lens 33 into a single linear beam (linear beam) having a linear cross-sectional shape and detects the detection point ( Irradiate with a linear spot in the vicinity of 6). Thus, the 2nd irradiation optical system is comprised by the cylindrical lens 44. As shown in FIG. Although not shown, the laser ultrasonic measuring apparatus 10a is provided by providing a mirror that is movable or detachable so as to bypass the diffraction grating 14 in the optical path of the laser beam for ultrasonic generation of the first configuration arrangement 10a of the laser ultrasonic measuring apparatus. The first configuration and the second configuration of the can be shared.

FIG. 5 illustrates ultrasonic waves generated in the plate thickness direction of the inspected object 5 by the second configuration arrangement 10b of the laser ultrasonic measuring device, reflected from the bottom surface of the inspected object 5, and returned to the upper surface again. Is a diagram showing a waveform of an electrical signal when the signal is detected. The measurement conditions in this measurement example are as follows.

Test object (5): thickness (d) 0.5mm × length (l) 50mm × width (w) 50mm, test object (steel grade SS400), physical properties longitudinal wave speed = 5,820m / s

Ultrasonic-generating laser light source: YAG laser (maximum pulse energy 450mJ / pulse (damped by ND filter), pulse interval 10ns

Laser pulse energy density: 2.0mJ / mm ² (thermoelastic region)

Linear beam size: 1mm × 0.5mm

Detection laser beam spot: diameter 1mm

FIG. 5A shows a time waveform of an electrical signal detected when irradiated with a pulsed laser beam of a linear spot, and FIG. 5B shows a time waveform shown in FIG. The distribution waveform of the switched frequency component is shown.

As shown in Fig. 5B, the frequency spectrum peak 75 can be detected at 5.82 MHz by irradiation with a linear pulsed laser light.

The detected spectrum 5.82 MHz is a frequency (i.e., resonant frequency) resonating in the height direction of the inspected object 5, and this resonant wave waveform? Is 2d with the thickness of the inspected object 5 as d. . At this time, since the resonant wave sound speed is CL = λ × f, CL = 2d × f = 2 × 0.5mm × 5.82MHz = 5,820m / s.

The calculated 5,820 m / s is the physical property value of the test object 5 and is consistent with the longitudinal wave speed value by a separate known ultrasonic measurement method. This result shows that the peak 75 of the frequency spectrum is a peak due to a resonance wave.

As described above, the phase velocity c of the symmetrical plate wave can be detected from the plate wave measurement by the first configuration arrangement 10a of the laser ultrasonic measuring apparatus. Further, the longitudinal wave sound velocity V _L can be detected from the measurement of the longitudinal wave by the second configuration arrangement 10b in the laser ultrasonic measuring apparatus. Using these measured values, the numerical values of the shear wave sound speed V _S and all other variables can be specified by the constant in Equation (2). The equation 2 is solved by numerical analysis, and the shear wave sound velocity V _S can be calculated as described below.

Numerical example of transverse sound velocity V _S

6 shows an example of numerical analysis of the shear wave sound velocity (V _S ).

6 shows a simulation result of the relationship between the frequency and the phase velocity of the A ₀ mode wave calculated using Equation 2 of the asymmetric wave plate wave. The simulated cases are the following four cases.

Case 1 (solid line): V _L = 5200 m / s ν = 0.29

Case 2 (dashed line): V _L = 5000m / s ν = 0.29

Case 3 (single dashed line): V _L = 4800 m / s ν = 0.29

Case 4 (dotted line): V _L = 5000m / s ν = 0.324

As shown, the shear wave sound velocity is obtained by calculating the longitudinal wave velocity from Equation 2 using the shear wave velocity as a manipulated variable, and modifying the quaternary shear wave velocity to minimize the error between the calculated value and the detection value of the longitudinal velocity (V _L ) of 5.820 m / s. You can get it.

Thus, A ₀ being denomination prepare sound speed (V _L) and a transverse acoustic wave velocity (V _S) Numerical Examples longitudinal speed of sound in advance to comply with a theoretical value (V _L) and a transverse acoustic wave velocity (V _S) a plurality of cases each of and obtained from each case The shear wave sound velocity can be specified by specifying a case where an error between the plate wave velocity in the mode and the detected wave velocity and the shear wave velocity as a variable of numerical analysis falls below a certain value.

7 is a flowchart showing the calculation process of the shear wave sound velocity.

First, in the form of the first configuration arrangement 10a of the laser ultrasonic measuring apparatus, the signal processing unit 60 performs a Fourier transform on the time waveform obtained from the detected light by frequency analysis (for example, FFT) to determine the plate wave in the wave wave A ₀ mode. The plate wave phase velocity A ₀ -v (r) (m / s) of the plate wave A ₀ mode is calculated at the frequency A ₀ -f (r) (MHz) and the frequency (step S101).

In the form of the second configuration arrangement 10b of the laser ultrasonic measuring apparatus, the signal processing unit 60 obtains the resonance frequency by Fourier transforming the time waveform obtained from the detection light by frequency analysis (for example, FFT). The plated longitudinal wave velocity V _L (m / s) of the inspected object 5 is measured using the thickness d (mm) of the inspected object 5 inputted before or after measurement using this resonance frequency. It calculates (step S102).

The signal processing unit 60 includes frequencies A ₀ -f (r) (MHz), phase speeds A ₀ -v (r) (m / s), and frequencies S 1 -f (r) (MHz) calculated as input parameters. It is input (step S103).

Further, the calculation processing unit 60 inputs the calculation start speed, end speed, and calculation step ΔV _S of the shear wave speed V _S as arbitrary calculation parameters (step S104).

The signal processing unit 60 calculates the wave wave frequency A ₀ − in the A ₀ mode calculated from the longitudinal wave speed V _L calculated in step S102 and the transverse wave speed V _S determined by the calculation parameter in accordance with the equation shown in Equation 2. f (Hz) (MHz) and plate wave phase velocity A ₀ -v (p) (m / s) are calculated (step S105).

Next, the plate wave frequency A ₀ -f (p) (MHz) and plate wave phase velocity A ₀ -v (p) (m / s) obtained by the theoretical formula using the calculation parameters, and the wave wave calculated in steps S101 and S102 The error between the frequency A ₀ -f (r) (MHz) and the wave wave phase velocity A ₀ -v (r) (m / s) is respectively calculated (step S106).

If the error is not within the specified threshold value (step S107) repeats the calculation steps (ΔV _S) minutes, to increase the transverse acoustic wave velocity (V _S), respectively (step S108), and steps S105 S106 again.

If the error is within the specified threshold (step S107), the shear wave sound velocity V _S at that time is set as an analysis value and the calculation is completed.

10 is an overall view of an example of a laser ultrasonic measuring apparatus 10c according to the present invention.

The ultrasonic light source 11 for ultrasonic generation irradiates the to-be-tested object 5 through the mirrors 31a and 31b to generate an ultrasonic wave to the to-be-tested object 5. The pulsed laser beam EL is a linear beam (linear beam) having a cross-sectional shape in the cylindrical lens 43, and then diffracted by the diffraction grating 14, thereby providing a plurality of linear spots on the inspected object 5. Is investigated.

In addition, the pulsed laser light EL retracts the mirror 31b by the mirror driver 15 so that the pulsed laser light EL is a linear beam from the cylindrical lens 44 and then through the mirror 31c. The test object 5 is irradiated with a single linear spot.

The portion to which the pulsed laser light EL of the inspected object 5 is irradiated is thermally expanded and then contracted to cause distortion, and ultrasonic waves propagate to the inspected object 5. Ultrasonic waves generated by a plurality of linear spots of electrons propagate in a plate wave by a detection material in the form of a plate.

From the laser light source 12 for ultrasonic detection, the continuous wave laser light DL passes through the lens 42 and the mirrors 41a and 41b, and the pulsed laser light as shown in Fig. 1A or 4B. By EL, it is irradiated to the detection part 6 of the to-be-tested object 5 which exists in a predetermined positional relationship with a linear spot.

Continuous wave laser light DL is irradiated to the measurement point on the to-be-tested object 5. The frequency of the laser reflected light RL of the laser light DL from the test object 5 is subjected to Doppler shift by surface vibration. As the laser light source 12 for ultrasonic detection, the laser reflected light RL is focused by the lens 43 through the mirrors 41c and 41d and enters the FP interferometer 13.

The FP interferometer 13 reciprocates (resonates) the incident laser light between mirrors 13a and 13b and transmits some amount of light in the rear mirror 13b. The spectrum of the laser beam passing through the FP interferometer 13 is composed of sharp peaks and slopes on both sides in a very narrow wavelength range. The distance between the mirrors 13a and 13b is slightly deviated from the resonance condition of the wavelength of the laser light, and the wavelength of the laser light is set to be located in the wavelength range of the corresponding slope. At this time, the Doppler shift of the laser reflected light is converted into the light intensity change when passing through the FP interferometer 13. The light passing through the FP interferometer 13 is incident on the photodetector 20 capable of a high-speed response made of an avalanche photodiode or the like. In the photo detector 20, the light intensity of the transmitted light is converted into an electrical signal S1 and input to the signal processor 60.

In the signal processing unit 60, the asymmetric wave plate frequency frequency calculating unit 61 may execute the processing described in step S101 of FIG. 9. The longitudinal sound velocity calculator 62 can execute the process described in step S102 of FIG. 9. The shear wave sound velocity calculating section 63 can execute the processes described in steps S103 to S108 in FIG. 9.

The signal processing unit 60 is composed of a processor, and the calculation units 61 to 63 may be mounted by executing a program stored in a memory (not shown). The calculation result of the calculator may be output to the display device 19.

The controller 50 controls the mirror driving unit 15 to change the shape of the laser ultrasonic measuring apparatus 10a from the laser ultrasonic measuring apparatus 10a to the laser ultrasonic measuring apparatus 10b or vice versa under the instruction of the signal processing unit 60. . In addition, the oscillation control unit 18 may be controlled to control the laser irradiation timing of the laser light source 11 for generating ultrasound. The controller 50 may be mounted in an electronic circuit or the like.

As described above, the laser ultrasonic measuring apparatus and the method according to the present embodiment are capable of non-destructive, non-connected and high-precision measurement of sound waves of longitudinal and transverse waves by a laser beam having a low laser pulse energy density that reduces damage to the surface of a material to be detected. I can do it. For this reason, the inspected object can be reduced by the use of 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 also applicable to the coexistence region ablation region of the ablation and thermoelastic change region.

BRIEF DESCRIPTION OF THE DRAWINGS It is a figure explaining the outline | summary of the 1st arrangement | positioning of a laser ultrasonic measuring apparatus.

FIG. 2 is a diagram showing an asymmetric wave plate wave detected by the first configuration arrangement of the laser ultrasonic measuring apparatus.

3 is a diagram for describing a dispersion relationship between a plurality of modes of a symmetric wave and an asymmetric wave.

It is a figure explaining the outline | summary of the 2nd structural arrangement of a laser ultrasonic measuring apparatus.

Fig. 5 is a diagram showing a waveform detected by the second configuration arrangement of the laser ultrasonic measuring apparatus.

Fig. 6 is a diagram showing the relationship between the frequency and the phase velocity of the A ₀ mode wave calculated using the asymmetric wave plate equation.

7 is a flowchart showing the calculation process of the shear wave sound velocity.

8 is an overall view of a laser ultrasonic measuring apparatus according to the present invention.

It is a figure explaining the laser ultrasonic measuring principle.

FIG. 10 is a diagram for explaining two types of generation principles of ultrasonic waves by laser light irradiation.

                <Explanation of symbols for the main parts of the drawings>

1: Laser Ultrasonic Measuring Device

5: test object

6 detection point

11: laser light source for ultrasonic generation

12: laser light source for ultrasonic detection

13: Fabry Perot Interferometer

15: mirror driving unit

18: oscillation control unit

19: display device

20: light detector

50: control unit

60: signal processing unit

61: asymmetric wave wave frequency calculation unit

62: longitudinal sound velocity calculation unit

63: shear wave sound velocity calculation unit

Claims (4)

Irradiating pulsed laser light to an object under inspection by a laser light source for generating an ultrasonic wave to generate ultrasonic waves, and irradiating the continuous wave laser light to the object under inspection using an ultrasonic detection laser light source, causing the laser reflected light to interfere with the interferometer. It is a laser ultrasonic measuring apparatus for detecting the ultrasonic wave propagated in the test object by detecting a change in light intensity due to the Doppler shift by A first irradiation optical system for converting the pulsed laser beam into a linear beam having a cross-sectional shape and then generating and irradiating an interference pattern of a plurality of linear spots on a test object with a diffraction grating; A second irradiation optical system for condensing and irradiating the pulsed laser light with a single linear spot on the inspection object; Thickness of the object under test generated by the plurality of linear spots generated by the first irradiating optical system and propagated within the object under test in the array direction of the linear spot and by the single linear spot. A detection optical system for concentrating and irradiating the continuous wave laser light to a predetermined inspection point on the test object to detect each longitudinal wave propagated to the target object, and causing the laser reflected light to enter the interferometer to cause interference; A light detector which receives a laser beam transmitted through the interferometer and outputs a change in light intensity as an electric signal; And a signal processor for inputting an electrical signal output from the light detector to derive a sound velocity of the object under test. The signal processing unit, An asymmetric wave wave frequency calculating unit configured to calculate a frequency of the asymmetric wave plate wave by performing a frequency analysis on the basis of an electrical signal of light intensity change by the wave wave ultrasonic wave generated by the first irradiation optical system; A longitudinal wave sound velocity calculating unit configured to calculate a longitudinal wave velocity by performing frequency analysis on the basis of an electrical signal of the change in light intensity generated by the longitudinal wave ultrasonic waves generated by the second irradiation optical system; And a transverse wave sound velocity calculating unit for calculating the transverse wave sound velocity using a predetermined equation based on the frequency of the asymmetric wave plate wave and the longitudinal wave sound velocity. The method of claim 1, The asymmetric wave wave frequency calculator calculates a frequency of the asymmetric wave wave by performing a frequency analysis on the basis of the electrical signal of the light intensity change by the wave ultrasonic wave generated by the first irradiation optical system, and calculates a frequency of the asymmetric wave wave wave and the plurality of It is to calculate the phase velocity of the asymmetric wave wave from the pitch of the linear spot, The longitudinal wave sound velocity calculation unit performs frequency analysis based on an electrical signal of light intensity change by longitudinal wave ultrasonic waves generated by the second irradiation optical system to derive a resonance frequency of ultrasonic waves in the plate thickness direction of the test object, The longitudinal wave speed is calculated from the plate thickness of the test object, The shear wave sound velocity calculation unit derives the shear wave sound velocity by performing an iterative operation based on a predetermined ultrasonic propagation theory using the frequency and phase speed of the asymmetric wave plate wave and the resonant frequency and longitudinal sound velocity of the longitudinal wave ultrasonic wave. Ultrasonic measuring device. Ultrasonic wave is generated by irradiating pulsed laser light to an inspected object with a laser light source for generating an ultrasonic wave, and continuous wave laser light is irradiated to the inspected object with an ultrasonic detection laser light source, causing the laser reflected light to interfere with the interferometer. It is a laser ultrasonic measuring method using a laser ultrasonic measuring device which detects the ultrasonic wave propagated in the test object by detecting a change in light intensity due to the Doppler shift by A first laser light irradiation step of converting the pulsed laser beam into a linear beam having a cross-sectional shape and then generating and irradiating an interference pattern of a plurality of linear spots on a test object with a diffraction grating; A second laser light irradiation step of condensing and irradiating the pulsed laser light into a single linear spot on the inspection object; The inspection object generated by the plurality of linear spots generated by the first laser light irradiation step and propagated in the inspection object in the arrangement direction of the linear spots and the single linear spot. An ultrasonic wave detection step of collecting and irradiating the continuous wave laser light to a predetermined inspection point on the test object to detect each longitudinal wave propagated in the thickness direction of the beam, and causing the laser reflected light to enter the interferometer to cause interference; A light detection step of receiving a laser beam transmitted through the interferometer and outputting a change in light intensity as an electric signal; A signal processing step of inputting an electrical signal output in the light detection step to derive a sound velocity of the inspected object; The signal processing step, An asymmetric wave plate wave frequency calculating step of calculating an asymmetric wave plate wave frequency by performing frequency analysis on the basis of an electrical signal of light intensity change by the plate wave ultrasonic wave generated by the first laser light irradiation step; A longitudinal wave sound velocity calculating step of calculating a longitudinal sound velocity by performing a frequency analysis on the basis of an electrical signal of light intensity change by the longitudinal wave ultrasonic waves generated by the second laser light irradiation step; And a transverse wave sound velocity calculating step of calculating the transverse wave sound velocity using a predetermined equation based on the frequency of the asymmetric wave plate wave and the longitudinal wave sound velocity. The method of claim 3, The asymmetric wave frequency calculation step is performed by frequency analysis based on the electrical signal of the light intensity change by the wave ultrasonic wave generated by the first laser light irradiation step to derive the frequency of the asymmetric wave frequency wave and the frequency of the asymmetric wave frequency wave And calculating the phase velocity of the asymmetric wave wave from the pitches of the plurality of linear spots, The longitudinal wave sound speed calculating step is performed by frequency analysis based on the electrical signal of the light intensity change by the longitudinal wave ultrasonic waves generated by the second laser light irradiation step to derive the resonant frequency of the ultrasonic wave in the plate thickness direction of the inspected object. The longitudinal wave speed is calculated from the resonance frequency and the plate thickness of the test object, The shear wave sound speed calculation step is a laser ultrasonic wave, characterized in that it is repeatedly calculated based on a predetermined ultrasonic wave propagation equation using the frequency and phase speed of the asymmetric wave plate wave and the resonance frequency and longitudinal wave speed of the longitudinal wave ultrasonic wave. How to measure.

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