JP5058196B2 - Apparatus and method for measuring phase transformation rate of material - Google Patents

Description

  The present invention relates to a laser ultrasonic measurement apparatus and method for detecting an ultrasonic wave generated by irradiating a material to be inspected with a laser, and more particularly to a laser ultrasonic measurement apparatus and method for measuring a phase transformation rate of a material.

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

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

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

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

  The principle of generation of ultrasonic waves by laser light irradiation will be described with reference to FIG. FIG. 14A shows a case in which ultrasonic waves are generated by ablation, which is a phenomenon of irradiating a part to be inspected with laser light and evaporating a part of the object to be inspected, and FIG. Is applied to the object to be inspected to cause a thermoelastic change in the object to be inspected to generate ultrasonic waves.

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

  The conventional laser ultrasonic method is a method used in an ablation region where ultrasonic waves are generated by damaging a target material. In this method, the intensity of the generated ultrasonic wave is strong, and the sound velocity of the longitudinal wave or the transverse wave of the ultrasonic wave is detected (the following non-patent documents 1 to 5). In addition, a method (hereinafter, Patent Documents 1 and 2) in which the phase transformation rate is utilized for measurement by utilizing the detected acoustic velocity of a longitudinal wave or a transverse wave has been proposed.

JP 11-166918 A JP 11-166919 A

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 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 “J.D. Achenbach,“ Wave propagation in elastic solids ”, Amsterdam, North-Holland, 1973” C.B.Scruby and L.E.Drain, “Laser Ultrasonics -Techniques and Applications”, ISBN0-7503-0050-7, Adam Hilger, p.289, 1990 J. Huang, Y. Nagata, S. Krishnaswamy, and J. D. Achenbach, “Laser-based ultrasonics for flaw detection”, Proc. Of IEEE Ultrasonics Symp., P1205-1209, 1994 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.

  In the thermoelastic region where ultrasonic waves are generated without damaging the surface of the target material, the directivity in the direction of propagation is greatly different because the generation mechanism of the ultrasonic waves is different. It was difficult to measure. Therefore, in order to obtain the phase transformation rate, it is necessary to use the longitudinal and transverse wave velocities detected in the ablation region.

  In view of the above-mentioned problems, the phase transformation rate is determined by measuring the frequency of the plate wave ultrasonic wave with zero group velocity in the thermoelastic region where the damage to the surface of the target material can be reduced, not the acoustic velocity of the longitudinal wave or the transverse wave. An object of the present invention is to provide a measuring device and a measuring method of a phase transformation rate of a material to be measured.

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

(1) An object to be inspected is irradiated with pulsed laser light from a laser light source for generating ultrasonic waves to generate ultrasonic waves, and the object to be inspected is irradiated with continuous wave laser light by a laser light source for detecting ultrasonic waves. A laser ultrasonic measurement apparatus that detects reflected ultrasonic waves propagated through the body to be inspected by detecting reflected light intensity change due to Doppler shift caused by the ultrasonic waves by interfering reflected light with an interferometer,
An irradiation optical system for irradiating the pulsed laser beam on the object to be inspected with a spot-like spot;
The continuous wave laser beam is irradiated on the object to be inspected so as to detect a plate wave ultrasonic wave with zero group velocity generated by the spot-like spot and propagated in the thickness direction of the object to be inspected, and the reflected laser beam is applied to the object. A detection optical system that enters and interferes with the interferometer;
A light detector that receives the laser light transmitted through the interferometer and outputs a change in light intensity as an electrical signal;
An electrical signal output from the light detection unit is input, and a signal processing unit that derives the frequency of the plate wave ultrasonic wave of the object to be inspected,
The signal processing unit
A frequency calculation unit that calculates a frequency of a symmetric plate wave by performing a frequency analysis based on an electrical signal of a light intensity change due to a plate wave ultrasonic wave generated by the irradiation optical system;
The phase transformation rate of the material to be inspected is calculated from the frequency of the symmetric plate wave and a calibration line that is created in advance and represents the relationship between the product of the frequency of the symmetric plate wave and the thickness of the object to be inspected and the phase transformation rate. And a phase transformation rate calculating unit.
(2) The signal processing unit is inputted with a phase transformation rate of the object to be measured separately, and represents a relationship between the product of the frequency of the symmetric plate wave and the thickness of the object to be examined and the phase transformation rate. The laser ultrasonic measurement device according to (1), further comprising a calibration line creation unit that creates a line.
(3) The laser ultrasonic measurement device according to (1) or (2), wherein the continuous wave laser beam is irradiated in an irradiation spot region of the pulse laser beam.
(4) The laser ultrasonic measurement apparatus according to any one of (1) to (3), wherein in the frequency analysis, a recording time for detecting a plate wave ultrasonic wave having a plurality of periods is set.
(5) The laser ultrasonic measurement apparatus according to any one of (1) to (4), wherein the frequency of the symmetric plate wave is a frequency of an S1 symmetric plate wave mode with a group velocity of zero.
(6) The laser ultrasonic wave according to any one of (1) to (5), wherein the phase transformation rate used for creating the calibration line is measured by microscopic observation or thermal expansion coefficient measurement. measuring device.
(7) The calibration line is a linear calibration line created from a frequency when the phase transformation rate is 0% and a phase transformation rate when the phase transformation rate is 100%. The laser ultrasonic measurement device according to any one of 1) to (6).
(8) The object to be inspected is irradiated with pulsed laser light from a laser light source for generating ultrasonic waves to generate ultrasonic waves, and the object to be inspected is irradiated with continuous wave laser light by a laser light source for detecting ultrasonic waves. A laser ultrasonic measurement method using a laser ultrasonic measurement device that detects reflected ultrasonic waves propagating through the body to be inspected by detecting reflected light intensity change due to Doppler shift caused by the ultrasonic waves by interfering reflected light with an interferometer Because
A laser light irradiation step of irradiating the pulsed laser light on the object to be inspected with a spot-like spot;
The continuous wave laser beam is irradiated on the object to be inspected so as to detect a plate wave ultrasonic wave with zero group velocity generated by the spot-like spot and propagated in the thickness direction of the object to be inspected, and the reflected laser beam is applied to the object. An ultrasonic detection step of making the interferometer enter and interfere;
A light detection step of receiving laser light transmitted through the interferometer and outputting a change in light intensity as an electrical signal;
Based on the electrical signal, the frequency calculation step of calculating the frequency of the symmetric plate wave by performing frequency analysis,
The phase transformation rate of the material to be inspected is calculated from the frequency of the symmetric plate wave and a calibration line that is created in advance and represents the relationship between the product of the frequency of the symmetric plate wave and the thickness of the object to be inspected and the phase transformation rate. And a phase transformation rate calculating unit step for performing the laser ultrasonic measurement method.
(9) A calibration line for inputting a phase transformation rate of the object to be inspected separately and creating a calibration line representing the relationship between the frequency of the product of the symmetrical plate wave and the thickness of the object to be inspected and the phase transformation rate The laser ultrasonic measurement method according to (8), further comprising a creation step.
(10) The laser ultrasonic measurement method according to (8) or (9), wherein the ultrasonic detection step is performed in an irradiation spot area of the pulsed laser light.
(11) The laser ultrasonic measurement method according to any one of (8) to (10), wherein in the frequency analysis, a recording time for detecting a plurality of plate wave ultrasonic waves is set.
(12) The laser ultrasonic measurement method according to any one of (8) to (11), wherein the frequency of the symmetric plate wave is a frequency of an S1 symmetric plate wave mode with zero group velocity.
(13) The laser ultrasonic wave according to any one of (8) to (12), wherein the phase transformation rate used for creating the calibration line is measured by microscopic observation or thermal expansion coefficient measurement. Measuring method.
(14) The calibration line is a linear calibration line created from a frequency when the phase transformation rate is 0% and a phase transformation rate when the phase transformation rate is 100%. The laser ultrasonic measurement method according to any one of 8) to (13).

  The above-described laser ultrasonic measurement apparatus and laser ultrasonic measurement method perform high-precision frequency measurement of plate wave ultrasonic waves with zero group velocity using laser light with low pulse energy density that does not damage the surface of the material to be inspected. be able to. As a result, it is possible to perform non-destructive and non-contact inspection of cracks and defects or material evaluation of inspection materials with higher accuracy.

FIG. 1A is a schematic configuration diagram of the laser ultrasonic measurement device 10 viewed from the side surface of the inspection object 5, and FIG. 1B is a view of the inspection object 5 viewed from the laser ultrasonic measurement device 10. FIG. FIG. 2A shows a detection waveform when the pulse laser beam is irradiated, and FIG. 2B shows a frequency characteristic obtained by Fourier transforming the time waveform shown in FIG. FIG. 3 is a diagram showing the relationship between the frequency of the S1 mode plate wave, the phase velocity, and the group velocity by theoretical calculation. FIG. 4 is a diagram showing FFT results of waveforms detected by the laser ultrasonic measurement apparatus 10 while changing the measurement temperature condition. FIG. 5 is a diagram showing the relationship between the temperature of the object to be inspected and the measured frequency of the S1 mode plate wave with zero group velocity. FIG. 6A is a diagram showing a relationship between the product of the phase transformation rate, the frequency of the S1 mode plate wave of zero group velocity, and the thickness of the object to be inspected by a linear calibration line, and FIG. It is a figure which shows the relationship between the product of the phase transformation rate, the frequency of the S1 mode plate wave of zero S group velocity, and the thickness of the object to be inspected by a nonlinear calibration line. FIG. 7 is a diagram showing a positional relationship between an ultrasonic wave generation laser spot and an ultrasonic wave detection laser spot when a laser ultrasonic measurement method is applied while moving an object to be inspected. FIG. 8 is a diagram showing the frequency characteristics of the detected waveform when the moving speed of the object to be inspected is 3 m / s. FIG. 9 is a diagram showing the frequency characteristics of the detected waveform when the moving speed of the test object is 5 m / s. FIG. 10 is a diagram showing the frequency characteristics of the detected waveform when the moving speed of the object to be inspected is 10 m / s. FIG. 11A is a flowchart for creating a calibration line indicating the relationship between the product of the frequency peak on the frequency spectrum and the thickness of the object to be inspected, and the phase transformation rate. FIG. 11B shows the detected frequency. It is a flowchart which calculates a phase transformation rate from the frequency peak on a spectrum, to-be-inspected object thickness, and a calibration line. FIG. 12 is a diagram for explaining the overall configuration of the laser ultrasonic measurement apparatus. FIG. 13 is a diagram for explaining the measurement principle of the laser ultrasonic method. FIG. 14 is a diagram for explaining two types of generation principles of ultrasonic waves by laser light irradiation.

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

  According to the theory of plate waves described in Non-Patent Document 3, plate waves include symmetric plate waves and asymmetric plate waves each having a plurality of modes. Here, the plate wave is a well-known ultrasonic propagation phenomenon in the field of ultrasonic applications such as ultrasonic flaw detection, and is a wave that appears to vibrate the entire plate by propagating the longitudinal wave and the transverse wave together. is there. An asymmetric plate wave is a plate wave that propagates with an asymmetric displacement from the center of the plate thickness, and a symmetric plate wave is a plate wave that propagates with a symmetric displacement from the plate thickness center.

  FIG. 1A is a schematic configuration diagram of the laser ultrasonic measurement device 10 viewed from the side surface of the inspection object 5. FIG. 1B is a view of the inspection object 5 as seen from the laser ultrasonic measurement device 10.

  A laser ultrasonic measurement apparatus 10 shown in FIG. 1A includes an ultrasonic wave generation laser light source 11, an ultrasonic detection laser light source 12, a laser interferometer 13, and a signal processing unit 60. FIG. 1B shows an ultrasonic wave generation laser spot region 6 that irradiates the generation laser beam and an ultrasonic wave detection laser spot region 7 that irradiates the detection laser beam.

  The laser light source 11 for generating ultrasonic waves irradiates the laser spot region 6 for generating ultrasonic waves with pulsed laser light via an optical system. When the ultrasonic wave detection laser light source 12 irradiates the ultrasonic wave detection laser spot region 7 smaller than the ultrasonic wave generation laser spot region 6 through the optical system, the ultrasonic wave generation laser beam is emitted. A plate wave vibration generated in the spot region 6 causes a Doppler shift in the wavelength of the detection laser beam. Then, the laser reflected light of the detection laser in which the Doppler shift has occurred is guided to a laser interferometer 13 such as a Fabry-Perot interferometer by an optical system (not shown) and caused to interfere in the laser interferometer 13. At this time, the resonance condition of the laser interferometer 13 is slightly shifted from the resonance condition corresponding to the original frequency of the detection laser. When the laser reflected light for detection that has been temporarily Doppler shifted is transmitted through the laser interferometer 13, the wavelength change due to the Doppler shift is converted into a change in light intensity in the transmitted laser light.

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

  The signal processing unit 60 takes the detected electric signal having a plate wave waveform as a digital signal by an A / D converter, and then performs FFT (Fast Fourier Transform) as frequency analysis, for example, to determine the distribution of the size of each frequency component. To derive. The peak frequency in the distribution is obtained, and the frequency of the plate wave with zero group velocity generated in the inspection object is detected.

2 and 3, it is shown that the laser ultrasonic measurement device 10 can detect a plate wave with zero group velocity.
FIG. 2 is a diagram showing a waveform of an electric signal when a plate wave is generated by pulse laser irradiation by the laser ultrasonic measurement device 10 and detected. FIG. 2A shows a detected waveform in the time domain detected when the pulse laser beam is irradiated. FIG. 2B shows frequency characteristics obtained by Fourier transforming the time waveform shown in FIG.
A time T1 in FIG. 2A is a time when the pulse laser for generating ultrasonic waves is irradiated. The frequency waveform shown in FIG. 2B is obtained by performing FFT on the time waveform detected during 90 μs after the pulse laser irradiation in FIG. The peak F1 on the frequency spectrum shown in FIG. 2B is the largest frequency component obtained by FFT.

The measurement conditions for the measurement are as follows.
Inspected object: Thickness (d) 2 mm, SUS304
Temperature during measurement: Room temperature Time range during which FFT was executed: 90 μs
Laser light source for ultrasonic generation: YAG laser (maximum pulse energy 450 mJ / pulse (attenuated by ND filter), pulse interval 10 ns
Laser pulse energy density: 1.8 mJ / mm ² (Thermoelastic region)
Laser light source for ultrasonic detection: power 500 mW, wavelength 532 nm
Laser beam spot for ultrasonic generation: 5mm in diameter
Detection laser beam spot: 1mm in diameter

  Non-Patent Document 3 describes a calculation formula for calculating the phase velocity of a plate wave. The plate wave Sn (n is an integer) in the symmetric mode is expressed by the following formula 1 in the relationship of “speed” and “frequency × thickness”. Further, the group velocity Cg is obtained by Equation 2.

FIG. 3 is a diagram showing the frequency of the S1 mode with zero group velocity by theoretical calculation. FIG. 3 shows the relationship between the phase velocity (c) and the frequency (f = ω / 2π) shown in Equation 1, and the relationship between the group velocity (Cg) and the frequency (f = ω / 2π) obtained by Equation 2. It is.
The frequency at the group velocity of zero in the S1 mode according to the theoretical calculation shown in FIG. 3 matches the detected frequency of 1.3 MHz (F1). From this, it can be seen that the detected frequency (F1) is the frequency of the S1 mode plate wave with zero group velocity. Therefore, it can be seen from the results of FIGS. 2 and 3 that the frequency of the S1 mode plate wave with zero group velocity in the thermoelastic region (irradiation condition without surface damage) can be measured at room temperature.
In SUS304, which is the object to be inspected 5, the longitudinal wave velocity (VL) and the transverse wave velocity (VS) used in Equation 1 are known, and the longitudinal wave velocity of the plate wave: 5,900 [m / s], the plate The shear wave velocity of the wave is 3,200 [m / s], and this value is used in the theoretical calculation of FIG. Further, SUS304, which is the object to be inspected 5, has a thickness of 2 mm, and this value is also used in the theoretical calculation of FIG.
Since the plate wave with zero group velocity stays at the point of occurrence and continues to vibrate in the plate thickness direction, for example, as shown in FIG. 2B, an FFT is applied to a long waveform of 90 μs so that multiple cycles can be detected. By doing this, you can see that the frequency can be measured.

FIG. 4 is a diagram showing the FFT result of the electric signal waveform when the ultrasonic wave measuring apparatus 10 generates a plate wave in the S1 plate wave mode while changing the measurement temperature condition and detects the plate wave. The measurement conditions in the measurement example are the same as the measurement conditions described with reference to FIG. 2 except for the following conditions.
Temperature conditions during measurement: The temperature was raised from room temperature to 900 ° C. (1 ° C./1 s), held for 5 minutes, and lowered to 500 ° C. (−20 ° C./min). The measurement is performed from 900 ° C. during sample air cooling, and the temperature is a measured value of a thermocouple attached to the sample surface.
Inspected object: High carbon steel (C = 0.85%), thickness 2 mm
Time domain in which FFT is performed: 200 μs after the irradiation timing of the pulse laser for generating ultrasonic waves

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

  FIG. 5 is a diagram showing the relationship between the temperature of the object to be inspected and the measured frequency of the S1 mode plate wave with zero group velocity. The detected value indicated by Δ is the frequency of the S1 mode plate wave at zero group velocity measured when the temperature is raised from room temperature to 900 ° C. The detection value indicated by ◯ is the frequency of the S1 mode plate wave with zero group velocity measured when the temperature is lowered from 900 ° C. to 500 ° C.

For example, the ratio when transforming from γ iron to α iron is referred to as the phase transformation rate of the steel material. The phase transformation rate includes a method of observing a phase transformation by performing image processing on an image taken with an electron microscope, or a method of calculating a phase transformation using thermal expansion.
In observing the phase transformation from the image using an electron microscope, a human may judge the transformation rate by looking at the image, or the phase transformation may be obtained by image recognition from the characteristics of the fine structure. For example, in the case of SUS, the ferrite structure is refined at the time of the γ → α transformation using oxides such as Ti 2 O 3, so that the transformation rate can be obtained by recognizing this ferrite refinement as an image. .

  Methods for obtaining phase transformation using thermal expansion include methods such as image processing measurement and laser measurement. For example, in image processing measurement, the coefficient of thermal expansion is obtained by measuring the displacement of a measurement location by image processing on an image of an object to be inspected.

S1f100 shown in FIG. 5 is a measurement frequency when the phase transformation rate is 100%, and S1f0 shown in FIG. 5 is a measurement frequency when the phase transformation rate is 0%.
By changing the temperature, the phase transformation rate of the steel material that is the object to be inspected changes. It can also be seen that the change in the phase transformation rate has a different tendency between when the temperature is raised and when the temperature is lowered.

Note that the non-patent document 6 shows that the frequency of the S1 mode plate wave with zero group velocity is expressed by Equation 3. On the other hand, there is a one-to-one correspondence between the longitudinal sound velocity _VL and the phase transformation rate, as shown in Documents 1, 2 and the like. Therefore, it can be seen that the product of the frequency of the S1 mode plate wave of zero group velocity and the thickness of the object to be inspected has information on the phase transformation rate.

As shown in FIG. 6A or FIG. 6B, a plurality of sample data is obtained by observing or calculating the phase transformation rate as described above at the phase transformation rate between S1f100 × d0 and S1f0 × d0. A simple calibration line. Here, d0 is the thickness of the inspection object used for obtaining the calibration line. FIG. 6A is a diagram showing the relationship between the phase transformation rate and the frequency of the S1 mode plate wave having zero group velocity with a linear calibration line, and FIG. 6B is a diagram showing the phase transformation rate and group velocity of zero. It is a figure which shows the relationship with the frequency of S1 mode plate wave with a nonlinear calibration line. As a result of taking a plurality of sample data at the phase transformation rate between S1f100 × d0 and S1f0 × d0 and obtaining a calibration line, the relationship between the phase transformation rate and the frequency of the S1 mode plate wave with zero group velocity is measured. When it can be regarded as linear from the viewpoint of accuracy, the linear calibration line of FIG. 6A is used, and when it cannot be regarded as linear, the nonlinear calibration line of FIG. 6B is used.
For example, as shown in FIG. 6 (a), when a linear calibration line is used, the phase transformation rate (P) of the measurement frequency S1f of an arbitrary inspection object (thickness d) is S1f100 × d0 and S1f0 × d0. Can be obtained by the following formula 4.
P (%) = 100 × (S1f0 × d0−S1f × d) / (S1f0 × d0−S1f100 × d0) (Equation 4)
When the nonlinear calibration line shown in FIG. 6B is used, the relationship between the product of the measured frequency and the thickness of the object to be inspected and the phase transformation rate is fitted with a function such as a polynomial, or By approximating the broken line, the relationship as shown in the following formula 5 is derived.
P (%) = F (S1f × d) (Formula 5)
Here, F (S1f × d) indicates a function of S1f × d. When measuring the phase transformation rate, the phase transformation rate can be obtained by measuring the measurement frequency S1f of the S1 mode plate wave of zero group velocity and substituting it into the function F together with the thickness of the object to be inspected. I can do it.
Thus, by obtaining the calibration line between the phase transformation rate and the frequency × thickness of the S1 mode plate wave of zero group velocity, and allowing the phase transformation rate to be calculated by a mathematical formula, the S1 mode plate wave of zero group velocity is calculated. By measuring the frequency (S1f), the phase transformation rate can be calculated from the frequency and thickness of S1f.

An example in which a laser ultrasonic measurement method is performed while moving an object to be inspected will be described with reference to FIG. The inspected object 5 is transported to a transport facility such as a conveyor. The measurement conditions are the same as those described with reference to FIG. 2 except for the measurement conditions described below.
Conveyance speed: V (mm / s)
Waveform recording time for FFT execution for S1f measurement: 200 μs
If the waveform recording time for executing FFT is short, the frequency resolution is lowered, and if it is long, the noise component is dominant and SN is lowered. As the object to be inspected moves, the ultrasonic spot generation laser spot region 7a moves so that the ultrasonic detection laser spot region 6a traverses, and the waveform recording time for performing FFT has been described with reference to FIG. It takes longer than the waveform recording time under the measurement conditions, and is preferably about 100 μs to 200 μs.

Since the inspection object 5 is transported to the transport facility, the ultrasonic detection laser spot region 6a has a rectangular shape as illustrated. At this time, the ultrasonic spot spot 7a for ultrasonic wave generation has a length that the ultrasonic spot spot for ultrasonic detection moves within the waveform recording time in order to place the ultrasonic spot spot 6a for ultrasonic detection in the spot area. It is preferable to make the longitudinal direction longer than X.
X = V × T
= V (mm / s) x Waveform recording time for FFT execution (s)
The length in the longitudinal direction of the irradiation spot area of the ultrasonic wave generating laser is preferably X or more. In addition, the position of the ultrasonic detection laser spot region 6a at the time of irradiation with the ultrasonic generation laser needs to be set on the downstream side of the object movement in the ultrasonic generation laser spot region 7a.

The relationship between the moving speed of the inspection object 5 and the frequency waveform detected from the inspection object 5 is shown using FIGS. FIG. 8 shows a detected frequency waveform when the inspection object 5 is moved at 3 [m / s], and FIG. 9 shows a detected frequency when the inspection object 5 is moved at 5 [m / s]. FIG. 10 is a frequency waveform detected when the inspection object 5 is moved at 10 [m / s].
The measurement conditions are the same as those described with reference to FIG. The frequency spectrums F5, F6, and F7 of about 1.3 MHz, which is the frequency of the S1 mode plate wave with zero group velocity, have an object movement speed of 3 [m / s] (FIG. 8) and 5 [m / s], respectively. (FIG. 9) It can be seen that even 10 [m / s] (FIG. 10) is detected.

  In this way, as shown in FIGS. 8 to 10, it can be seen that the frequency of the S1 mode plate wave with zero group velocity can be measured even when the inspection object 5 is moved. From this result, even when the steel material is actually moved by a conveyor or the like, it is possible to measure the frequency of the S1 mode plate wave with zero group velocity, and thus the group velocity zero detected from the steel material thus moved. Using the frequency of the S1 mode plate wave, the phase transformation rate can be measured as shown in FIGS.

As described above, the laser ultrasonic measurement apparatus and method according to the present embodiment measure the phase transformation rate with high accuracy by using a laser beam with a low pulse energy density that does not damage the surface of the material to be inspected. Can do.
Further, the present invention is not limited to the use of such a weak pulse laser, and can be used even in a coexistence region of ablation and a thermoelastic change region, or an ablation region.

  As described above, the laser ultrasonic measurement apparatus and method according to the present embodiment perform FFT on the detected ultrasonic waveform in the thermoelastic region that generates ultrasonic waves without damaging the surface of the target material. To calculate the frequency peak and measure the phase transformation rate from the frequency value.

  Measuring the phase transformation rate is important information for building materials in the metal manufacturing process. For example, in the production of high-strength steel sheets, there is a production method in which the structure control for transformation from γ iron to α iron is performed to increase the strength, but in the production, the ratio when transforming from γ iron to α iron, that is, the transformation rate It is possible to manufacture a steel plate with less variation in strength with the same strength by adjusting well. From this point of view, it is extremely useful that the phase transformation rate can be easily measured and that the phase transformation rate can be obtained for the steel material being conveyed as in the actual metal production process.

FIG. 11 is a flowchart showing a calculation process of the phase transformation rate. FIG. 11A is a flow for creating a calibration line indicating the relationship between the product of the frequency peak on the frequency spectrum and the thickness of the object to be inspected and the phase transformation rate. FIG. 11B is a flow for calculating the phase transformation rate from the product of the detected frequency peak on the frequency spectrum and the thickness of the object to be inspected and the calibration line.
In the flow shown in FIG. 11A, the laser ultrasonic measurement apparatus 10 calculates the frequency of the S1 mode plate wave with zero group velocity generated from the object to be inspected as described in FIGS. S101). Next, the laser ultrasonic measurement device 10 acquires the phase transformation rate of the object to be inspected (S102). Here, as described above, a phase transformation rate measured in advance with an electron microscope or calculated with a coefficient of thermal expansion is acquired.

  When the predetermined phase transformation rate is calculated in step S102, as shown in FIG. 6, the laser ultrasonic measurement device 10 determines the phase transformation rate of the object to be inspected, the frequency peak on the frequency spectrum, and the thickness of the object to be inspected. A calibration line is created from (S103). In this way, a calibration line is created.

  In the flow shown in FIG. 11B, the laser ultrasonic measurement apparatus 10 calculates the frequency of the S1 mode plate wave with zero group velocity generated from the object to be inspected as described in FIGS. S151). Next, the laser ultrasonic measurement device 10 calculates the phase transformation rate corresponding to the frequency peak on the detected frequency spectrum using the calibration line and the thickness of the object to be inspected created in step S103 (S152).

FIG. 12 is an overall view of a laser ultrasonic measurement apparatus 10a according to an embodiment. As illustrated, the laser ultrasonic measurement device 10 a includes an ultrasonic wave generation laser light source 11, an ultrasonic detection laser light source 12, a laser interferometer 13, and a signal processing unit 60.
The ultrasonic wave generation laser light source 11 irradiates the inspection object 5 with the high-power pulse laser light EL via the mirrors 31a and 31b in order to generate an ultrasonic wave in the inspection object 5. The pulsed laser light EL is converged by the lens 33 and irradiated to the ultrasonic detection laser spot region 7 as a spot-like spot. The portion of the inspection object 5 irradiated with the pulsed laser light EL is thermally expanded and then contracted to generate distortion, and the ultrasonic wave propagates to the inspection object 5.

  The ultrasonic wave detection laser light source 12 irradiates the ultrasonic wave detection laser spot region 7 as a dot spot through the mirrors 41a and 41b. The continuous wave laser beam DL is irradiated to a measurement point on the inspection object 5. The frequency of the laser reflected light RL of the continuous wave laser light DL from the object to be inspected 5 undergoes a Doppler shift due to surface vibration. The laser reflected light RL is converged by the lens 43 via the mirrors 41 c and 41 d and enters the laser interferometer 13.

  The laser interferometer 13 causes the incident laser light to reciprocate between the mirrors 13a and 13b to interfere (resonate), and transmit a part of the light quantity through the mirror 13b. The spectrum of the laser light transmitted through the laser interferometer 13 is composed of a sharp peak in an extremely narrow wavelength region and slopes on both sides. The distance between the mirrors 13a and 13b is slightly shifted from the resonance condition of the wavelength of the laser beam, and is set so that the wavelength of the laser beam is located in the wavelength region of the slope. At this time, the Doppler shift of the laser reflected light is converted into a light intensity change when passing through the laser interferometer 13. Then, the light transmitted through the laser interferometer 13 is incident on a light-sensitive detector 20 composed of an avalanche photodiode or the like. In the photodetector 20, the light intensity of the transmitted light is converted into an electrical signal E 1 and input to the signal processing unit 60.

  In the signal processing unit 60, the frequency calculation unit 61 can execute the processing described in steps S101 and S151 in FIG. The phase transformation rate storage unit 62 stores a phase transformation rate measured or calculated in advance. The thickness information storage unit 65 stores the value of the inspected object thickness measured in advance. This value can be obtained by measurement using a radiation measuring instrument or measurement using a measure. The calibration line creation unit 63 acquires the phase transformation rate stored in the phase transformation rate storage unit 62 as described in step S102 of FIG. 11, and creates the calibration line as described in step S103 of FIG. be able to. The phase transformation rate calculation unit 64 can execute the process described in step S152 of FIG.

  The signal processing unit 60 includes a processor, and the functions provided by the frequency calculation unit 61, the calibration line creation unit 63, and the phase transformation rate calculation unit 64 described above are performed by the processor in a memory or an external storage device (not shown). You may implement | achieve by running the stored program. The phase transformation rate storage unit 62 and the thickness information storage unit 65 can be implemented by a memory or an external storage device. The calculation result of the phase transformation rate calculation unit 64 can be output to the display device 19. Note that the signal processing unit 60 may be connected to an input / output device and a communication device (not shown). The measured phase transformation rate can be stored in the phase transformation rate storage unit 62 via an input device or a communication device.

  Note that, when the laser ultrasonic measurement device 10a is arranged in a metal manufacturing process, the laser ultrasonic measurement device 10a may not have a calibration line creation function, and may obtain a phase transformation rate using a calibration line already created. In that case, the function of the calibration line creation unit 63 may be executed by another device (having a processor, a memory, a communication unit, and an external storage device) connected to the laser ultrasonic measurement device 10a. The signal processing unit 60 does not have the phase transformation rate storage unit 62 and the calibration line creation unit 63, but uses the data of the calibration line received from the other device via the network or the recording medium, and uses the phase transformation rate data. The calculation unit 64 can execute the process described in step S152 of FIG.

DESCRIPTION OF SYMBOLS 5 Inspected object 6, 6a Laser spot area for ultrasonic wave generation 6a Laser spot area for ultrasonic wave generation 7, 7a Laser spot area for ultrasonic wave detection 10, 10a Laser ultrasonic measurement device 11 Laser light source for ultrasonic wave generation 12 Ultrasonic wave Laser light source for detection 13 Laser interferometer 13a, 13b, 31a, 41a, 41b, 41c Mirror 19 Display device 20 Photo detector 33, 43 Lens 60 Signal processing unit 61 Frequency calculation unit 62 Phase transformation rate storage unit 63 Calibration line creation unit 64 Phase transformation rate calculation unit 65 Thickness information storage unit

Claims (14)

The object to be inspected is irradiated with pulsed laser light from a laser light source for generating ultrasonic waves to generate ultrasonic waves, and the object to be inspected is irradiated with continuous wave laser light by a laser light source for detecting ultrasonic waves. A laser ultrasonic measurement device that detects an ultrasonic wave propagated through the inspected body by detecting a change in light intensity caused by Doppler shift caused by the ultrasonic wave by interfering with an interferometer,
An irradiation optical system for irradiating the pulsed laser beam on the object to be inspected with a spot-like spot;
The continuous wave laser beam is irradiated on the object to be inspected so as to detect a plate wave ultrasonic wave with zero group velocity generated by the spot-like spot and propagated in the thickness direction of the object to be inspected, and the reflected laser beam is applied to the object. A detection optical system that enters and interferes with the interferometer;
A light detector that receives the laser light transmitted through the interferometer and outputs a change in light intensity as an electrical signal;
An electrical signal output from the light detection unit is input, and a signal processing unit that derives the frequency of the plate wave ultrasonic wave of the object to be inspected,
The signal processing unit
A frequency calculation unit that calculates a frequency of a symmetric plate wave by performing a frequency analysis based on an electrical signal of a light intensity change due to a plate wave ultrasonic wave generated by the irradiation optical system;
The phase transformation rate of the material to be inspected is calculated from the frequency of the symmetric plate wave and a calibration line that is created in advance and represents the relationship between the product of the frequency of the symmetric plate wave and the thickness of the object to be inspected and the phase transformation rate. And a phase transformation rate calculating unit.   The signal processing unit receives a phase transformation rate of the inspected object measured separately, and creates a calibration line representing the relationship between the product of the frequency of the symmetric plate wave and the inspected object thickness and the phase transformation rate The laser ultrasonic measurement apparatus according to claim 1, further comprising a calibration line creation unit that performs the calibration.   3. The laser ultrasonic measurement apparatus according to claim 1, wherein the continuous wave laser beam is irradiated into an irradiation spot area of the pulse laser beam. 4.   The laser ultrasonic measurement apparatus according to claim 1, wherein in the frequency analysis, a recording time for detecting plate wave ultrasonic waves of a plurality of periods is set.   5. The laser ultrasonic measurement apparatus according to claim 1, wherein the frequency of the symmetric plate wave is a frequency of an S1 symmetric plate wave mode having a group velocity of zero.   The laser ultrasonic measurement device according to any one of claims 1 to 5, wherein the phase transformation rate used for creating the calibration line is measured by microscopic observation or thermal expansion coefficient measurement.   The calibration line is a linear calibration line created from a frequency when the phase transformation rate is 0% and a phase transformation rate when the phase transformation rate is 100%. The laser ultrasonic measurement device according to any one of 6. The object to be inspected is irradiated with pulsed laser light from a laser light source for generating ultrasonic waves to generate ultrasonic waves, and the object to be inspected is irradiated with continuous wave laser light by a laser light source for detecting ultrasonic waves. A laser ultrasonic measurement method using a laser ultrasonic measurement device that detects an ultrasonic wave propagated through the inspected body by detecting a change in light intensity caused by Doppler shift caused by the ultrasonic wave by interfering with an interferometer. ,
A laser light irradiation step of irradiating the pulsed laser light on the object to be inspected with a spot-like spot;
The continuous wave laser beam is irradiated on the object to be inspected so as to detect a plate wave ultrasonic wave with zero group velocity generated by the spot-like spot and propagated in the thickness direction of the object to be inspected, and the reflected laser beam is applied to the object. An ultrasonic detection step of making the interferometer enter and interfere;
A light detection step of receiving laser light transmitted through the interferometer and outputting a change in light intensity as an electrical signal;
Based on the electrical signal, the frequency calculation step of calculating the frequency of the symmetric plate wave by performing frequency analysis,
The phase transformation rate of the material to be inspected is calculated from the frequency of the symmetric plate wave and a calibration line that is created in advance and represents the relationship between the product of the frequency of the symmetric plate wave and the thickness of the object to be inspected and the phase transformation rate. And a phase transformation rate calculating unit step for performing the laser ultrasonic measurement method.   A calibration line creation step of creating a calibration line that represents the relationship between the phase transformation rate and the product of the frequency of the symmetric plate wave and the thickness of the test object, and the phase transformation rate of the specimen to be measured separately input; The laser ultrasonic measurement method according to claim 8, further comprising:   10. The laser ultrasonic measurement method according to claim 8, wherein the ultrasonic detection step is performed in an irradiation spot area of the pulsed laser light.   The laser ultrasonic measurement method according to any one of claims 8 to 10, wherein in the frequency analysis, a recording time for detecting a plate wave ultrasonic wave having a plurality of periods is set.   The laser ultrasonic measurement method according to any one of claims 8 to 11, wherein the frequency of the symmetric plate wave is a frequency of an S1 symmetric plate wave mode with a group velocity of zero.   The laser ultrasonic measurement method according to any one of claims 8 to 12, wherein the phase transformation rate used for creating the calibration line is measured by microscopic observation or thermal expansion coefficient measurement.   The calibration line is a linear calibration line created from a frequency when the phase transformation rate is 0% and a phase transformation rate when the phase transformation rate is 100%. 14. The laser ultrasonic measurement method according to any one of items 13.

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