JP2012220221A - Measuring method of poison ratio and measuring apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a measuring method of poison ratio utilizing ultrasonic excitation caused by thermo-elastic effects, with which the surface of an object to be examined is prevented from being damaged and no laser irradiated mark is generated in a laser ultrasonic method, and a measuring apparatus.SOLUTION: A measuring method of poison ratio includes generating an ultrasonic wave by irradiating an object to be examined with pulse oscillation laser light for ultrasonic wave generation, further, irradiating the object to be examined with laser light for ultrasonic detection having a wavelength different from that of the pulse oscillation laser light for ultrasonic wave generation, calculating a strength waveform of ultrasonic waves generated in the object to be examined utilizing the laser light for ultrasonic detection Doppler-shifted by vibration of ultrasonic waves generated in the object to be examined, performing frequency analysis on the strength waveform of ultrasonic waves, calculating a frequency of board ultrasonic waves in Si mode of which the group velocity is zero generated in the object to be examined and a resonant frequency of longitudinal waves, and calculating a poison ratio from the calculated frequency of board ultrasonic waves in the Si mode of which the group velocity is zero and the calculated resonant frequency of the longitudinal waves.

Description

  The present invention relates to a laser ultrasonic method in which an ultrasonic wave is generated on an inspection object in a non-contact manner using a laser, and an ultrasonic wave is generated without damaging the surface of the inspection object using a thermoelastic effect. The invention relates to a method for measuring Poisson's ratio.

  The Poisson's ratio is a ratio of transverse strain and longitudinal strain when stress is applied to a material, and is an important material strength index in the same way as Young's modulus in grasping deformation in an elastic deformation region. By appropriately adjusting the Poisson's ratio, a material with less variation in strength can be manufactured. From this point of view, measuring the Poisson's ratio is important.

  In general, the Poisson's ratio is determined by a tensile test. That is, a tensile test piece is cut from a material, a strain gauge is attached, a tensile tester is used to pull at a constant speed, and a strain in the tensile direction and a strain in the vertical direction are measured to obtain a Poisson's ratio. Ask.

  However, in this method, the load measurement accuracy of the testing machine and the influence of the initial strain generated when the test piece is attached to the tensile testing machine affect the measurement accuracy. Moreover, since the tensile test is a test of a destructive method, there is a drawback that the Poisson's ratio cannot be measured for the actual product.

  Several methods for measuring the Poisson's ratio of a non-destructive object have been proposed. For example, in Patent Document 1 and Patent Document 2, ultrasonic waves that are incident on an object to be inspected from an ultrasonic probe and are propagated through the object to be inspected using a contact ultrasonic probe. Longitudinal sound velocity and transverse wave sound velocity (hereinafter simply referred to as “longitudinal wave sound velocity” and “transverse wave sound velocity”, respectively) are measured. From the measured longitudinal wave sound velocity and shear wave sound velocity, and the density of the inspected object, Young A method for measuring rate and Poisson's ratio is disclosed. These methods have an advantage that the Young's modulus and Poisson's ratio of the actual product can be measured without destroying the actual product. However, the contact-type measurement method is somewhat difficult from the viewpoint of speeding up the measurement.

  For example, the method using a piezoelectric probe requires an ultrasonic transmission medium between the object to be inspected and the transducer, but the function of the transmission medium is degraded at high temperatures. Further, in the method using the electromagnetic ultrasonic probe, it is usually necessary to bring the probe close to the object to be inspected up to about several millimeters. Therefore, each method has a drawback that it cannot be used in a poor environment such as, for example, a steel plate production line or a hot rolling process.

  On the other hand, a non-contact measurement method (hereinafter referred to as “laser ultrasonic method”), in which an ultrasonic wave is generated using a pulsed laser and an ultrasonic wave propagated through the body to be inspected is detected using a continuous wave laser. Various applications have been proposed.

  Regarding the method for measuring the Poisson's ratio by the laser ultrasonic method, Non-Patent Document 1 reports a method for measuring the Poisson's ratio, longitudinal wave velocity and transverse wave velocity using ultrasonic excitation by ablation.

Non-Patent Document 1 discloses that a plate generated by irradiating a Q switch Nd: YAG laser beam having a pulse output of about 5.1 mJ / mm ^{2 with} a fluence (amount of energy per unit area) on the object to be inspected. The frequency of the S1 mode (hereinafter referred to as “S1f”) and the frequency of the A2 mode (hereinafter referred to as “A2f”) of the group velocity zero of the wave ultrasonic wave are converted to a double wave Nd: YAG laser having a continuous wave output. The experimental results are described in which the Poisson's ratio, the longitudinal sound velocity, and the transverse wave velocity are calculated from the detected values.

  The non-contact type laser ultrasonic method does not require a destructive tensile test, so various applications are expected as an on-line non-destructive inspection in a production line that requires high speed and high reliability. .

  In the method of exciting ultrasonic waves by laser, the surface of the object to be inspected is irradiated with high energy laser light, and ultrasonic waves are excited by the thermoelastic effect due to instantaneous temperature rise, and the object to be inspected by higher energy. There is a method of exciting an ultrasonic wave using a pressure wave generated when a part of the surface of the material is vaporized (ablated).

  FIG. 1A schematically shows the principle of ultrasonic excitation utilizing ablation, and FIG. 1B schematically shows the principle of ultrasonic excitation utilizing a thermoelastic effect.

As shown in FIG. 1A, when the object 1 is irradiated with the laser beam 2, a part of the object evaporates due to high energy (see 3 in the figure). At this time, an ultrasonic wave 4 is generated as a pressure wave generated as a reaction force. Here, in the figure, the arrow of the ultrasonic wave 4 indicates the directivity of the generated ultrasonic wave. On the surface of the object 1, a part of the object evaporates, and an irradiation mark of laser light is generated. However, when irradiating a steel material, if the fluence is about ² mJ / mm ² or less, irradiation marks due to ablation do not occur on the object.

  As shown in FIG. 1B, when the object 1 is irradiated with the laser beam 2, the surface temperature of the object 1 instantaneously increases due to the rapid heating by the laser, and a temperature increase region 5 is formed. An ultrasonic wave 4 is generated with thermal expansion and contraction in the region 5. Here, in the figure, the arrow of the ultrasonic wave 4 indicates the directivity of the generated ultrasonic wave.

  The directivity of ultrasonic propagation of ultrasonic excitation by the thermoelastic effect is described in Non-Patent Document 2, for example.

Japanese Patent Laid-Open No. 5-133861 Japanese Patent Laid-Open No. 5-126805

Dominique Clorennec, etc, ‘Local and non-contact measurements of bulk acoustic wave velocities in thin isotropic plates and shells using zero group velocity Lamb modes’, Journal of Applied Physics, 101, 034908, 2007. C.B.Scruby and L.E.Drain, “Laser Ultrasonic-Techniques and Applications”, ISBN0-7503-0050-7, Adam Hilger, p.289, 1990 A. Gibson and J.S. Popovics, “Lamb Wave Basis for Impact-Echo Method Analysis”, J. Eng. Mech., Vol. 131 (4), 438-443, (2005).

  For example, in the conventional technique for measuring Poisson's ratio by the laser ultrasonic method described in Non-Patent Document 1, ultrasonic excitation by ablation is used, but irradiation marks are generated on the surface of the object to be inspected by ablation. For this reason, there is a problem that the measurement by this method cannot be performed in applications where irradiation marks are not allowed, and the applications are limited.

  However, if the same measurement is performed using ultrasonic excitation by the thermoelastic effect, there is a problem that it is difficult to detect the generated ultrasonic wave and A2f of the plate wave ultrasonic wave cannot be measured. That is, due to the difference in directivity of ultrasonic propagation, ultrasonic detection using the thermoelastic effect makes it difficult to detect ultrasonic waves, and in particular, measurement of A2f with a poor S / N is practically impossible.

  In view of the above circumstances, in the laser ultrasonic method, the present invention does not cause damage to the surface of an object to be inspected, does not cause laser irradiation traces, and measures Poisson's ratio using ultrasonic excitation by a thermoelastic effect. It is an object to provide a method and a measurement device.

  The present inventors diligently studied a method for receiving an ultrasonic wave excited by a thermoelastic effect and calculating a Poisson's ratio.

  As a result, by configuring an appropriate measurement system using an ultrasonic wave generation laser and an ultrasonic detection laser having different wavelengths, the frequency of the S1 mode of the plate wave ultrasonic wave with zero group velocity and the longitudinal frequency It was found that the wave resonance frequency can be detected, and the Poisson's ratio can be calculated from the detected frequency.

  Furthermore, it has been found that measurement accuracy can be increased by using a pulsed laser as the ultrasonic detection laser.

  The present invention has been made based on the above findings, and the gist is as follows.

(1) A method for measuring a Poisson's ratio of an object to be inspected,
Irradiating the object to be inspected with pulsed laser light for generating ultrasonic waves, and generating ultrasonic waves on the object to be inspected;
Irradiating the object to be inspected with ultrasonic wave detecting laser light having a wavelength different from that of the pulse oscillation laser light for generating ultrasonic waves;
The reflected light of the ultrasonic detection laser beam that has undergone Doppler shift due to the vibration of the ultrasonic wave generated by irradiating the object to be inspected with pulsed laser light for ultrasonic generation is received, and the amount of the Doppler shift is received. Outputting light of a high intensity,
Calculating an intensity waveform of an ultrasonic wave generated on the object to be inspected from the amount of Doppler shift, using light having an intensity corresponding to the amount of Doppler shift;
Performing frequency analysis of the intensity waveform of the ultrasonic wave, calculating the frequency of the plate wave ultrasonic wave in the S1 mode with zero group velocity generated in the inspection object, and the resonance frequency of the longitudinal wave;
A Poisson's ratio measuring method comprising: calculating a Poisson's ratio from the calculated frequency of the S1 mode plate wave ultrasonic wave with zero group velocity and the longitudinal resonance frequency.

  (2) The method for measuring Poisson's ratio according to (1), wherein the ultrasonic detection laser beam is a pulsed laser beam.

  (3) The Poisson's ratio measuring method according to (2) above, wherein the peak output of the ultrasonic detection laser beam is 100 to 1000 W, and the pulse width is 50 μs to 1 ms.

  (4) When the ultrasonic wave generation pulsed laser beam is irradiated onto the object to be generated to generate an ultrasonic wave, the ultrasonic wave generation pulsed laser beam is irradiated with a spot-like spot. The Poisson's ratio measuring method according to any one of (1) to (3).

  (5) The Poisson's ratio measuring method according to (4), wherein the ultrasonic detection laser beam is irradiated into the spot-like spot region where the ultrasonic wave generation pulse oscillation laser beam is irradiated.

  (6) receiving the ultrasonic detection laser light that has been subjected to Doppler shift due to vibration of ultrasonic waves generated by irradiating the object to be inspected with pulse generation laser light for ultrasonic generation, and depending on the amount of Doppler shift The method of outputting light having a high intensity is a method in which the laser beam for ultrasonic detection subjected to the Doppler shift is interfered by an interferometer, and light having an intensity corresponding to the amount of the Doppler shift is output from the interferometer. The Poisson's ratio measuring method according to any one of (1) to (5), wherein:

(7) An apparatus for measuring a Poisson's ratio of an object to be inspected,
A laser irradiation unit for generating ultrasonic waves for irradiating a pulsed laser beam for generating ultrasonic waves to generate ultrasonic waves on the object to be inspected;
An ultrasonic detection laser irradiating unit that irradiates the inspected object with an ultrasonic detection laser beam having a wavelength different from that of the ultrasonic wave generation pulse oscillation laser beam;
The reflected light of the ultrasonic detection laser beam that has undergone Doppler shift due to the vibration of the ultrasonic wave generated by irradiating the object to be inspected with pulsed laser light for ultrasonic generation is received, and the amount of the Doppler shift is received. A receiver that outputs light of high intensity;
A first processing unit that calculates an intensity waveform of an ultrasonic wave generated on the object to be inspected from the amount of Doppler shift, using light having an intensity corresponding to the amount of Doppler shift;
Frequency analysis of the intensity waveform of the ultrasonic wave, a second processing unit that calculates the frequency of the S1 mode plate wave ultrasonic wave of zero group velocity generated in the object to be inspected, and the resonance frequency of the longitudinal wave;
A Poisson's ratio measuring apparatus comprising a third processing unit that calculates a Poisson's ratio from the calculated frequency of the S1 mode plate wave ultrasonic wave with zero group velocity and the resonance frequency of the longitudinal wave.

  (8) The Poisson's ratio measuring device according to (7), wherein the ultrasonic detection laser irradiation unit includes a pulsed laser light source.

  (9) The Poisson's ratio measuring device according to (8), wherein the pulsed laser light source can irradiate laser light having a peak output of 100 to 1000 W and a pulse width of 50 μs to 1 ms.

  (10) The Poisson according to any one of (7) to (9), wherein the ultrasonic wave generating laser irradiation unit is capable of irradiating the surface of the object to be inspected with pulsed laser light with a spot-like spot. Ratio measuring device.

  (11) The ultrasonic detection laser irradiating unit can irradiate a laser beam in a spot-like spot region of a pulsed laser beam irradiated by the ultrasonic wave generating laser irradiating unit on the surface of the object to be inspected. (10) The Poisson's ratio measuring device.

  (12) The receiving unit inputs and interferes with the ultrasonic detection laser light that has undergone Doppler shift due to ultrasonic vibration generated by irradiating the object to be inspected with pulse generation laser light for ultrasonic generation. The Poisson's ratio measuring device according to any one of (7) to (11), wherein the interferometer outputs light having an intensity corresponding to the amount of Doppler shift.

  According to the present invention, since the ultrasonic wave is excited by the thermoelastic effect using a laser beam having a low energy that does not cause ablation, the surface of the object to be inspected is not damaged, and a laser beam irradiation mark is generated. Without doing so, the Poisson's ratio can be measured in a non-contact and non-destructive manner.

It is a figure which illustrates typically the principle of ultrasonic excitation using the ablation by a laser beam. It is a figure which illustrates typically the principle of ultrasonic excitation using the thermoelastic effect by a laser beam. It is a figure which shows the relationship between the group velocity of a plate wave ultrasonic wave, and a frequency x board thickness. It is a figure which shows the relationship between Poisson's ratio and (beta) 1. It is a figure which shows the relationship between Poisson's ratio and (beta) 2. It is a figure explaining the pulse waveform of the laser beam for ultrasonic generation in the measuring method of this invention, and the laser beam for ultrasonic detection. It is a figure explaining the other example of the pulse waveform of the laser beam for ultrasonic generation in the measuring method of this invention, and the laser beam for ultrasonic detection. It is a figure which shows the relationship between the irradiation position of the preferable ultrasonic wave generation laser and ultrasonic detection laser at the time of detecting the plate wave ultrasonic wave of zero group velocity in this invention. It is a figure which shows an example of the relationship between the frequency of a Fabry-Perot interferometer, and the transmittance | permeability. It is a block diagram which shows the outline of the measuring apparatus which measures Poisson's ratio using the measuring method of this invention. It is a block diagram which shows the other example of the outline of the measuring apparatus which measures Poisson's ratio using the measuring method of this invention. It is a figure which shows the intensity | strength waveform of the ultrasonic wave detected in the Example of this invention. It is a figure which shows the result of having processed the intensity | strength waveform of the ultrasonic wave detected in the Example of this invention by a fast Fourier transform (FFT). It is a figure which shows the space | interval with the peak next to each peak as a result of processing the intensity waveform of the ultrasonic wave detected in the Example of this invention by fast Fourier transform (FFT). It is a figure which shows the relationship between the temperature measured in the Example of this invention, and Poisson's ratio.

  Hereinafter, the principle of the measurement method using ultrasonic excitation by the thermoelastic effect in the present invention will be described in comparison with the measurement method using ultrasonic excitation by ablation (hereinafter referred to as “conventional method”).

  In the conventional method, ultrasonic waves are generated and propagated to the object by ultrasonic excitation using ablation. At this time, when the object to be inspected is a relatively thin plate, in addition to the longitudinal wave and the transverse wave, a wave called a plate wave ultrasonic wave, which is an elastic wave propagating along the plane in the solid plane layer, is generated.

  There are many modes in the plate wave ultrasonic wave, which are classified into a symmetric mode (S0, S1, S2,...) And an asymmetric mode (A0, A1, A2,...) Depending on the symmetry of vibration. Further, the phase velocity of the plate wave ultrasonic wave differs depending on the mode, and also depends on the product of the frequency and the plate thickness. That is, the phase velocity has the characteristic of velocity dispersion. A group velocity exists due to the phase velocity dispersion, and the group velocity also depends on the product of the frequency and the plate thickness. It is known that there is a relationship shown in FIG. 2 between the group velocity [m / s] of the plate wave ultrasonic wave and the product of the frequency and the plate thickness.

  The plate wave ultrasonic wave is a mode in which the S1 mode and the A2 mode are easy to detect from the viewpoint of S / N and attenuation of the ultrasonic wave, and the frequency measurement of the plate wave ultrasonic wave is mainly performed in the S1 mode and the A2 mode. Done about.

It is known that the relationship between the S1f and A2f of the plate wave ultrasonic wave, the Poisson's ratio ν, the longitudinal wave sound velocity V _L , and the transverse wave sound velocity V _S is expressed by Equation 1. β1 (ν) and β2 (ν) are known functions using the Poisson's ratio ν as a parameter (Non-Patent Document 3). FIG. 3A shows the relationship between Poisson's ratio and β1 (ν), and FIG. 3B shows the relationship between Poisson's ratio and β2 (ν).

Equation 3 is obtained from the relationship between the Poisson's ratio ν, the longitudinal wave sound velocity V _L , and the transverse wave sound velocity V _S shown in Equation 1 and Equation 2. S 1 f, and, if Motomare value of A2f, from equation 3, can be calculated Poisson's ratio [nu, then from equation 1, can be calculated longitudinal wave acoustic velocity _{V L} and shear wave velocity _{V S.}

That is, in the conventional method, plate wave ultrasonic waves generated by ablation are detected using a continuous wave laser or the like, and the detected intensity waveform is processed by a fast Fourier transform (hereinafter referred to as “FFT”). Thus, S1f and A2f of the plate wave ultrasonic wave are simultaneously measured to calculate the Poisson's ratio ν, the longitudinal wave sound velocity V _L , and the transverse wave sound velocity V _S.

  Next, the measurement method of the present invention will be described.

When the ultrasonic wave is excited by irradiating the object to be inspected with an ultrasonic wave generation laser beam, a longitudinal wave, a transverse wave, and a plate wave ultrasonic wave are generated. In the present invention, S1f of the plate wave ultrasonic wave and the resonance frequency of the longitudinal wave are measured using ultrasonic excitation by the thermoelastic effect. Hereinafter, the resonance frequency of the order n longitudinal wave, referred to as f _n.

The resonance frequency _{f 1} of the first order longitudinal wave, when the thickness d of the longitudinal wave acoustic velocity _{V L} and the object to be _inspected, is f _{1 =} V L / 2d. That is, S1f and _{f 1} is determined, from Equation 1 β1 (ν) is obtained. The Poisson ratio ν can be calculated from the relationship between the Poisson ratio and β1 (ν) shown in FIG. 3A.

As described above, measurement of A2f is practically impossible by measurement using ultrasonic excitation by the thermoelastic effect. However, using a laser and a laser for ultrasound detection for different ultrasound generated wavelengths, if configure the appropriate measurement system according to the present invention, plate wave ultrasound S 1 f, and the longitudinal wave resonance frequency f ₁ Measurement is possible. That is, without measuring the A2f, S 1 f, by measuring the vertical resonant frequency f _1, the point which enables calculation of the Poisson's ratio using ultrasonic excitation by thermoelastic effect, is a feature of the present invention .

  Specifically, the object to be inspected is irradiated with ultrasonic wave generation pulsed laser light to generate ultrasonic waves on the object to be inspected. Further, the object to be inspected is irradiated with ultrasonic detection laser light having a wavelength different from that of the ultrasonic wave generation pulse oscillation laser light.

When the laser beam for ultrasonic detection is reflected by the inspection object, it undergoes a Doppler shift due to the vibration of the ultrasonic waves generated on the inspection object, and the frequency (= wavelength ⁻¹ ) changes. The magnitude of the Doppler shift is expressed by Δf = 2V / λ where V is the surface displacement speed of the object 11 and λ is the wavelength of the ultrasonic detection laser.

That is, since the wavelength of the ultrasonic detection laser beam changes depending on the vibration state of the ultrasonic wave, the intensity of the ultrasonic wave generated on the inspection object from the reflected light wavelength of the ultrasonic detection laser light reflected by the inspection object. Waveform can be calculated. Then, by treating the intensity waveform by FFT, it is possible to obtain S 1 f, the _{f 1.}

  Lasers with different wavelengths are used for the pulse oscillation laser light for ultrasonic generation and the laser light for ultrasonic detection. This is the generation of ultrasonic waves into the laser light for ultrasonic detection received by the ultrasonic receiver. This is to prevent the pulsed laser beam for use from being mixed and becoming noise. That is, as long as the two laser beams have different wavelengths, the wavelength to be used is not limited.

  In order to improve the detection sensitivity of the ultrasonic wave generated on the object to be inspected, the amount of the ultrasonic detection laser light may be increased. For this purpose, it is effective to use a high-power detection laser. However, a continuous wave laser light source having a large output has a disadvantage in industrial use because it has a large apparatus size and is expensive.

  Therefore, the ultrasonic detection laser is pulsed laser light and oscillated in synchronization with the timing of intensity waveform detection, so that the ultrasonic detection sensitivity can be reduced without increasing the average output of the ultrasonic detection laser so much. Can be improved.

  This will be described with reference to FIG. 4A. FIG. 4A shows an example of the pulse waveforms of the ultrasonic wave generation laser beam and the ultrasonic wave detection laser beam. The solid line indicates the pulse waveform 6 of the ultrasonic wave generation laser beam, and the broken line indicates the ultrasonic wave detection laser beam. A pulse waveform 7 is shown. The alternate long and short dash line represents the continuous wave waveform 8 of the ultrasonic detection laser beam.

  The ultrasonic wave generated on the object to be inspected becomes the largest when the object to be inspected is irradiated with the ultrasonic wave generating pulsed laser beam. In other words, the ultrasonic detection laser can detect the ultrasonic wave with the highest accuracy when the object to be inspected is irradiated at the same time. The contribution to sound wave detection is small.

  When the ultrasonic detection laser has a continuous wave output, the output of the laser is constant with respect to time, but for the detection of ultrasonic waves that are emitted except when the pulsed laser light for ultrasonic generation is being output. Lasers have a small contribution to ultrasonic detection.

  When the ultrasonic detection laser has a pulse output, the output of the laser varies with time. Even when an ultrasonic detection laser with a low average output power is used, the output of the ultrasonic detection laser light is a high-power continuous wave output laser at the time around the peak output. The output can be equal to or more than the case. As a result, even if the average output power is low, the ultrasonic wave detection sensitivity can be made equal to or higher than that when a high-power continuous-wave laser is used.

  In order to oscillate the ultrasonic detection laser light in synchronization with the timing of intensity waveform detection, for example, by adjusting the laser light source while checking two pulses using a photo receiver or the like, it is easily synchronized. be able to.

  When the ultrasonic detection laser beam is a pulsed laser beam, the pulse width is preferably 50 μs or more and 1 ms or less, and the pulse peak output is preferably 100 W or more and 1000 W or less.

  When the pulse width is 50 μs or less, the effective waveform time region in the frequency analysis of the intensity waveform is 50 μs or less, the frequency resolution is lowered, and the frequency peak of S1f and the frequency peak of longitudinal wave resonance cannot be clearly obtained. If the pulse width is 1 ms or more, it is technically difficult to realize device fabrication.

  When the peak output of the pulse is 100 W or less, the amount of light of the ultrasonic detection laser light is reduced, and in particular, it becomes difficult to detect the longitudinal wave resonance frequency peak. In order to set the peak output of the pulse to 1000 W or more, it is technically difficult to realize device fabrication.

  The ultrasonic detection laser beam can be measured most efficiently and accurately when the intensity of the ultrasonic wave generation laser beam reaches the peak. However, even if the timing of the peak is slightly deviated, if the pulse of the ultrasonic wave generation laser beam is within the pulse of the ultrasonic wave detection laser beam, compared to the case where the ultrasonic wave detection laser beam is a continuous wave output, It is possible to perform highly accurate measurement.

  Moreover, if the pulse repetition frequency of the ultrasonic detection laser beam is the same as the pulse repetition frequency of the ultrasonic generation laser beam, the most efficient and accurate measurement can be performed, but there are restrictions on the laser light source. In some cases, the same pulse repetition frequency is not necessarily required. For example, as shown in FIG. 4B, even if the pulse repetition frequency of the ultrasonic detection laser light is set to be twice that of the ultrasonic generation laser light, the ultrasonic detection laser light is output as a continuous wave. It is possible to perform measurement with higher accuracy than in the case of. Any other pulse repetition frequency may be used as long as the measurement of the present invention is possible.

  As shown in FIG. 5, it is preferable that the irradiation region on the surface of the object to be inspected is irradiated with a laser beam for ultrasonic wave generation with a spot-like spot, and the laser beam for ultrasonic detection is irradiated into the irradiation region. By irradiating the laser beam for ultrasonic generation and the laser beam for ultrasonic detection almost at the same position, the wave that travels in the plane direction is not detected by the ultrasonic detection laser, and it can be detected by moving Only waves that are not, that is, plate waves with zero group velocity and longitudinal waves.

  In addition, there are multiple modes in the plate wave mode with zero group velocity, but the displacement is the largest in the direction perpendicular to the laser irradiation surface of the object to be inspected, and can be detected in the thermoelastic region of the laser ultrasonic method The only mode is the plate wave frequency of the S1 mode with zero group velocity.

  Furthermore, if the laser beam for ultrasonic detection is irradiated into the spot-like spot area of the laser for ultrasonic wave generation, the laser beam for ultrasonic wave detection is efficiently subjected to Doppler shift due to the generated ultrasonic wave. Zero plate waves and longitudinal waves can be detected with high accuracy.

  The amount of Doppler shift of ultrasonic detection laser light that has undergone Doppler shift includes, for example, a method in which the ultrasonic detection laser light is interfered with an interferometer such as a Fabry-Perot interferometer.

  The Fabry-Perot interferometer serves as a wavelength filter. The transmittance of the Fabry-Perot interferometer varies greatly depending on the frequency of light, as shown in FIG.

  The frequency of the laser beam for ultrasonic detection incident on the Fabry-Perot interferometer slightly changes depending on the amount of Doppler shift received from the ultrasonic wave propagating through the inspection object, that is, the surface displacement speed of the inspection object. By introducing and outputting ultrasonic detection laser light having a changed frequency to a Fabry-Perot interferometer, the change in frequency can be converted into a relatively large change in light intensity.

  Then, the vibration state of the surface of the object to be inspected can be obtained by measuring the intensity change of the ultrasonic detection laser beam that has passed through the Fabry-Perot interferometer.

  In the present invention, the method used for measuring the amount of Doppler shift is not limited to the method using a Fabry-Perot interferometer.

  For example, ultrasonic detection laser light that has undergone Doppler shift is introduced into an optical fiber grating or microstructured optical fiber that has a lossy wavelength characteristic that is steep near the wavelength of the ultrasonic detection laser light, A method of calculating the frequency spectrum of the ultrasonic wave by using the intensity as the intensity corresponding to the amount of Doppler shift can be employed.

The apparatus for measuring the S1f of plate wave ultrasonic waves and the longitudinal wave resonance frequency f ₁ according to the present invention and obtaining the Poisson's ratio from the results is as follows: an ultrasonic generation laser irradiating unit, an ultrasonic detection laser beam Are composed of an ultrasonic detection laser irradiation unit, a receiving unit, a first processing unit, a second processing unit, and a third processing unit.

  The ultrasonic wave generating laser irradiation unit includes a pulsed laser light source for generating ultrasonic waves by irradiating the surface of the inspection object with pulsed laser light for generating ultrasonic waves and exciting the ultrasonic waves. Furthermore, an irradiation optical system including a lens, a mirror, and the like is provided as necessary.

  The ultrasonic detection laser irradiation unit includes a continuous wave laser light source or a pulsed laser light source that irradiates the surface of the inspection object with laser light for ultrasonic detection. Furthermore, an irradiation optical system including a lens, a mirror, and the like is provided as necessary.

  The receiving unit receives ultrasonic detection laser light that has undergone Doppler shift due to ultrasonic vibration generated by irradiating the object to be inspected with pulse generation laser light for ultrasonic generation, and has an intensity corresponding to the amount of Doppler shift. It has a mechanism to output the light.

  The first processing unit includes an electronic computer or the like, and calculates a frequency spectrum from information on the amount of Doppler shift output from the receiving unit. Information output from the receiving unit may be input to the first processing unit as an electrical signal via an optical / electrical converter.

The second processing unit is composed of an electronic computer or the like, performs frequency analysis of the frequency spectrum calculated by the first processing unit, identifies a frequency peak, and plate wave ultrasonic frequency S1f and longitudinal wave resonance frequency f _1. Is calculated.

The third processing unit includes an electronic computer or the like, and calculates the Poisson's ratio ν of the object to be inspected from the plate wave ultrasonic frequency S1f and the longitudinal wave resonance frequency f ₁ obtained by the second processing unit. .

  The first processing unit, the second processing unit, and the third processing unit do not need to be physically configured separately from each other. For example, a program for performing each processing is performed in the same electronic computer. You may have it.

  Hereinafter, an example of the Poisson's ratio measuring method according to the present invention will be described in more detail with reference to the drawings.

  FIG. 7A shows an outline of the measuring apparatus of the present invention.

  For the ultrasonic wave generation laser light source 21 of the ultrasonic wave generation laser irradiation section, for example, a pulse output Q-switched double wave Nd: YAG laser can be used.

The laser beam GL emitted from the laser light source 21 for generating ultrasonic waves is irradiated on the surface of the inspection object 11 through the mirror 22a, the filter 23, and the mirror 22b, the beam diameter of which is enlarged by the condenser lens 24. At this time, the transmittance of the filter 23 is determined or the output of the laser light source 21 for generating ultrasonic waves is adjusted so that the fluence in the irradiation region on the surface of the object to be inspected is about ² mJ / mm ² or less. .

  The pulse repetition frequency of the ultrasonic wave generation laser beam GL is about 10 to 200 Hz, although it depends on the characteristics of the Q-switch Nd: YAG laser having a pulse output to be used. Further, the irradiation angle of the laser beam GL irradiated on the surface of the inspection object 11 is practically within ± 30 ° in the direction perpendicular to the surface. These are not particularly limited.

  On the surface of the object 11 to be inspected, the laser beam GL for generating ultrasonic waves is irradiated to cause a rapid temperature rise, and the ultrasonic waves are excited by the thermoelastic effect.

  As described above, the ultrasonic detection laser light DL emitted from the ultrasonic detection laser light source 31 is applied to the same position as the irradiation position of the ultrasonic generation laser light GL on the inspection object 11.

  For plate wave ultrasonic waves with a frequency at which the group velocity is zero, the energy of the plate wave ultrasonic waves does not propagate through the body to be inspected, but resonates locally at the irradiation position of the laser beam that is the source of generation. When the irradiation position of the laser beam DL is made coincident with the irradiation position of the laser beam GL for generating ultrasonic waves, it is possible to measure the frequency of the plate wave ultrasonic wave whose group velocity is zero.

  Similarly, the longitudinal wave resonates locally at the irradiation position of the ultrasonic wave generation laser beam GL, so that the irradiation position of the ultrasonic wave detection laser beam DL coincides with the irradiation position of the ultrasonic wave generation laser beam GL. The longitudinal frequency can be measured.

  The wavelength of the ultrasonic detection laser beam DL changes due to Doppler shift caused by the ultrasonic wave generated on the inspection object 11 by the ultrasonic wave generation laser beam GL. That is, the reflected light of the ultrasonic detection laser beam DL has information on the ultrasonic waves generated on the inspection object 11.

  This information is transmitted via the reception unit 41 to a frequency spectrum calculation unit 51a that is a first processing unit, a frequency peak identification calculation unit 51b that is a second processing unit, and a Poisson ratio calculation unit 51c that is a third processing unit. To calculate the Poisson's ratio.

  You may provide the display apparatus 61 which outputs a result as needed.

  FIG. 7B is a diagram showing another example of the measurement apparatus of the present invention, and shows an outline of the measurement apparatus when a Fabry-Perot interferometer is used as a receiving unit.

  In the measurement apparatus of FIG. 7B, the laser beam DL emitted from the ultrasonic detection laser light source 31 is transmitted by the polarization beam splitter (hereinafter referred to as “PBS”) 34a, the P-polarized component DLP is transmitted, and the S-polarized light is transmitted. The component DLS is reflected. The P-polarized component DLP of the laser beam DL is irradiated on the surface of the inspection object 11 via the mirrors 32a and 32b.

  The P-polarized component DLP of the ultrasonic detection laser beam DL irradiated on the inspection object 11 is Doppler shift Δf = according to the surface displacement speed V of the inspection object 11 due to the ultrasonic wave generated on the inspection object 11. 2V / λ is received and reflected. λ is the wavelength of the laser beam for ultrasonic detection.

  The reflected laser light passes through the PBS 34b through the condenser lens 33 and the mirrors 32c and 32d, and enters the Fabry-Perot interferometer 42.

  The irradiation angle of the P-polarized component DLP of the laser beam DL irradiated to the inspection object 11 is not particularly limited, but is practically within ± 30 ° in the direction perpendicular to the surface of the inspection object 11. .

  On the other hand, the S-polarized component DLS of the laser beam DL divided by the PBS 34 a is reflected by the PBS 34 b and enters the Fabry-Perot interferometer 42.

  The P-polarized component DLP of the laser beam DL incident on the Fabry-Perot interferometer 42 has a slight frequency depending on the amount of Doppler shift received from the ultrasonic wave propagating through the inspection object 11, that is, the surface displacement speed of the inspection object 11. However, this is converted into a relatively large change in transmitted light intensity by transmitting it through the Fabry-Perot interferometer 42.

  Then, by measuring the intensity change of the P-polarized component DLP of the laser beam DL transmitted through the Fabry-Perot interferometer 42, the vibration state of the surface of the device under test 11 can be obtained.

  Here, the magnitude of the Doppler shift Δf is approximately 0.01 to 0.1 Hz. Therefore, it is preferable to use a laser light source with high frequency stability for the ultrasonic light source 31 for ultrasonic detection. Although there is no particular limitation, it is preferable that the frequency drift is about 100 Hz / s or less.

  The transmission characteristics of the Fabry-Perot interferometer 42 are preferably about 1 to 10 MHz for FWHM (Full Width Half Max) and about 100 MHz to 1 GHz for FSR (Free Spectral Range). Note that FWHM is a width value in the range of x where f (x) is a value equal to or greater than half of the maximum value when a certain function f (x) indicates the shape of a mountain-shaped local function. It is. FSR is an abbreviation for the free spectral region, and is a value defined as the difference between adjacent resonance peak frequency values (see FIG. 6).

  The S-polarized component DLS of the laser beam DL is emitted from the Fabry-Perot interferometer 42, reflected by the PBS 43, and converted into an electric signal ES1 corresponding to the intensity of the laser beam by an APD (avalanche photodiode) 44a. To the stabilization circuit 45.

  The intensity of the S-polarized component DLS of the laser beam DL incident on the APD 44a should not change because the path of the laser beam on the way is always in the same state. If the intensity changes, the distance between two reflecting mirrors (not shown) constituting the resonator of the Fabry-Perot interferometer 42 changes due to disturbance such as external vibration, and the characteristics of the Fabry-Perot interferometer 42 are changed. This may be caused by a change in the frequency or a fluctuation in the oscillation frequency of the ultrasonic light source 31 for ultrasonic detection.

  In that case, the reflection mirror of the Fabry-Perot interferometer 42 is optimally positioned by the electric signal ES2 from the stabilization circuit 45 so that the intensity of the S-polarized component DLS of the laser beam DL incident on the APD 44a is constant. Adjust so that For example, a piezo element or the like can be used to adjust the position of the reflecting mirror.

  The P-polarized component DLP of the laser beam DL is emitted from the Fabry-Perot interferometer 42, passes through the PBS 43, is converted into an electric signal ES3 by the APD 44b, and is sent to the calculation unit 51.

  The intensity waveform calculated by the frequency spectrum calculation unit 51a is, for example, as shown in FIG. When this is processed by FFT in the frequency peak identification calculation unit 51b, it is as shown in FIG.

  FIG. 10 shows the interval (frequency) between each peak in FIG. 9 and the adjacent peak. FIG. 10 shows that only the first peak is a different component.

From Equation 1, since S1f = β1V _L / 2d and β1 <1 as shown in FIG. 3A, S1f is smaller than the frequency of the longitudinal wave. From this, it can be seen that the first peak is for S1f and the other is for longitudinal waves.

  As can be seen from Equation 1 and FIG. 3A, a peak indicating the frequency of the primary longitudinal wave should be observed near the peak indicating S1f, but this is not clear in FIG. This is due to the frequency characteristics of the Fabry-Perot interferometer when a Fabry-Perot interferometer is used as the receiving unit.

However, the nature of the longitudinal wave, and the frequency of the n-order longitudinal wave, the frequency difference of the (n + 1) order longitudinal wave, even if the order of the longitudinal wave increases is constant, because the difference is equal to f _1, 1 The fact that the peak indicating the next longitudinal wave frequency is not directly observed is not particularly a problem. Of course, other receivers that can observe the primary longitudinal wave frequency may be used.

In the present invention, f ₁ may be obtained from a peak indicating the frequency of any order longitudinal wave. When a peak indicating three or more orders of longitudinal waves can be observed, the average value of the frequency differences from the adjacent longitudinal waves may be set as f ₁ .

  Hereinafter, the present invention will be described more specifically with reference to examples.

  The Poisson's ratio of the steel material was measured using the measuring device shown in FIG. 7B. As the steel material, S55C having a thickness of 2 mm was used.

  A pulse-switched Q-switched double wave Nd: YAG laser was used as the ultrasonic light source laser light source, and a pulse-output Q-switch Nd: YAG laser was used as the ultrasonic wave detection laser light source. The measurement conditions are shown in Table 1.

The fluence of the ultrasonic wave generating laser beam on the object to be inspected was set to 1.8 mJ / mm ² . This is energy that does not cause ablation. Further, the distance between the object to be inspected and the detection system in Table 1 means the shortest distance between the object to be inspected and the detection system, and in the example of FIG. 7B, is the distance between the object to be inspected 11 and the condenser lens 33. .

As a result of the measurement, ultrasonic waves having the intensity waveform shown in FIG. 8 are observed, and when the intensity waveform is FFT processed, the result is as shown in FIG. 9, and the interval between each peak and the adjacent peak is as shown in FIG. It was. From this result, it was found that the plate wave frequency S1f = 0.905 MHz and the longitudinal wave resonance frequency f ₁ = 0.979 MHz with zero group velocity, and from FIG. 3A, the Poisson's ratio ν was calculated to be 0.287.

[Comparative example]
In order to check whether the results obtained in the examples are appropriate, the Poisson's ratio ν was calculated by a known method that uses ablation. In the comparative example, the intensity of the pulsed laser beam for generating ultrasonic waves applied to the object to be inspected is 3.5 mmJ / mm ² , which is strong enough to cause minute ablation.

  The measurement results were S1f = 0.902, A2f = 1.571, and the Poisson's ratio ν was calculated to be 0.291.

  When the surface of the inspected object measured in the comparative example was observed, irradiation marks due to ablation were observed.

  From the above results, the measurement results using the measurement method according to the present invention and the measurement results according to the conventional method are in good agreement, and the measurement method of the present invention does not cause laser irradiation marks on the object to be inspected. It was confirmed that appropriate measurement results were obtained.

  The steel material was taken out after being held at 1000 ° C. in a heating furnace, and the Poisson ratio was measured every 0.2 seconds by the Poisson ratio measuring method of the present invention while naturally cooling from about 900 ° C.

  The results are shown in FIG. In FIG. 11, the horizontal axis represents temperature and the vertical axis represents Poisson's ratio. That is, the measurement in this example is measured in order from the point plotted at the upper right to the point plotted at the lower left. The vicinity of 650 ° C. is a transformation region, and the temperature rises due to transformation heat. FIG. 11 shows that the relationship between temperature and Poisson's ratio changes before and after transformation.

  When the object to be inspected after the measurement was observed, no irradiation marks due to laser light irradiation were observed.

  As mentioned above, although this invention was demonstrated using the Example, it cannot be overemphasized that the aspect of this invention is not restrict | limited to said Example.

  According to the present invention, in the laser ultrasonic method, it is possible to calculate the Poisson's ratio without causing ultrasonic irradiation using the thermoelastic effect and generating a laser irradiation mark on the surface of the object. Applicable to destructive inspection.

  Furthermore, since the present invention is a non-destructive and non-contact measuring method, the present invention is applied online during, for example, a metal manufacturing process, and online, physical property values such as the Poisson's ratio of the actual product being manufactured can be obtained. It can be measured, and can be used to feed back to manufacturing conditions immediately during manufacturing.

DESCRIPTION OF SYMBOLS 1 Object 2 Laser beam 3 Evaporation of object 4 Ultrasound 5 Temperature rise area 6 Pulse waveform of laser beam for ultrasonic generation 7 Pulse waveform of laser beam for ultrasonic detection 8 Continuous wave waveform of laser beam for ultrasonic detection 9 Irradiation spot of laser beam for ultrasonic generation 10 Irradiation spot of laser beam for ultrasonic detection 11 Inspected object 21 Laser light source for ultrasonic generation 22a, 22b Mirror 23 Filter 24 Condensing lens 31 Laser light source for ultrasonic detection 32a, 32b , 32c, 32d Mirror 33 Condensing lens 34a, 34b Polarizing beam splitter 41 Receiving unit 42 Fabry-Perot interferometer 43 Polarizing beam splitter 44a, 44b Avalanche photodiode 45 Stabilization circuit 51 Calculation unit 51a Frequency spectrum calculation unit 51b Frequency peak Identification calculation unit 51c Poisson's ratio calculation 61 Display device DL Laser light for ultrasonic detection DLP P-polarized component of ultrasonic detection laser light DLS S-polarized component of ultrasonic detection laser light GL Laser light for ultrasonic generation

Claims (12)

A method for measuring the Poisson's ratio of an object under test,
Irradiating the object to be inspected with pulsed laser light for generating ultrasonic waves, and generating ultrasonic waves on the object to be inspected;
Irradiating the object to be inspected with ultrasonic wave detecting laser light having a wavelength different from that of the pulse oscillation laser light for generating ultrasonic waves;
The reflected light of the ultrasonic detection laser beam that has undergone Doppler shift due to the vibration of the ultrasonic wave generated by irradiating the object to be inspected with pulsed laser light for ultrasonic generation is received, and the amount of the Doppler shift is received. Outputting light of a high intensity,
Calculating an intensity waveform of an ultrasonic wave generated on the object to be inspected from the amount of Doppler shift, using light having an intensity corresponding to the amount of Doppler shift;
Performing frequency analysis of the intensity waveform of the ultrasonic wave, calculating the frequency of the plate wave ultrasonic wave in the S1 mode with zero group velocity generated in the inspection object, and the resonance frequency of the longitudinal wave;
A Poisson's ratio measuring method comprising: calculating a Poisson's ratio from the calculated frequency of the S1 mode plate wave ultrasonic wave with zero group velocity and the longitudinal resonance frequency.   2. The Poisson's ratio measuring method according to claim 1, wherein the ultrasonic detection laser beam is a pulsed laser beam.   3. The Poisson's ratio measurement method according to claim 2, wherein a peak output of the ultrasonic detection laser beam is 100 to 1000 W and a pulse width is 50 μs to 1 ms.   2. The pulse generation laser beam for generating ultrasonic waves is irradiated with a spot-like spot when generating the ultrasonic waves by irradiating the object to be inspected with the pulse generation laser beam for generating ultrasonic waves. 4. The method for measuring a Poisson's ratio according to any one of items 3.   5. The Poisson's ratio measuring method according to claim 4, wherein the ultrasonic detection laser beam is irradiated into the spot-like spot region where the ultrasonic wave generation pulse oscillation laser beam is irradiated.   The ultrasonic detection laser beam that has been subjected to Doppler shift due to vibration of ultrasonic waves generated by irradiating the object to be inspected with pulse generation laser light for ultrasonic generation is received, and has an intensity corresponding to the amount of Doppler shift The method of outputting light is a method of causing the ultrasonic detection laser beam subjected to the Doppler shift to interfere with an interferometer, and outputting light having an intensity corresponding to the amount of the Doppler shift from the interferometer. The method for measuring a Poisson's ratio according to any one of claims 1 to 5. A device for measuring the Poisson's ratio of an object under test,
A laser irradiation unit for generating ultrasonic waves for irradiating a pulsed laser beam for generating ultrasonic waves to generate ultrasonic waves on the object to be inspected;
An ultrasonic detection laser irradiating unit that irradiates the inspected object with an ultrasonic detection laser beam having a wavelength different from that of the ultrasonic wave generation pulse oscillation laser beam;
The reflected light of the ultrasonic detection laser beam that has undergone Doppler shift due to the vibration of the ultrasonic wave generated by irradiating the object to be inspected with pulsed laser light for ultrasonic generation is received, and the amount of the Doppler shift is received. A receiver that outputs light of high intensity;
A first processing unit that calculates an intensity waveform of an ultrasonic wave generated on the object to be inspected from the amount of Doppler shift, using light having an intensity corresponding to the amount of Doppler shift;
Frequency analysis of the intensity waveform of the ultrasonic wave, a second processing unit that calculates the frequency of the S1 mode plate wave ultrasonic wave of zero group velocity generated in the object to be inspected, and the resonance frequency of the longitudinal wave;
A Poisson's ratio measuring apparatus comprising a third processing unit that calculates a Poisson's ratio from the calculated frequency of the S1 mode plate wave ultrasonic wave with zero group velocity and the resonance frequency of the longitudinal wave.   The Poisson's ratio measuring device according to claim 7, wherein the ultrasonic detection laser irradiation unit includes a pulsed laser light source.   The Poisson's ratio measuring device according to claim 8, wherein the pulsed laser light source is capable of irradiating laser light having a peak output of 100 to 1000 W and a pulse width of 50 µs to 1 ms.   The Poisson's ratio according to any one of claims 7 to 9, wherein the ultrasonic wave generation laser irradiation unit can irradiate the surface of the object to be inspected with a pulsed laser beam with a spot-like spot. Measuring device.   The ultrasonic detection laser irradiating unit can irradiate a laser beam in a spot-like spot region of a pulsed laser beam irradiated on the surface of an inspection object by the ultrasonic wave generating laser irradiating unit. Item 15. The Poisson's ratio measuring device according to Item 10.   The receiving unit inputs and interferes with the ultrasonic detection laser beam which has been subjected to Doppler shift due to ultrasonic vibration generated by irradiating the object to be inspected with pulse generation laser light for ultrasonic generation, and performs Doppler shift. The Poisson's ratio measuring device according to any one of claims 7 to 11, wherein the interferometer outputs light having an intensity corresponding to the amount of light.

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