KR20100106120A - Laser ultrasonic measuring apparatus and laser ultrasonic measuring method - Google Patents

Abstract

PURPOSE: A laser ultrasonic measuring apparatus and a laser ultrasonic measuring method thereof are provided to facilitate frequency measurement of plate-ultrasonic wave of zero group velocity with high precision. CONSTITUTION: A laser ultrasonic measuring apparatus comprises an irradiation optical system, a detection optical system, an optical detecting unit, and a signal processing unit(60). The irradiation optical system irradiates pulse laser light on test materials. The detection optical system emits continuous wave laser light on the test material. The optical detecting unit receives the light laser and outputs light intensity variation as electric signals. The signal processing unit receives electric signals outputted from the optical detecting unit and draws out the frequency of the plate-ultrasonic wave of the test materials. The signal processing unit comprises a frequency output unit(61) and a phase transformation ratio output unit(65).

Description

Laser ultrasonic measuring apparatus 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 a measuring method for detecting ultrasonic waves generated by irradiating a laser on a subject, and more particularly, to a laser ultrasonic measuring apparatus and a measuring method for measuring a phase transformation rate of a material.

The laser ultrasonography transmits an ultrasonic wave by using thermal stress generated by irradiating pulsed laser beam to an inspected object or vaporization stress generated by vaporization of the inspector itself near the surface, and transmits other laser beams to the receiving point continuously. It is a technique of detecting the ultrasonic wave propagated inside the subject under test by receiving the displacement induced by the ultrasonic wave by using the straightness or coherence.

The laser ultrasonic method is capable of performing non-contact detection of material cracks and inherent defects or evaluation of material properties using ultrasonic waves, and application to various material evaluation fields is expected.

With reference to FIG. 13, the principle of the laser ultrasonic method is demonstrated.

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

Δf = 2V / λ

Where V = surface displacement velocity and λ = laser wavelength

The measuring instrument using the laser ultrasonic method is equipped with a laser interferometer, such as a Fabry-Perot interferometer. The Fabriferot interferometer acts as a filter that vibrates and transmits only a specific frequency. For example, when there are defects inside the steel, the Doppler shift amount shows a different value from the conventional steel because the surface vibration is different from that of the conventional 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 in the inspection material can be performed.

In the laser ultrasound method, the state of the inspection object can be observed without bringing anything into contact with the inspection object as long as it is within the reach of the laser light. By using an optical fiber or a mirror, a laser beam can perform ultrasonic inspection relatively easily also to the to-be-tested object which is difficult to contact or access.

The generation principle of the ultrasonic wave by laser light irradiation is demonstrated with reference to FIG. FIG. 14 (a) is a case where 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 FIG. 14 (b) is a test object irradiated with laser light to the test object. This is the case where ultrasonic waves are generated by generating a thermoelastic change.

In the case of ablation, irrigation stresses cause seismic waves, causing irradiation marks on the material. On the other hand, in the case of thermoelasticity, elastic waves are generated due to thermal expansion and contraction due to rapid heating by a laser, but no irradiation traces occur on the material. In the case of thermoelasticity, the material is not damaged, but in the case of thermoelasticity compared with the ablation conditions, the ultrasonic sound pressure generated by the low fluence level is nearly two orders of magnitude lower.

Conventional laser ultrasonication is a method used in an ablation area that damages an object and generates ultrasonic waves. In this method, the intensity of the generated ultrasonic waves is strong and the sound velocity of the longitudinal waves or the transverse waves of the ultrasonic waves is detected (Non Patent Literatures 1 to 5). Moreover, the method (following patent document 1 and 2) which utilizes the sound velocity of the detected longitudinal wave or transverse wave and utilizes a phase transformation rate for a measurement is 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, “'Laser-ultrasonics (LUS) for microstructure characterization”. Research report IM-2003-113, Swedish institute for metals research, 2003.

[Non-Patent Document 3] J.D. Achenbach, “Wave propagation in elastic solids”, Amsterdam, North-Holland, 1973

[Non-Patent Document 4] C.B. Scruby and L.E. Drain, “Laser Ultrasonics-Technologies and Applications”, ISBN 0-7503-0050-7, Adam Hilger, p. 289, 1990.

[Non-Patent Document 5] J. Huang, Y. Nagata, S. Krishnaswamy, and J. D. Achenbach, “Laser-based ultrasonics for flaw detection”, Proc. of IEEE Ultrasonics Symp., p 1205-1209, 1994.

[Patent Document 1] Japanese Patent Laid-Open No. 11-166918

[Patent Document 2] Japanese Patent Application Laid-Open No. 11-166919

In the thermoelastic region in which ultrasonic waves are generated without damaging the surface of the target material, since the ultrasonic generating mechanisms are different, the directivity of the generated propagation direction is greatly different, and as a result, it is difficult to measure the sound velocity of longitudinal and transverse waves. For this reason, it was necessary to use the sound velocity of the longitudinal wave or the transverse wave detected in the ablation region in order to obtain the phase transformation rate.

In view of the above-described problems, a device for measuring the phase transformation rate of a material for measuring the phase transformation rate by measuring the plate ultrasonic wave frequency of zero group velocity in the thermoelastic region where the damage to the surface of the target material can be reduced, not the sound velocity of longitudinal or transverse waves. And an object of the measurement method.

The laser ultrasonic measuring apparatus and laser ultrasonic measuring method of the present invention provided to solve the above problems are as described in the following (1) to (14).

(1) Irradiating pulsed laser light to an inspected object with a laser light source for generating an ultrasonic wave to generate an ultrasonic wave, and irradiating a continuous wave laser light to the inspected object with an ultrasonic detection laser light source and interfering the laser reflected light with an interferometer. In the laser ultrasonic measuring apparatus for detecting the ultrasonic wave propagated inside the test object by detecting a change in light intensity due to the Doppler shift

An irradiation optical system for irradiating the pulsed laser light on a test subject with a point spot;

A detection optical system for irradiating the continuous wave laser light onto the inspected object so as to detect the plate wave ultrasonic wave generated by the point spot and propagating in the thickness direction of the inspected object, and causing the laser reflected light to enter the interferometer and interfering therewith;

A light detector for receiving a laser beam transmitted through the interferometer and outputting a change in light intensity as an electric signal;

An electric signal outputted from the photodetector is input to provide a signal processor for deriving a frequency of the plate wave ultrasonic wave of the inspected object;

The signal processing unit,

A frequency calculator for calculating a symmetric wave frequency by performing frequency analysis on the basis of an electrical signal of light intensity change caused by the wave wave ultrasonic wave generated by the irradiation optical system;

And a phase transformation rate calculating unit for calculating a phase transformation rate of the inspected object from a calibration line indicating a relationship between the product of the symmetric wave frequency, the pre-made symmetric wave frequency, the thickness of the inspected object, and the phase transformation rate. To provide.

(2) The signal processing unit further includes a calibration line preparation unit for inputting a phase transformation rate of the test subject separately measured and preparing a calibration line indicating a product of the symmetric wave frequency and the thickness of the test subject and the phase transformation rate. The laser ultrasonic measuring apparatus described in (1) is provided.

(3) The laser ultrasonic measuring apparatus according to (1) or (2), wherein the continuous wave laser light is irradiated into the irradiation spot region of the pulse laser light.

(4) The frequency analysis provides the laser ultrasonic measuring apparatus according to any one of (1) to (3), wherein the recording time for detecting the plural cycle wave ultrasonic waves is set.

(5) Said symmetric plate wave frequency is the symmetric plate wave mode frequency of group speed zero, The laser ultrasonic measuring apparatus in any one of (1)-(4) characterized by the above-mentioned.

(6) The laser ultrasonic measuring apparatus according to any one of (1) to (5), wherein the phase transformation rate used for preparing the correction line is measured by microscopic tissue observation or thermal expansion coefficient measurement.

(7) The correction line is a linear correction line created from a frequency when the phase transformation rate is 0% and a frequency when the phase transformation rate is 100%. Provided is a laser ultrasonic measuring device.

(8) Irradiating pulsed laser light to an inspected object with a laser light source for generating an ultrasonic wave to generate ultrasonic waves, and irradiating the continuous wave laser light to the inspected object with an ultrasonic detection laser light source and interfering the laser reflected light with an interferometer. In the laser ultrasonic measuring method using a laser ultrasonic measuring device for detecting the change in the light intensity due to the Doppler shift by the ultrasonic wave to detect the ultrasonic wave propagated inside the object,

A laser light irradiation step of irradiating the pulsed laser light on a test subject with a point spot;

An ultrasonic detection step of irradiating the continuous wave laser light onto the inspected object so as to cause interference with the interferometer by detecting the continuous wave laser light generated by the point spot and propagating in the thickness direction of the inspected object in the plate wave ultrasonic wave;

A photodetection step of receiving a laser beam transmitted through the interferometer and outputting a change in light intensity as an electric signal;

A frequency calculating step of calculating a symmetric wave frequency by performing frequency analysis on the basis of the electrical signal;

And a phase transformation rate calculating part step of calculating a phase transformation rate of the subject under test from a calibration line indicating a relationship between the product of the symmetric wave frequency, the pre-created symmetric wave frequency and the thickness of the subject, and the phase transformation rate. Provide a method.

(9) further comprising a calibration line preparation process for inputting a phase transformation rate of the test object separately measured and preparing a calibration line indicating a relationship between the product of the symmetric wave frequency and the thickness of the test object and the phase transformation rate; The laser ultrasonic measuring method described in (8) is provided.

(10) The laser ultrasonic measuring method according to (8) or (9), wherein the ultrasonic detecting step is irradiated into the irradiation spot region of the pulsed laser light.

(11) The frequency analysis provides the laser ultrasonic measuring method according to any one of (8) to (10), wherein a recording time for detecting a plurality of cycle wave ultrasonic waves is set.

(12) The symmetric plate wave frequency is a symmetric plate wave mode frequency of group velocity zero, the laser ultrasonic measurement method according to any one of items (8) to (11).

(13) The method for measuring the ultrasonic wave according to any one of (8) to (12), wherein the phase transformation rate used for preparing the correction line is measured by microscopic tissue observation or thermal expansion coefficient measurement.

(14) The correction line described in any one of (8) to (13), wherein the correction line is a linear correction line created from a frequency when the phase transformation rate is 0% and a frequency when the phase transformation rate is 100%. It provides a laser ultrasonic measuring method.

In the laser ultrasonic measuring apparatus and the laser ultrasonic measuring method, frequency measurement of a plate wave ultrasonic wave with a group velocity of zero can be performed with high precision by a laser beam having a low pulse energy density without damage to the surface of an inspected object. As a result, inspection or material evaluation of cracks and defects of inspection materials with higher precision can be performed non-destructively and non-contact.

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. However, embodiments of the present invention may be modified in various other forms, and the scope of the present invention is not limited to the embodiments described below. The shape and the size of the elements in the drawings may be exaggerated for clarity and the same elements are denoted by the same reference numerals in the drawings.

In the theory of the plate wave described in the said nonpatent literature 3, a plate wave has the symmetric plate wave and the asymmetric plate wave which have several modes, respectively. Here, the plate wave is an ultrasonic wave propagation phenomenon that is well known in the field of ultrasonic use, such as ultrasonic flaw detection, and is a wave that appears to vibrate the entire plate by transmitting the longitudinal wave and the transverse wave as a single body. Asymmetric wave is a wave wave that transmits asymmetrical displacement from the center of plate thickness, and symmetric plate wave is a wave wave that transmits while generating symmetrical displacement from the center of plate thickness.

FIG. 1A is a schematic configuration diagram of the laser ultrasonic measuring apparatus 10 seen from the side of the inspected object 5. FIG. 1 (b) is a view of the inspected object 5 from the laser ultrasonic measuring apparatus 10.

The laser ultrasonic measuring apparatus 10 shown in FIG. 1 (a) has a laser light source 11 for ultrasonic generation, a laser light source 12 for ultrasonic detection, a laser interferometer 13, and a signal processor 60. 1 (b) shows an ultrasonic wave generation laser spot region 6 for irradiating laser light for generation and an ultrasonic wave detection laser spot region 7 for irradiating detection laser light.

The ultrasonic wave generation light source 11 irradiates the laser spot region 6 for ultrasonic generation with the pulsed laser light through an optical system. Ultrasonic detection laser light source 12 irradiates the laser spot region 6 for ultrasonic generation, which is smaller than the ultrasonic spot laser region 6 for ultrasonic generation, through the optical system. Doppler shift occurs in the wavelength of the laser beam for detection by the vibration of the generated plate wave. The laser reflected light of the laser for detection in which the Doppler shift has occurred is guided to an optical laser interferometer 13 such as a Febriferot interferometer by an optical system (not shown) and interferes with the laser interferometer 13. At this time, the resonance condition of the laser interferometer 13 is slightly deviated from the resonance condition corresponding to the original frequency of the detection laser. When the Doppler-shifted detection laser reflected light is transmitted through the laser interferometer 13, the wavelength change due to the Doppler shift is converted into the change in the light intensity in the transmitted laser light.

Then, the transmitted laser light is detected by a photodetector (not shown) constituted by a known photodetector having a good sensitivity to the wavelength of the laser light and converted into an electrical signal. Can be detected as an electrical signal (voltage waveform). Ultrasonic detection in such a laser ultrasonic method is described in the non-patent document 5, for example. The signal processor 60 may derive the desired measurement information by performing signal processing on the electric signal as described below.

The signal processing unit 60 converts the electric signal of the detected wave wave into a digital signal using an A / D converter, and then performs FFT (Fast Fourier Transform) by frequency analysis, for example, to distribute the distribution of the magnitude of each frequency component. To derive. The peak frequency is obtained from the distribution to detect the plate wave frequency of zero group velocity in the subject.

2 and 3, it is shown that the laser ultrasonic measuring apparatus 10 can detect the plate wave at the group velocity zero.

FIG. 2 is a diagram showing waveforms of electric signals when a wave wave is generated and detected by pulsed laser irradiation by the laser ultrasonic measuring apparatus 10. Fig. 2A shows the detected waveform of the detected time domain when irradiated with pulsed laser light. FIG. 2 (b) shows a frequency characteristic obtained by Fourier transforming the time waveform shown in FIG.

The time T1 of FIG. 2 (a) is a time when the ultrasonic wave generation laser beam is irradiated. The frequency waveform shown in FIG. 2 (b) is obtained by performing FFT on the time waveform detected between 90 kHz after the pulse laser irradiation of FIG. The peak F1 on the frequency spectrum shown in Fig. 2B is the largest frequency component obtained by the FFT.

The measurement conditions of the measurement are as follows.

Subject under test: Thickness (d) 2mm, SUS304

Temperature during measurement: room temperature

Time domain with FFT: 90 ms

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

Laser pulse energy density: 1.8 mJ / mm2 (thermoelastic region)

Ultrasonic Detection Laser Light Source: Power 500 ㎽, Wavelength 532 nm

Laser Beam Spot for Ultrasonic Generation: Diameter 5㎜

Detection laser beam spot: diameter 1 mm

Non-patent document 3 describes a calculation formula for calculating the phase velocity of the plate wave. The plate wave Sn (n is an integer) of the symmetric mode is represented by the following equation (1) in the relationship between "speed" and "frequency x thickness". In addition, the group speed Cg is calculated | required by Formula (2).

3 is a diagram showing the frequency of the S1 mode of the group speed zero by logical calculation. 3 shows the relationship between the phase speed c and the frequency f = ω / 2π shown in Equation 1 and the relationship between the group speed Cg and the frequency f = ω / 2π obtained by Equation 2.

The frequency at group speed zero in S1 mode by the logic calculation shown in FIG. 3 coincides with the detected frequency 1.3 MHz (F1). Thus, it can be seen that the detected frequency F1 is the S1 mode fan wave frequency of group speed zero. Thus, it can be seen from the results of Figs. 2 and 3 that the S1 mode plate wave frequency at zero group velocity in the thermoelastic region (irradiation condition without surface damage) can be measured at room temperature.

In addition, in SＵS304, which is the test object 5, the longitudinal sound velocity VL and the transverse sound velocity VS used in Equation 1 are known, the longitudinal wave velocity of the plate wave is 5,900 [m / s], and the transverse sound velocity of the plate wave is 3,200. [m / s], this value is used in the logical calculation of FIG. In addition, SUS304 which is the to-be-tested object 5 is 2 mm in thickness, and this value is used for the logical calculation of FIG.

Since the plate wave with the group velocity of zero stops at the generation point and continues to oscillate in the plate thickness direction, the FFT is performed for a long time waveform of 90 Hz as shown in FIG. It can be seen that the frequency can be measured.

FIG. 4 is a diagram showing an FFT result of an electric signal waveform when generating a wave wave in an S1 wave wave mode by changing the measurement temperature condition by the laser ultrasonic measuring apparatus 10. The measurement conditions in this measurement example are the same as the measurement conditions described with reference to FIG. 2 except for the conditions shown below.

Measurement temperature condition: It heated up to 900 degreeC from room temperature (1 degreeC / 1s), hold | maintains for 5 minutes, and it temperature-falls to 500 degreeC (-20 degreeC / min). Measurements are taken during sample air cooling at 900 ° C, and temperature is the measured value of the thermocouple attached to the sample surface.

Subject: High carbon steel (C = 0.85%), thickness 2mm

Time domain with FFT: 200 ms after irradiation timing of pulse generator for ultrasonic generation

The peaks shown by F2, F3, and F4 are the peak frequencies detected when the temperatures are 529 ° C, 609 ° C, and 711 ° C, respectively. These values are the S1 mode wave frequency at group speed zero at high temperatures. As shown in F2 to F4, it can be seen that the S1 mode plate wave frequency of the group speed zero changes with the measured temperature.

It is a figure which shows the relationship between the temperature of a test subject and the S1 mode plate | board frequency of the measured group speed zero. The detection value shown by (triangle | delta) is S1 mode pan wave frequency of group speed zero measured when heated up to 900 degreeC from room temperature. The detection value shown by (circle) is S1 mode fan wave frequency of group speed zero measured when it cooled from 900 degreeC to 500 degreeC.

For example, the ratio at which iron is transformed from γ iron to α iron is called the phase transformation rate of steel. As the image transformation rate, there is a method of measuring the image transformation rate by performing image processing on an image photographed with an electron microscope, or a method of calculating the image transformation rate using thermal expansion.

Observing the image transformation in the image using an electron microscope may be performed by the human being to determine the transformation rate by looking at the image, or may have a characteristic structure with fine structure, so the image transformation rate may be determined by image recognition. For example, in the case of SＵS, the ferrite structure becomes fine at the time of?-? Transformation using carbides such as Ti ₂ O ₃ , so that the transformation rate can be obtained by image recognition of the miniaturization of the ferrite.

As a method of obtaining the phase transformation rate using thermal expansion, there are methods such as image processing measurement and laser measurement. For example, in the image processing measurement, the thermal expansion coefficient is obtained by measuring the displacement of the measurement location with the image processing on the image of the inspected object.

S1f100 shown in FIG. 5 is a measurement frequency when a phase transformation rate is 100%, and S1f0 shown in FIG. 5 is a measurement frequency when a phase transformation rate is 0%.

By changing the temperature, the phase transformation rate of the steel to be inspected changes. In addition, it turns out that the change of this phase transformation rate differs in the tendency at the time of temperature rising from the case of temperature rising.

In addition, the non-patent document 6 shows that the S1 mode plate | board wave frequency of group speed zero becomes (3). On the other hand, the longitudinal sound velocity (V _L ) and the phase transformation rate have 1: 1 correspondence, as shown in Patent Literatures 1 and 2 and the like. Therefore, it can be seen that the value of the product of the S1 mode plate wave frequency of the group velocity zero and the thickness of the subject has information about the phase transformation rate.

[Non-Patent Document 6] 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.

은 정수이다.Where S1f is the S1 mode wave frequency at group velocity zero,

Is an integer.

Further, as shown in Fig. 6 (a) or 6 (b), a plurality of pieces of sample data are taken by observation or calculation of the above-described phase transformation rate at the phase transformation rate between S1f100xd0 and S1f0xd0. Can be obtained. However, dO is the thickness of the subject used to obtain the calibration line. Fig. 6 (a) is a diagram showing the relationship between the phase transformation rate and the S1 mode wave frequency at group speed zero with a linear calibration line, and Fig. 6 (b) is a nonlinear relationship between the phase transformation rate and S1 mode wave frequency at group speed zero. It is a figure shown with the correction line of. In addition, as a result of obtaining a calibration line by taking a plurality of sample data at a phase transformation rate between S1f100xd0 and S1f0xd0, the relationship between the phase transformation rate and the S1 mode wave frequency of group speed zero may be regarded as linear from the viewpoint of measurement accuracy. In this case, the linear calibration line of FIG. 6 (a) is used, and when it cannot be regarded as linear, the nonlinear calibration line of FIG. 6 (b) is used.

For example, as shown in Fig. 6A, when the linear calibration line is used, the phase transformation rate P of the measurement frequency S1f of an arbitrary subject (thickness d) is used by S1f100xd0 and S1f0xd0. It can be obtained by the following equation (4).

P (%) = 100 x (S1f0 x d0-S1f x d0) / (S1f0 x d0-S1f100 x d0)

In addition, in the case of using the nonlinear calibration line shown in Fig. 6 (b), the relationship between the product of the measured frequency, the thickness of the subject, and the phase transformation rate is obtained by fitting or linearly irradiating from a function such as a polynomial equation. Draw the same relationship.

P (%) = F (S1f × d)

Here, F (S1fxd) shows that it is a function of S1fxd. In the case of measuring the phase transformation rate, the phase transformation rate can be obtained by measuring the measurement frequency S1f of the S1 mode plate wave at the group speed zero and substituting this function (F) together with the thickness of the subject.

Thus, by calculating the phase transformation rate by the equation by calculating the correction line of the phase transformation rate and the group speed zero S1 mode wave frequency × thickness, the S1 mode wave frequency (S1f) of the group speed zero is measured to measure the frequency and thickness of S1f. The phase transformation rate can be calculated from

With reference to FIG. 7, the example which performs a laser ultrasonic measuring method while moving a test subject is demonstrated. The inspected object 5 is conveyed to conveyance facilities, such as a conveyor. The measurement conditions are the same as those described with reference to FIG. 2 except for the measurement conditions shown below.

Carrying Speed: V (mm / s)

Waveform recording time for FFT execution for S1f measurement: 200 ms

In addition, if the waveform recording time for FFT execution is short, the frequency resolution decreases, and if it is long, the noise component becomes dominant, and the SN decreases. Since the object under test moves as the ultrasonic spot laser spot region 7a for ultrasonic generation crosses the ultrasonic spot laser region 6a, the waveform recording time for the FFT execution is the waveform under the measurement conditions described with reference to FIG. It is preferable that the length is about 100 ms to about 200 ms longer than the recording time.

Since the inspected object 5 is conveyed to a conveyance facility, the laser spot area | region 6a for ultrasonic detection becomes rectangular as shown. At this time, the ultrasonic spot laser spot region 7a for generating ultrasonic waves is longer than the length X at which the ultrasonic spot laser spot region for movement is moved within the waveform recording time in order to put the ultrasonic spot laser spot region 6a in the spot region. Long is preferred.

Ｘ = V × T = V (mm / s) × Waveform recording time for executing FFT (s)

It is preferable that the length of the longitudinal direction of the irradiation spot area | region of an ultrasonic wave generation laser shall be more than Ｘ. In addition, it is necessary to set the position of the ultrasonic detection laser spot area | region 6a at the time of the ultrasonic generation laser irradiation downstream of the test subject movement in the ultrasonic generation laser spot area | region 7a.

8-10, the relationship between the moving speed of the test subject 5 and the frequency waveform detected from the test subject 5 is shown. 8 is a detected frequency waveform when the inspected object 5 is moved to 3 [m / s], and FIG. 9 is a detected frequency waveform when the inspected object 5 is moved to 5 [m / s]. 10 is a frequency waveform detected when the test subject 5 is moved to 10 [m / s].

Measurement conditions are the measurement conditions described with reference to FIG. 4 except that the conveyance speeds are different. In the frequency spectrums F5, F6, and F7 of about 1.3 MHz, which are S1 mode wave frequencies of group speed zero, the moving speeds of the subject were 3 [m / s] (Fig. 8), 5 [m / s] (Fig. 9), and 10, respectively. It can be seen that even m / s (FIG. 10) was detected.

As shown in Fig. 8 to Fig. 10, even when the subject 5 is moved, it can be seen that the S1 mode plate wave frequency of zero group speed can be measured. As a result, even when the steel is actually moved by a conveyor or the like, it is possible to measure the S1 mode wave frequency at the group speed zero, and by using the S1 mode wave frequency at the group speed zero detected from the moved steel in this way, FIGS. As shown in FIG. 6, the phase transformation rate can be measured.

As described above, the laser ultrasonic measuring apparatus and the method according to the present embodiment can measure the phase transformation rate with high precision by laser light having a low pulse energy density without damage to the surface of the inspected object.

In addition, the present invention is not limited to the use of such a weak pulse laser, but can also be used in the coexistence region and ablation region of the ablation and thermoelastic change regions.

As described above, the laser ultrasonic measuring apparatus and the method according to the present embodiment calculate the frequency peak by performing FFT on the detected ultrasonic waveform in the thermoelastic region that generates ultrasonic waves without damaging the surface of the target material. The transformation rate can be measured from the frequency value.

Measuring phase transformation is important information to make materials in the metal manufacturing process. For example, in the manufacture of high strength steel sheet, there is a manufacturing method for making high strength by performing the structure control of transforming from iron to α iron, but in this manufacturing, the strength by controlling the ratio, i.e. It is possible to produce a steel sheet with less variation in strength to the same extent. From this point of view, it is very useful to be able to easily measure the phase transformation rate, and to be able to obtain the phase transformation rate with respect to the steel material being conveyed like the actual metal manufacturing process.

11 is a flowchart showing the calculation process of the phase transformation rate. FIG. 11 (a) is a flow which produces the correction line which shows the relationship between the product of the frequency peak in a frequency spectrum, the thickness of a test subject, and a phase transformation rate. Fig. 11B is a flow for calculating the phase transformation rate from the product of the frequency peak on the detected frequency spectrum and the thickness of the subject and the calibration line.

In the flow shown in FIG. 11A, the laser ultrasonic measuring apparatus 10 calculates the S1 mode plate wave frequency of the group velocity zero generated from the object under test as described with reference to FIGS. 2 and 4 (S101). Next, the laser ultrasonic measuring apparatus 10 obtains the phase transformation rate of the subject under test (S102). Here, as mentioned above, the phase transformation rate which was previously measured by an electron microscope or calculated by thermal expansion rate is obtained.

When the predetermined phase transformation rate is calculated in step S102, as shown in FIG. 6, the laser ultrasonic measuring apparatus 10 creates a calibration line with the phase transformation rate of the inspected object, the frequency peak on the frequency spectrum, and the inspected object thickness (S103). In this way, a calibration line is created.

In the flow shown in FIG. 11B, the laser ultrasound measuring apparatus 10 calculates the S1 mode plate wave frequency of the group velocity zero generated in the subject as described in FIGS. 2 and 4 (S151). Next, the laser ultrasonic measuring apparatus 10 calculates an image transformation rate corresponding to a frequency peak on the detected frequency spectrum using the calibration line and the thickness of the object under test prepared in step S103 (S152).

12 is an overall view of a laser ultrasonic measuring apparatus 10a according to one embodiment. As illustrated, the laser ultrasonic measuring apparatus 10a includes a laser light source 11 for generating an ultrasonic wave, a laser light source 12 for detecting an ultrasonic wave, a laser interferometer 13, and a signal processor 60.

The ultrasonic light source 11 for ultrasonic generation irradiates the test object 5 with the high output pulsed laser light EL through the mirrors 31a and 31b to generate an ultrasonic wave to the test object 5. The pulsed laser light EL is focused by the lens 33 and irradiated to the laser spot region 7 for ultrasonic detection as a point spot. The portion to which the pulsed laser light EL of the inspected object 5 is irradiated is thermally expanded, and after that, distortion occurs to cause the ultrasonic wave to be transmitted to the inspected object 5.

The continuous wave laser beam DL is irradiated from the ultrasonic wave detection laser light source 12 to the ultrasonic spot laser spot region 7 for ultrasonic wave through the mirrors 41a and 41b. 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 continuous wave laser light DL from the inspected object 5 is Doppler shifted by surface vibration. The laser reflected light RL is focused by the lens 43 through the mirrors 41c and 41d and enters the laser interferometer 13.

The laser interferometer 13 reciprocates (resonates) the incident laser light by reciprocating between the mirrors 13a and 13b, and transmits part of the light amount to the mirror 13b. The spectrum of the laser beam passing through the laser interferometer 13 is composed of a sharp peak in a very narrow wavelength range and slopes on both sides. The distance between Miller 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 region 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 laser interferometer 13. The light transmitted through the laser interferometer 13 is incident on the photodetector 20 capable of a high speed response made of an avalanche photodiode or the like. In the photodetector 20, the light intensity of the transmitted light is converted into an electric signal E1 and input to the signal processor 60.

In the signal processor 60, the frequency calculator 61 can execute the processes described in steps S101 and S151 in FIG. The phase transformation rate storage part 62 stores the phase transformation rate previously measured or calculated. The thickness information storage part 63 stores the value of the thickness of the subject to be measured in advance. In addition, this value is possible by the measurement by a radiometer and the measurement using a measure. As described in step S102 of FIG. 11, the calibration line preparation unit 64 obtains the phase transformation rate stored in the phase transformation rate storing unit 62, and can create a calibration line as described in step S103 of FIG. 11. The phase transformation rate calculation part 65 can perform the process demonstrated by step S152 of FIG.

The signal processor 60 is composed of a processor. The frequency calculator 61, the calibration line generator 64, and the phase transformation calculator 65 may store a program stored in a memory or an external storage device in which the processor is not shown. You may implement by executing. The phase transformation rate storage unit 62 may be mounted in a memory or an external storage device. The calculation result of the phase transformation rate calculator 65 may be output to the display device 19. In addition, an input / output device and a communication device (not shown) may be connected to the signal processing unit 60. The measured phase transformation rate may be stored in the phase transformation rate storing unit 62 through an input device or a communication device.

When the laser ultrasonic measuring apparatus 10a is disposed in the metal fabrication process, the laser ultrasonic measuring apparatus 10a may not have a function of creating a calibration line, and may calculate a phase transformation rate using a calibration line prepared in advance. In this case, the function of the calibration line preparation unit 64 may be performed by another device (having a processor, memory, communication means, and external storage device) connected to the laser ultrasonic measuring apparatus 10a for data. In addition, the signal processing unit 60 does not have the phase transformation rate storing unit 62 and the calibration line preparing unit 64, and uses the data of the calibration line received through the network or the recording medium in the separate apparatus, and calculates the phase transformation rate calculation unit. 65 can execute the process described in step S152 of FIG.

The present invention is not limited by the above-described embodiment and the accompanying drawings. It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims, .

FIG. 1 (a) is a schematic configuration diagram of the laser ultrasonic measuring apparatus 10 seen from the side of the inspected object 5, and FIG. 1 (b) shows the laser inspected object 5 from the laser ultrasonic measuring apparatus 10. FIG. to be.

Fig. 2 (a) shows the detection waveform when the pulsed laser light is irradiated, and Fig. 2 (b) shows the frequency characteristic obtained by Fourier transforming the time waveform shown in Fig. 2 (a).

3 is a diagram showing the relationship between the S1 mode wave frequency, phase velocity, and group velocity by logic calculation.

4 is a view showing the FFT result of the waveform detected by changing the measurement temperature conditions by the laser ultrasonic measuring apparatus 10.

It is a figure which shows the relationship between the temperature of a test subject and the S1 mode plate | board frequency of the measured group speed zero.

Fig. 6 (a) is a diagram showing a linear calibration line showing the relationship between the phase transformation rate and the S1 mode plate frequency of group velocity zero and the thickness of the subject, and Fig. 6 (b) shows the S1 mode of phase transformation rate and S group velocity zero. It is a figure which shows the relationship of the product of a plate wave frequency and the thickness of a subject with a nonlinear correction line.

FIG. 7 is a view showing the positional relationship between the laser spot for ultrasonic generation and the laser spot for ultrasonic detection when the laser ultrasonic measuring method is applied while moving the inspected object. FIG.

8 is a diagram showing the frequency characteristics of the detected waveform when the moving speed of the subject is 3 m / s.

9 is a diagram showing the frequency characteristics of the detection waveform when the moving speed of the subject is 5 m / s.

10 is a diagram showing the frequency characteristics of the detected waveform when the moving speed of the object under test is 10 m / s.

Fig. 11A is a flowchart for creating a calibration line showing the relationship between the product of the frequency peak on the frequency spectrum and the thickness of the test object and the phase transformation rate, and Fig. 11B is the frequency peak and test thickness on the detected frequency spectrum. Is a flowchart for calculating the phase transformation rate from and the calibration line.

It is a figure explaining the whole structure of a laser ultrasonic measuring apparatus.

It is a figure explaining the measuring principle of a laser ultrasonic method.

It is a figure for demonstrating the two types of generation | occurrence principle of the ultrasonic wave by laser beam irradiation.

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

5:: test subject

6, 6a: Laser spot area for ultrasonic generation

7, 7a: Laser spot area for ultrasonic detection

10, 10a: laser ultrasonic measuring device

11:: laser light source for ultrasonic generation

12 ms: laser light source for ultrasonic detection

13: Laser interferometer

13a, 13b, 31a, 41a, 41b, 41c

19: Display device

20: Photodetector

33, 43 ': Lens

60Hz: Signal processing part

61 Hz: Frequency output section

62: phase transformation rate storing part

63: thickness information storage

64: Calibration line preparation department

65 °: Phase transformation rate calculation unit

Claims (14)

Irradiating pulsed laser light to an object under examination by a laser light source for generating an ultrasonic wave to generate ultrasonic waves, and irradiating a continuous wave laser light to the inspected object with an ultrasonic detection laser light source to interfer the laser reflected light in an interferometer, thereby performing a Doppler by the ultrasonic wave. In the laser ultrasonic measuring apparatus for detecting a change in the light intensity due to the shift to detect the ultrasonic wave propagated inside the test object, An irradiation optical system for irradiating the pulsed laser light on a test subject with a point spot; A detection optical system that irradiates the continuous wave laser light onto the inspected object and causes the laser reflected light to enter and interfere with the interferometer so as to detect a plate wave ultrasonic wave having a group velocity of zero generated by the point spot and propagated in the thickness direction of the inspected object; A light detector for receiving a laser beam transmitted through the interferometer and outputting a change in light intensity as an electric signal; An electrical signal outputted from the photodetector is input, and includes a signal processing unit for deriving a frequency of the pan wave ultrasonic wave of the object under test; The signal processor A frequency calculator for calculating a symmetric wave frequency by performing frequency analysis on the basis of an electrical signal of light intensity change caused by the wave wave ultrasonic wave generated by the irradiation optical system; And a phase transformation rate calculating unit for calculating a phase transformation rate of the inspected object from a calibration line indicating a relationship between the product of the symmetric wave frequency, the pre-made symmetric wave frequency, the thickness of the inspected object, and the phase transformation rate. . The method of claim 1, The signal processing unit may further include a calibration line preparing unit configured to input a phase transformation rate of the inspected object separately and to generate a calibration line indicating a relationship between the product of the symmetric wave frequency and the thickness of the inspected object and the phase transformation rate. Laser ultrasonic measuring device. The method according to claim 1 or 2, And the continuous wave laser beam is irradiated into the irradiation spot region of the pulsed laser beam. The method according to claim 1 or 2, In the frequency analysis, the recording time for detecting the phantom ultrasonic waves of a plurality of cycles is set, characterized in that the laser ultrasonic measuring apparatus. The method according to claim 1 or 2, The symmetric plate wave frequency is a laser ultrasonic measuring apparatus, characterized in that the group speed zero symmetric plate wave mode frequency. The method according to claim 1 or 2, The phase transformation rate used to prepare the correction line is laser ultrasonic measuring apparatus, characterized in that the measurement by microscopic tissue observation or thermal expansion coefficient measurement. The method according to claim 1 or 2, And the calibration line is a linear calibration line prepared from a frequency when the phase transformation rate is 0% and a frequency when the phase transformation rate is 100%. Irradiating pulsed laser light to an inspected object with a laser light source for generating an ultrasonic wave to generate ultrasonic waves, irradiating continuous wave laser light to the inspected object with an ultrasonic detection laser light source, and interfering with the laser reflected light in an interferometer. In the laser ultrasonic measuring method using a laser ultrasonic measuring device for detecting a change in light intensity due to the shift, and detecting the ultrasonic wave propagated inside the test object, A laser light irradiation step of irradiating the pulsed laser light on a test subject with a point spot; An ultrasonic detection step of irradiating the continuous wave laser light onto the inspected object so as to detect the plate wave ultrasonic wave generated by the point spot and propagating in the thickness direction of the inspected object on the inspected object and interfering the laser reflected light by interferometer; A photodetection step of receiving a laser beam transmitted through the interferometer and outputting a change in intensity as an electrical signal; A frequency calculating step of calculating a symmetric wave frequency by performing frequency analysis on the basis of the electric signal; And a phase transformation rate calculating part step of calculating a phase transformation rate of the subject under test from a calibration line indicating a relationship between the product of the symmetric wave frequency, the pre-created symmetric wave frequency and the thickness of the subject, and the phase transformation rate. Way. The method of claim 8, And a calibration line preparation process for inputting a phase transformation rate of the test subject separately measured and preparing a calibration line indicating a relationship between the product of the symmetric wave frequency and the thickness of the test subject and the phase transformation rate. How to measure. The method according to claim 8 or 9, Ultrasonic detection process is irradiated into the irradiation spot area of the pulsed laser light, characterized in that the laser ultrasonic measuring method. 10. The method according to claim 8 or 9, In the frequency analysis, the recording time for detecting the phantom ultrasonic wave of a plurality of cycles is set. 10. The method according to claim 8 or 9, The symmetric plate wave frequency is a laser ultrasonic wave measuring method, characterized in that the group speed zero symmetric wave mode. 10. The method according to claim 8 or 9, Phase transformation rate used to prepare the correction line is a laser ultrasonic measuring method, characterized in that the measurement by microscopic tissue observation or thermal expansion coefficient measurement. 10. The method according to claim 8 or 9, And the calibration line is a linear calibration line prepared from a frequency when the phase transformation rate is 0% and a frequency when the phase transformation rate is 100%.

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