JP5528395B2 - Thin plate stress measuring method and measuring apparatus - Google Patents

Description

  The present invention relates to a laser ultrasonic method in which an ultrasonic wave is generated on an inspection object in a non-contact manner using a laser, and an ultrasonic wave is generated without damaging the surface of the inspection object using a thermoelastic effect. The present invention relates to a method for measuring the magnitude of stress applied to a thin plate such as residual stress in a non-destructive and non-contact manner.

  Thin plate products are used in a wide range of fields, including automobiles, digital home appliances, building materials, houses, beverage cans, and transformers. For example, since a thin plate used for an electronic component needs to be finely processed, it is generally processed by etching. When residual stress is present in the thin plate, the etching rate varies between a portion where the residual stress is present and a portion where the residual stress is not present, resulting in uneven etching. Further, since the residual stress is released by the etching process, there arises a problem that a dimensional error occurs or the product is curved.

  As a method for measuring the magnitude of the residual stress of a thin plate in a nondestructive manner, a stress measurement method using X-rays is known (for example, Patent Document 1, Patent Document 2, etc.). X-ray stress measurement is a measurement method that utilizes the fact that the position of the X-ray diffraction peak changes according to the distortion of the crystal lattice at the X-ray irradiation position. Usually, the surface of the thin plate is irradiated with X-rays at a plurality of positions perpendicular to the rolling direction of the thin plate, and based on the obtained diffraction peak positions, the distortion of the crystal lattice is obtained, and the obtained strain is converted. Then, the magnitude of the stress is obtained.

  On the other hand, a non-contact type measurement method (hereinafter referred to as “laser ultrasonic method”) that generates ultrasonic waves using a pulsed laser and detects ultrasonic waves propagated through the body to be inspected using a continuous wave laser. Various applications have been proposed.

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

Non-Patent Document 1 discloses that a plate generated by irradiating an object to be inspected with a pulsed Q-switch Nd: YAG laser beam having a fluence (energy amount per unit area) of about 5.1 mJ / mm ² and ultrasonically exciting it. The frequency of the S1 mode with zero group velocity (hereinafter referred to as “S1f”) and the frequency of the A2 mode with zero group velocity (hereinafter referred to as “A2f”) using a continuous wave double wave Nd: YAG laser are used. And the experimental results of calculating the Poisson's ratio, longitudinal wave velocity, and transverse wave velocity from the detected values.

JP-A-5-72061 JP-A-6-102103

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 manufacture of a steel plate, warpage of the steel plate due to the influence of residual stress may be a problem. Therefore, a method for measuring the magnitude of residual stress of a steel sheet in a non-destructive and non-contact manner is desired.

  The stress measurement method using X-rays takes time to obtain a plurality of diffraction peaks, so that the time required to measure the residual stress distribution becomes long. Moreover, since the measurement by X-ray can only know the distortion of the surface layer of about several μm to several tens of μm of the thin plate, the magnitude of the residual stress in the whole thin plate cannot be measured accurately. Furthermore, since X-rays of radiation that is dangerous to the human body are used, skill is required for operation of the X-ray irradiation apparatus, and measurement cannot be performed safely and easily.

  Although it is conceivable to measure the magnitude of residual stress by applying the laser ultrasonic method, in the measurement by the laser ultrasonic method described in Non-Patent Document 1, an irradiation mark is generated on the surface of the object to be inspected by ablation. Therefore, in an application where irradiation marks are not allowed, measurement by this method cannot be performed, and the application is limited.

  In view of the above circumstances, the present invention is a method for measuring the stress of a thin plate that accurately measures the magnitude of stress applied to the thin plate, such as residual stress, in a non-destructive, non-contact manner, at high speed and safely, and It is an object to provide a measuring device.

  The present inventors diligently studied a method for measuring the magnitude of stress applied to a thin plate, such as residual stress of the thin plate, using a laser ultrasonic method using ultrasonic excitation by a thermoelastic effect.

  In the laser ultrasonic method, in order not to damage the surface of the object to be inspected and to cause no laser irradiation trace, in the measurement method using ultrasonic excitation by the thermoelastic effect, it is difficult to detect the generated ultrasonic wave, Although it is difficult to measure A2f of plate wave ultrasonic waves, it is possible to measure S1f.

  As a result of the study by the present inventors, ultrasonic waves are generated by a laser ultrasonic method on an object to be inspected with tensile stress, the ultrasonic intensity waveform is detected, and the detected ultrasonic intensity waveform is subjected to a fast Fourier transform. (Fast Fourier Transform: hereinafter referred to as “FFT”), it was found that the frequency peak of S1f was separated and two frequency peaks were observed. Furthermore, it has been found that the magnitude of this frequency separation correlates with the magnitude of tensile stress.

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

(1) A method for measuring the magnitude of stress applied to a thin plate,
Irradiating the object to be inspected with pulsed laser light for generating ultrasonic waves, and generating ultrasonic waves on the object to be inspected;
Irradiating the object to be inspected with ultrasonic wave detecting laser light having a wavelength different from that of the pulse oscillation laser light for generating ultrasonic waves;
The reflected light of the ultrasonic detection laser beam that has undergone Doppler shift due to the vibration of the ultrasonic wave generated by irradiating the object to be inspected with pulsed laser light for ultrasonic generation is received, and the amount of the Doppler shift is received. Outputting light of a high intensity,
Calculating an intensity waveform of an ultrasonic wave generated on the object to be inspected from the amount of Doppler shift, using light having an intensity corresponding to the amount of Doppler shift;
The frequency analysis of the intensity waveform of the ultrasonic wave is performed, and two frequencies S1f1 and S1f2 of the plate wave ultrasonic wave of the S1 mode with zero group velocity generated on the inspection object observed separately are calculated. Steps,
Calculating the magnitude of the stress applied to the object to be inspected from the calculated two frequencies using the relationship between the two frequencies S1f1 and S1f2 created in advance and the magnitude of the stress. A method for measuring stress of a thin plate characterized by

(2) The step of calculating the magnitude of the stress applied to the object to be inspected is calculated in advance by calculating the magnitude ΔS1f1 / S1f of separation of the two frequencies from the calculated two frequencies S1f1 and S1f2. The step of calculating the magnitude of the stress applied to the object to be inspected using the relationship between the magnitude of the separation of the two frequencies ΔS1f1 / S1f and the magnitude of the stress. Stress measurement method for thin plates.
Here, S1f1 <S1f2 and ΔS1f = S1f2−S1f1.

  (3) The thin plate stress measurement method according to (1) or (2), wherein the ultrasonic detection laser beam is irradiated into a spot region where the ultrasonic wave generation pulse oscillation laser beam is irradiated.

  (4) receiving the ultrasonic detection laser beam that has undergone Doppler shift due to ultrasonic vibration generated by irradiating the object to be inspected with pulse generation laser light for ultrasonic generation, and depending on the amount of Doppler shift The method of outputting light having a high intensity is a method in which the laser beam for ultrasonic detection subjected to the Doppler shift is interfered by an interferometer, and light having an intensity corresponding to the amount of the Doppler shift is output from the interferometer. The thin plate stress measurement method according to any one of (1) to (3), wherein

  (5) The stress measurement method for a thin plate according to any one of (1) to (4), wherein the ultrasonic detection laser beam is a pulsed laser beam.

(6) An apparatus for measuring the magnitude of stress applied to a thin plate,
An irradiation optical system for irradiating a pulsed laser beam for generating ultrasonic waves to generate ultrasonic waves on the object to be inspected;
A detection optical system for irradiating the object to be inspected with a pulsed laser beam for generating ultrasonic waves and a laser beam for detecting ultrasonic waves having a different wavelength;
The reflected light of the ultrasonic detection laser beam that has undergone Doppler shift due to the vibration of the ultrasonic wave generated by irradiating the object to be inspected with pulsed laser light for ultrasonic generation is received, and the amount of the Doppler shift is received. A receiver that outputs light of high intensity;
The frequency analysis of the intensity waveform of the ultrasonic wave is performed, and two frequencies S1f1 and S1f2 of the plate wave ultrasonic wave of the S1 mode with zero group velocity generated on the inspection object observed separately are calculated. A frequency spectrum calculation unit;
A stress calculation unit that calculates the magnitude of the stress applied to the object to be inspected using the relationship between the two frequencies S1f1 and S1f2 obtained in advance and the magnitude of the stress from the two calculated frequencies. A thin plate stress measuring device comprising:

(7) The stress calculation unit calculates the separation frequency ΔS1f1 / S1f of the two frequencies from the calculated two frequencies S1f1 and S1f2, and obtains the separation frequency ΔS1f1 / S1f of the two frequencies obtained in advance. The stress measurement apparatus for a thin plate according to (6), wherein the magnitude of the stress applied to the object to be inspected is calculated using the relationship between the stress and the magnitude of the stress.
Here, S1f1 <S1f2 and ΔS1f = S1f2−S1f1.

  (8) The detection optical system can irradiate a laser beam into a spot region of a pulsed laser beam irradiated on the surface of the object to be inspected by the irradiation optical system (6) or (7) ) Thin plate stress measurement device.

  (9) The reception unit inputs and interferes with the ultrasonic detection laser light that has undergone Doppler shift due to ultrasonic vibration generated by irradiating the object to be inspected with pulse generation laser light for ultrasonic generation. The thin plate stress measuring apparatus according to any one of (6) to (8), wherein the apparatus is an interferometer that outputs light having an intensity corresponding to the amount of Doppler shift.

  (10) The thin plate stress measuring apparatus according to any one of (6) to (9), wherein the ultrasonic detection laser beam is a pulsed laser beam.

  (11) Further, the separately measured magnitude ΔS1f / S1f1 of the two frequencies and the magnitude of the stress are input, and the stress calculation unit adds the magnitude of the stress to the object to be inspected based on the input values. A calibration line creating unit that creates a calibration line that represents a relationship between the magnitude of the separation of two frequencies ΔS1f / S1f1 and the magnitude of the stress, which is used to calculate the magnitude of the stress. 6)-(10) a thin plate stress measurement device.

  According to the present invention, using a laser beam having a low energy that does not cause ablation, the surface of the object to be inspected is not damaged, is non-contact and non-destructive, and is also safe at high speed. The magnitude of the stress applied to the thin plate can be measured.

  As a result, information on residual stress can be obtained online during the production of thin plates, so that it can be used as control information for the correction process of steel sheets, for example, and control for reducing residual stress can be optimized. .

It is a figure which shows the outline of the stress measuring apparatus of the thin plate of this invention. It is a figure which shows the intensity | strength waveform of the ultrasonic wave detected with the stress measuring apparatus of the thin plate of this invention. It is a figure which shows the result of having carried out the FFT process of the intensity waveform of the ultrasonic wave detected with the thin-plate stress measuring device of this invention. It is a figure which shows the relationship between the magnitude | size of the stress added to the thin plate, and the magnitude | size of isolation | separation of two frequencies detected with the stress measuring apparatus of the thin plate of this invention. It is a figure which shows the relationship between the irradiation position of the preferable ultrasonic wave generation laser and ultrasonic detection laser at the time of detecting the plate wave ultrasonic wave of zero group velocity in this invention. It is a figure which shows an example of the relationship between the frequency of a Fabry-Perot interferometer, and the transmittance | permeability. It is a figure which shows the relationship between the group velocity of a plate wave ultrasonic wave, and a frequency x board thickness. It is a figure which shows the relationship between the magnitude | size of the stress measured with the measuring method of this invention, and the magnitude | size of the stress measured with the strain gauge.

  An outline of the stress measurement method for a thin plate of the present invention will be described with reference to the stress measurement apparatus for a thin plate of the present invention shown in FIG. Here, the thin plate will be basically described as meaning a thin steel plate, but the present invention can be widely applied to any metal material having elasticity.

  When the ultrasonic wave is excited by irradiating the inspection object 11 with the ultrasonic wave generation laser light GL irradiated from the ultrasonic wave generation pulse oscillation laser light source 21 of the irradiation optical system, the longitudinal wave and the transverse wave are generated inside the inspection object 11. And plate wave ultrasonic waves are generated.

  When the inspection object 11 is irradiated with the ultrasonic detection laser light DL emitted from the ultrasonic detection laser light source 31 of the detection optical system, the ultrasonic detection laser light DL is generated on the inspection object 11. The sound wave receives and reflects the Doppler shift Δf = 2V / λ according to the surface displacement speed V of the inspection object 11. λ is the wavelength of the laser beam for ultrasonic detection.

  The reflected light of the ultrasonic detection laser beam DL subjected to the Doppler shift is received by the receiving unit 41 and then sent to the signal processing unit 51. The signal processing unit 51 calculates a frequency spectrum from information on the amount of Doppler shift, and performs frequency analysis of the frequency spectrum by FFT processing.

  Specifically, for example, the frequency spectrum is calculated as shown in FIG. When this is subjected to FFT processing, the results shown in FIGS. 3A to 3C are obtained. The results shown in FIG. 2 and FIG. 3 are the results of measurement using 1 mm-thick SS400 as the object to be inspected. 3A is a measurement result in a state in which no tensile stress is applied, (b1) is a measurement result in a state in which a tensile stress of 3 MPa is applied to the object to be inspected, and (b2) is (b1). ) Is an enlarged view of the frequency region of S1f, and (c) is a measurement result in a state in which a tensile stress of 15 MPa is applied to the object to be inspected.

  From FIG. 3, it can be seen that the frequency peak of S1f is separated into two frequencies in a state where tensile stress is applied to the object to be inspected.

  As a result of the study by the present inventors, when the magnitude of the tensile stress applied to the object to be inspected is changed, as shown in FIG. 4, there is a correlation between the magnitude of separation of the two frequencies and the magnitude of the tensile stress. I found out. The magnitude of the separation can be expressed as ΔS1f / S1f1, for example, where the two separated frequencies are S1f1, S1f2 (S1f1 <S1f2), ΔS1f = S1f2-S1f1. The vertical axis in FIG. 4 is ΔS1f / S1f1. The magnitude of the separation between the two frequencies may be evaluated using another formula such as ΔS1f / S1f2.

  A calibration line that is a relationship between the magnitude of the separation between the two frequencies (for example, ΔS1f / S1f1) and the magnitude of the applied stress is obtained in advance, for example, by adding it as a database. For an object to be inspected whose magnitude of stress is unknown, the intensity waveform detected by the laser ultrasonic method is FFT processed to calculate the separation size of the two frequencies. The magnitude of the applied stress can be obtained.

  Depending on the component composition of the steel sheet, the relationship between the magnitude of the separation of the two frequencies and the magnitude of the stress may differ, so it is advisable to prepare multiple databases so that the optimum database can be referenced during measurement. .

  The stress measuring device of the present invention will be described in more detail.

  The pulse generation laser light source 21 for generating ultrasonic waves is a pulse oscillation laser light source, and for example, a Q switch Nd: YAG laser can be used.

  The pulse repetition frequency of the ultrasonic wave generation laser beam GL is about 10 to 200 Hz, although it depends on the characteristics of the ultrasonic wave generation pulse oscillation laser light source 21 to be used. In the irradiation optical system, mirrors 22a and 22b, an ND filter 23, a condensing lens 24, and the like are arranged between the ultrasonic generation pulse oscillation laser light source 21 and the inspection object 11, and the ultrasonic generation laser light GL is arranged. The traveling direction, intensity, beam diameter, etc. can be adjusted. The irradiation angle of the ultrasonic wave generation laser beam GL irradiated on the surface of the inspection object 11 is practically within ± 30 ° in the direction perpendicular to the surface. These are not particularly limited.

  As the ultrasonic detection laser light source 31, for example, a continuous wave second harmonic Nd: YAG laser can be used. Laser light having different wavelengths is used for the ultrasonic generation laser light GL and the ultrasonic detection laser light DL. This is to prevent the ultrasonic wave generation laser beam GL from being mixed with the ultrasonic wave detection laser beam DL received by the receiving unit 41 and causing noise. That is, the wavelength used is not limited as long as the two laser beams have different wavelengths.

  The magnitude of the Doppler shift that the ultrasonic detection laser beam DL receives from the ultrasonic waves generated on the inspection object 11 is about 0.01 to 0.1 Hz. Therefore, it is preferable to use a laser light source with high frequency stability for the ultrasonic light source 31 for ultrasonic detection, and the frequency drift of the ultrasonic light source 31 for ultrasonic detection is preferably 100 Hz / s or less.

  In the detection optical system, mirrors 32a, 32b and the like can be arranged between the ultrasonic detection laser light source 31 and the inspection object 11, and the traveling direction of the ultrasonic detection laser light DL can be adjusted. The irradiation angle of the ultrasonic detection laser beam DL irradiated on the surface of the inspection object 11 is practically within ± 30 ° in the direction perpendicular to the surface. These are not particularly limited.

  As the ultrasonic detection laser light source 31, a pulsed laser light source can be used. In this case, the ultrasonic detection laser light DL is oscillated in synchronization with the detection timing of the intensity waveform, so that the ultrasonic wave detection sensitivity can be reduced without increasing the average output of the ultrasonic detection laser light DL so much. Can be improved.

  As shown in FIG. 5, the irradiation area on the surface of the object to be inspected is irradiated with an ultrasonic wave generation laser beam, and the ultrasonic detection laser beam is within the irradiation area of the irradiation spot 25 of the ultrasonic wave generation pulse oscillation laser beam. It is preferable to irradiate the ultrasonic detection laser beam so that the irradiation spot 35 enters. By irradiating the laser beam for ultrasonic generation and the laser beam for ultrasonic detection almost at the same position, the wave traveling in the plane direction is not detected by the ultrasonic detection laser, and a plate wave with zero group velocity that does not move is detected. It can be detected with high accuracy.

  For the receiving unit 41, for example, a Fabry-Perot interferometer can be used. The Fabry-Perot interferometer serves as a wavelength filter. The transmittance of the Fabry-Perot interferometer varies greatly depending on the frequency of light, as shown in FIG.

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

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

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

  An adjustment mechanism using reference light may be provided so that the reflection mirror can be adjusted to an optimal position when the reflection mirror of the Fabry-Perot interferometer changes due to disturbance such as external vibration. A piezo element or the like can be used to adjust the reflection mirror.

  The receiving unit of the stress measuring device of the present invention is not limited to a Fabry-Perot interferometer.

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

  Information on the amount of Doppler shift received by the receiving unit 41 is sent to the signal processing unit 51. At this time, for example, it may be converted into an electric signal by an optical / electrical converter such as an avalanche photodiode and input to the signal processing unit 51 as an electric signal.

  The signal processing unit 51 includes a frequency spectrum calculation unit 51a and a stress calculation unit 51b.

  The frequency spectrum calculation unit 51a is composed of an electronic computer or the like, calculates a frequency spectrum from information on the amount of Doppler shift output from the reception unit, further performs frequency analysis of the calculated frequency spectrum, and separates it into two. The frequencies S1f1 and S1f2 of the S1 mode plate wave ultrasonic waves with zero group velocity to be observed are calculated.

  The stress calculation unit 51b is composed of an electronic computer or the like, and the stress applied to the object 11 to be inspected using the relationship between the magnitude of the separation of two frequencies and the magnitude of the stress obtained in advance from the calculated S1f1 and S1f2. The size of is calculated.

  The signal processing unit 51 can further include a calibration line creation unit 51c as necessary. The calibration line creation unit 51c is composed of an electronic computer or the like, and inputs the magnitude of separation of two frequencies separately measured (for example, ΔS1f / S1f1) and the magnitude of the applied stress, and based on the inputted value. Create a calibration line between the magnitude of the separation of the two frequencies and the magnitude of the stress. The created calibration line is used for stress calculation in the stress calculation unit 51b.

  As shown in FIG. 4, the relationship between the magnitude of the separation between the two frequencies and the magnitude of the stress is a linear relationship. Therefore, the relationship between the magnitude of the separation between the two frequencies and the magnitude of the stress is calibrated. It is easy to reflect on

  The frequency spectrum calculation unit 51a, the stress calculation unit 51b, and the calibration line creation unit 51c do not have to be physically separate devices, and have, for example, programs for performing respective processes in the same electronic computer. It doesn't matter.

  Further, a display device 61 that outputs the calculation result may be provided as necessary.

  The waveform resolution required in the stress measurement method of the present invention can be determined from the required resolution of the stress to be measured. That is, when the necessary resolution of the stress to be measured is determined, the necessary resolution of ΔS1f / S1f1 is determined from the already obtained calibration line and the resolution of the stress to be measured.

  Since the group velocity of the plate wave ultrasonic wave has the relationship shown in FIG. 7 with the frequency × plate thickness, S1f × d (plate thickness) at the time of creating the calibration line and the plate thickness of the object to be inspected are approximate S1f. A value is calculated, and ΔS1f is calculated from the value.

  Then, at least the necessary FFT frequency resolution Δfr is Δfr = ΔS1f / 2, and therefore the time domain T of the waveform necessary for the FFT processing is obtained as T = 1 / Δfr.

  The stress measuring method of the present invention is suitable for measuring the magnitude of stress such as residual stress added to a thin plate having a thickness of about 0.1 to 3 mm. In addition, if the metal material has elasticity, the mechanism of ultrasonic generation by the thermoelastic effect is similar to that of the thin steel plate, and ultrasonic waves can be transmitted, so the method of the present invention is applicable.

  A tensile stress was applied using SS400 having a thickness of 1 mm after cold rolling as a test material, and the magnitude was measured and compared by the stress measurement method of the thin plate of the present invention and the strain gauge method which is a known measurement method.

  Regarding the measurement method of the present invention, a calibration line that is a relationship between the magnitude of the tensile stress and ΔS1f / S1f1 was previously obtained in advance, and the relationship of FIG. 4 was obtained.

  A Q-switched Nd: YAG laser was used as a pulse oscillation laser light source for ultrasonic wave generation in the measurement method of the present invention, and a continuous wave, second harmonic Nd: YAG laser was used as a laser light source for ultrasonic detection. The measurement conditions are shown in Table 1.

The fluence of the ultrasonic wave generating laser beam on the object to be inspected was set to 1.8 mJ / mm ² . This is energy that does not cause ablation. Further, the distance between the object to be inspected and the detection system in Table 1 means the shortest distance between the object to be inspected and the detection system, and in the example of FIG. 1, is the distance between the object to be inspected 11 and the condenser lens 24. .

  Measurement by the strain gauge method was performed by providing a strain gauge in the vicinity of laser irradiation under the above measurement conditions.

  FIG. 8 shows the result of matching the magnitude of the stress calculated from the measured ΔS1f / S1f1 value on the vertical axis and the magnitude of the stress measured by the strain gauge on the horizontal axis based on the calibration line obtained in advance. Show.

  The dotted line in the graph of FIG. 8 is an auxiliary line indicating the accuracy required for measurement ± 5 MPa. All the data were within the range of ± 5 MPa, and it was found that the required measurement accuracy was satisfied.

  From the above results, the measurement result using the measurement method according to the present invention and the measurement result using the strain gauge method which is a well-known measurement method are in good agreement, and according to the stress measurement method of the present invention, It was confirmed that an appropriate measurement result could be obtained without causing laser irradiation marks on the body.

  In addition, although the case of the tensile stress was shown in the embodiment, the case of the compressive stress is also obtained based on the relationship between the measured separation of the two frequencies and the magnitude of the stress applied to the object to be inspected. It is possible to measure the magnitude of the compressive stress from the calibration line and the magnitude of the separation of the two frequencies measured.

  According to the present invention, in the laser ultrasonic method, ultrasonic excitation is performed using the thermoelastic effect, and the residual stress of the object, etc. is applied to the object to be inspected without causing a laser irradiation mark on the surface of the object. Since the magnitude of the applied stress can be calculated, it can be applied to non-destructive inspection of various materials. In the embodiments and examples for carrying out the invention, a thin steel plate is taken as an example of an object to be inspected, but the present invention can be applied to any metal material having elasticity such as aluminum or copper.

  Since the present invention is a non-destructive, non-contact measuring method, the present invention is applied online during the manufacturing process of a metal such as cold working, and the residual stress of the actual object being manufactured is online. Measurement is possible. As a result, for example, it can be utilized as control information for the straightening process of the steel sheet, and the control for reducing the residual stress can be optimized.

  Furthermore, gas cutting or the like for confirming the residual stress is not required, so that the yield is improved and the residual stress of the finished product can be guaranteed. Therefore, industrial applicability is great.

DESCRIPTION OF SYMBOLS 11 Test object 21 Pulse generation laser light source for ultrasonic generation 22a, 22b Mirror 23 ND filter 24 Condensing lens 25 Irradiation spot of pulse generation laser light for ultrasonic generation 31 Laser light source for ultrasonic detection 32a, 32b Mirror 35 Ultrasonic wave Irradiation spot of detection laser beam 41 Receiver 51 Signal processor 51a Frequency spectrum calculator 51b Stress calculator 51c Calibration line generator 61 Display device DL Ultrasound detection laser beam GL Ultrasound generation laser beam

Claims (11)

A method for measuring the magnitude of stress applied to a thin plate,
Irradiating the object to be inspected with pulsed laser light for generating ultrasonic waves, and generating ultrasonic waves on the object to be inspected;
Irradiating the object to be inspected with ultrasonic wave detecting laser light having a wavelength different from that of the pulse oscillation laser light for generating ultrasonic waves;
The reflected light of the ultrasonic detection laser beam that has undergone Doppler shift due to the vibration of the ultrasonic wave generated by irradiating the object to be inspected with pulsed laser light for ultrasonic generation is received, and the amount of the Doppler shift is received. Outputting light of a high intensity,
Calculating an intensity waveform of an ultrasonic wave generated on the object to be inspected from the amount of Doppler shift, using light having an intensity corresponding to the amount of Doppler shift;
The frequency analysis of the intensity waveform of the ultrasonic wave is performed, and two frequencies S1f1 and S1f2 of the plate wave ultrasonic wave of the S1 mode with zero group velocity generated on the inspection object observed separately are calculated. Steps,
Calculating the magnitude of the stress applied to the object to be inspected from the calculated two frequencies using the relationship between the two frequencies S1f1 and S1f2 created in advance and the magnitude of the stress. A method for measuring stress of a thin plate characterized by In the step of calculating the magnitude of the stress applied to the object to be inspected, the two frequency separation magnitudes ΔS1f / S1f1 are calculated from the two calculated frequencies S1f1 and S1f2, and the two obtained in advance are calculated. 2. The thin plate according to claim 1, which is a step of calculating the magnitude of stress applied to the object to be inspected using the relationship between the magnitude of frequency separation ΔS1f / S1f1 and the magnitude of stress. Stress measurement method.
Here, S1f1 <S1f2 and ΔS1f = S1f2−S1f1.   The thin plate stress measurement method according to claim 1, wherein the ultrasonic detection laser beam is irradiated into a spot region where the ultrasonic wave generation pulse oscillation laser beam is irradiated.   The ultrasonic detection laser beam that has been subjected to Doppler shift due to vibration of ultrasonic waves generated by irradiating the object to be inspected with pulse generation laser light for ultrasonic generation is received, and has an intensity corresponding to the amount of Doppler shift. The method of outputting light is a method of causing the ultrasonic detection laser beam subjected to the Doppler shift to interfere with an interferometer, and outputting light having an intensity corresponding to the amount of the Doppler shift from the interferometer. The thin plate stress measurement method according to any one of claims 1 to 3.   The thin plate stress measurement method according to claim 1, wherein the ultrasonic detection laser beam is a pulsed laser beam. An apparatus for measuring the magnitude of stress applied to a thin plate,
An irradiation optical system for irradiating a pulsed laser beam for generating ultrasonic waves to generate ultrasonic waves on the object to be inspected;
A detection optical system for irradiating the object to be inspected with a pulsed laser beam for generating ultrasonic waves and a laser beam for detecting ultrasonic waves having a different wavelength;
The reflected light of the ultrasonic detection laser beam that has undergone Doppler shift due to the vibration of the ultrasonic wave generated by irradiating the object to be inspected with pulsed laser light for ultrasonic generation is received, and the amount of the Doppler shift is received. A receiver that outputs light of high intensity;
The frequency analysis of the intensity waveform of the ultrasonic wave is performed, and two frequencies S1f1 and S1f2 of the plate wave ultrasonic wave of the S1 mode with zero group velocity generated on the inspection object observed separately are calculated. A frequency spectrum calculation unit;
A stress calculation unit that calculates the magnitude of the stress applied to the object to be inspected using the relationship between the two frequencies S1f1 and S1f2 obtained in advance and the magnitude of the stress from the two calculated frequencies is provided. A thin plate stress measuring device characterized by the above. The stress calculation unit calculates the separation frequency ΔS1f / S1f1 of the two frequencies from the calculated S1f1 and S1f2, and calculates the separation frequency ΔS1f / S1f1 of the two frequencies obtained in advance and the magnitude of the stress. 7. The thin plate stress measuring apparatus according to claim 6, wherein the magnitude of the stress applied to the object to be inspected is calculated using the relationship.
Here, S1f1 <S1f2 and ΔS1f = S1f2−S1f1.   8. The thin plate according to claim 6, wherein the detection optical system is capable of irradiating a laser beam in a spot region of a pulsed laser beam irradiated by the irradiation optical system onto a surface of an object to be inspected. Stress measuring device.   The receiving unit inputs and interferes with the ultrasonic detection laser beam which has been subjected to Doppler shift due to ultrasonic vibration generated by irradiating the object to be inspected with pulse generation laser light for ultrasonic generation, and performs Doppler shift. The thin plate stress measuring apparatus according to claim 6, wherein the interferometer outputs light having an intensity corresponding to the amount of the thin plate.   The thin plate stress measuring apparatus according to claim 6, wherein the ultrasonic detection laser beam is a pulsed laser beam.   Furthermore, the magnitude of separation of two frequencies ΔS1f / S1f1 measured separately and the magnitude of stress are input, and the magnitude of stress applied to the object to be inspected by the stress calculation unit based on the input values. 11. A calibration line creation unit that creates a calibration line that represents the relationship between the magnitude of the separation of two frequencies ΔS1f / S1f1 and the magnitude of stress used to calculate The thin plate stress measuring apparatus according to any one of the preceding claims.

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