JP2008020329A - Remote non-contact sonic speed/heat conductivity measuring method based on measurement of reflected light of pulse laser induced elastic wave attenuation process - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the problem that it is difficult to measure an activation material and a high-temperature material because contact measurement is inevitable theoretically in a presently used sonic speed measuring method and methods for measuring heat conductivity showing the heat propagation efficiency through the material surface of a metal are also variously present but the contact measurement of a temperature is inevitable in these methods in order to precisely detect the temperature. <P>SOLUTION: A sonic wave is generated along the surface or inside of a substance by a theory of an induction light scattering method using a short pulse laser and the sonic speed of the sound wave, which is formed from the time response of reflected or diffracted light when probe light is applied to the sound wave formed place, and heat conductivity are measured at the same time. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a method for measuring remote non-contact sound velocity (acoustic wave propagation velocity) and thermal conductivity by measuring reflected light in a pulse laser induced elastic wave decay process on a material surface. More specifically, the present invention non-contact measures the sound velocity and thermal conductivity on the material surface from the stimulated light scattering method using a short pulse laser beam, for example, the work hardening layer that causes stress corrosion cracking of the reactor material The present invention relates to a measurement method for detecting the presence (position and / or depth) of a denatured portion and a method for verifying the integrity of a target mercury container of a spallation neutron source that is deteriorated by irradiation.

  In the method of the present invention, when the substance to be measured is a transparent solid such as a lath material, the position and / or depth of the specific portion where the denatured portion exists is controlled by controlling the portion where the laser beam is collected, It can be measured by thermal conductivity.

  When a metal structure is subjected to machining, tensile stress remains on the surface layer. In addition, when a metal structure is irradiated with radiation such as gamma rays or neutron irradiation, atoms are ejected, and are destroyed by hardening and embrittlement.

  Since the modified part of the work hardened layer is exposed to the water environment for a long time, stress corrosion cracking occurs, so that the tensile stress of the surface layer of the stainless steel for nuclear power remains by focusing the ultrashort pulse laser to prevent this. A method has been proposed in which the work hardened layer is removed by evaporation and the residual stress inside the material is changed from the tension side to the compression side. Development of countermeasure technology against stress corrosion cracking including stainless steel equipment inside a nuclear reactor is in progress, and a patent has been filed (for example, Patent Document 1). In order to effectively implement this countermeasure technique, it is necessary to know the position and depth of the work hardened layer to be evaporated and removed by the ultrashort pulse laser.

  Also, as a means of measuring solid elastic anomalies, the contact method is an ultrasonic method that measures the speed of sound (the propagation speed of sound waves) from the propagation time of sound waves from an ultrasonic transducer, and a standard hardness probe is pressed against the material. There is a Vickers hardness method to measure. Non-contact methods include the Brillouin scattering method using laser light and the X-ray stress measurement method using hard X-rays. Is not suitable.

There are two methods for measuring the thermal conductivity of solids: the laser flash method as a contact method, the hot-wire probe method, and the thermograph method as a non-contact method. There is no means to do it.
JP 2005-131704 (Stress relief method using stainless steel surface with ultrashort pulse laser light)

  The ultrasonic method, which is the most widely used diagnostic method for elastic abnormality, is a measurement method that measures the speed of sound from the propagation time and propagation distance of a sound wave by contacting or bonding an ultrasonic transducer to the measurement object. Since contact measurement is unavoidable, activation material and high temperature material are difficult to measure. Further, since the measurement frequency is limited to the low frequency side and there is a limit to miniaturization of the ultrasonic vibrator, it is impossible to detect material modification in a minute region.

  The Vickers hardness method is a method of measuring the elastic modulus of a material by pressing a probe having a standard hardness against the material. For this reason, destruction by cutting out a part of material surface is required, and contact of a probe is inevitable.

  The Brillouin scattering method, which is a non-contact elasticity measurement method, irradiates a material with a single wavelength laser beam, and measures the Doppler shift of the wavelength of the scattered light with an interferometer, which is a high-precision spectrometer. It is a method of measuring the rate. High-precision light sources and spectrometers that are indispensable for this measurement are extremely delicate and expensive for the measurement environment, and it is impossible to carry out measurements on the site where a nuclear reactor or a spallation neutron source is installed or to make a compact device.

  Similarly, the X-ray stress method, which is a non-contact elasticity measurement method, is a method in which a material is irradiated with hard X-rays from an X-ray tube or synchrotron radiation, and the stress of the material is measured from changes in diffraction lines. Measurements outside the radiation control area are dangerous because the wires are extremely harmful to the human body, and the equipment becomes large.

  Similarly, a laser flash thermal conduction method is used as a means for measuring the thermal conductivity representing the efficiency of heat propagation on the metal material surface. This is a method of measuring the thermal conductivity by recording the time change of the temperature in another place after raising the temperature of the material locally with laser light, but in order to detect the temperature accurately, Temperature measurement in contact is essential. In addition, a size for heat propagation through the sample is also required, and measurement of a minute region is difficult.

  There is a thermographic method for measuring the radiant heat temperature from a material as a non-contact thermal conductivity measurement, but it is difficult to increase the accuracy of the measurement temperature and is unsuitable for measuring a minute region.

Means for Solving the Invention

  In the present invention, a sound wave is generated on the surface or inside of a substance based on the principle of the stimulated light scattering method using a short pulse laser, and the reflected light or diffracted light (signal to the photodetector) when the probe light is incident on the site. This is a method for simultaneous measurement of the propagation speed and thermal conductivity of a sound wave generated from the time response (used as light).

In particular, the present invention relates to a new laser-based method for inspecting the thickness of a work-hardened layer causing stress corrosion cracking in a non-contact, rapid and non-destructive manner. That is, the present invention uses a pulsed laser as a light source, condenses the laser light after being divided into two and irradiates the material surface, induces an elastic wave due to the interference of the laser light on the material surface having the work hardening layer, The process of the disappearance and attenuation of the micro undulations on the surface caused by this elastic wave is observed by the intensity change of the reflected or diffracted light by the laser beam of another wavelength, the change is converted into an electrical signal, and the time change of the signal waveform Thus, the sound velocity and thermal conductivity of the work hardened layer are observed with the same optical system.
[The invention's effect]

  According to the present invention, it is possible to simultaneously measure the elastic modulus and thermal conductivity of a substance such as a metal material modified by an elastic wave induced by a pulse laser, and provide a lot of information for predicting deterioration of the material. This elastic modulus is obtained by converting the propagation velocity of the sound wave obtained from the experimental result by the following formula 2. Furthermore, the present invention provides a method for on-site observation of radioactive materials and high-temperature materials, which has been difficult to measure in the past, because it can be measured from a remote location in a non-contact manner and the apparatus can be easily downsized. Combination with anti-corrosion cracking technology is possible.

  In order to solve the above-mentioned problem, the present invention generates an elastic wave by combining interference fringes generated by irradiating the same part at the same time with an ultrashort pulse laser divided into two, and generates an elastic wave. The decay process associated with the propagation is measured as the time variation of the reflection intensity of another laser beam. A series of measurements can be performed from a remote location without contact with the substance. In the initial time domain, information on the acoustic velocity (acoustic wave velocity) of the elastic wave propagating through the substance, and heat in the long time domain It contains conductivity information and can be measured simultaneously with the same device. Thus, a method for detecting the presence (position and / or depth) of a surface-modified portion is provided (claim 1).

  FIG. 1 is a diagram showing reflected light measurement in a pulse laser induced elastic wave attenuation process according to the present invention. After a short pulse laser 1 is divided into two by a polarization beam splitter, each passes through a long focus condenser lens 3. The surface of the sample 4 to be measured is condensed and irradiated. At that time, an interference fringe due to the irradiation laser divided into two is generated on the irradiation surface, an elastic wave is induced on the sample surface by this interference fringe, and a micro undulation is generated on the sample surface by the induced elastic wave. Decays and disappears. The irradiation surface is irradiated with a continuous light laser 2 of another wavelength as detection light, the reflected light or signal light is detected by the photodetector 5 and converted into an electrical signal, and the time change of this electrical signal is converted into a digital oscillograph 6. And by displaying and recording using the personal computer 7, the attenuation and disappearance process of the micro undulations can be observed as the intensity change of the reflected light or signal light.

  In addition, the present invention can detect the presence (position and / or depth) of a work-hardened layer by simultaneous measurement of the sound velocity (acoustic wave propagation velocity) and thermal conductivity. A method adapted to a plate-like sample having a residual stress made of a stainless steel material of a shroud or a recirculation system pipe is provided.

  The present invention also provides a target for a spallation neutron source that is exposed to fast neutrons for a long time in the detection of the presence (position and / or depth) of the surface degradation layer of the irradiated material by simultaneous measurement of the sound velocity and thermal conductivity. A method (claim 3) adapted to the stainless steel material of the inner wall of the container is provided.

  Hereinafter, the present invention will be described in detail by way of examples. Residual stress representing the hardening of the material: T can be expressed by the following relationship when expressed by the generalized Hooke's law.

Here, c represents an elastic constant, and S represents the magnitude of strain. This elastic constant can be measured as a change in sound velocity: V by the following equation.

Here, ρ is the density of the medium. By measuring the speed of sound in this way, a change in residual stress can be detected.

  The time width of the ultrashort pulse laser used is set to be equal to or less than the vibration period time of the generated elastic wave, and the time vibration of the elastic wave can be observed. The wavelength of the elastic wave is determined by the wavelength of the light divided into two and the crossing angle. When the wavelength of light is 1064 nm and the crossing angle is 3.2 °, the wavelength of the elastic wave is 19.2 μm.

  It is assumed that the light divided into two is collected at the crossing position so that only a minute region can be measured and the signal light intensity is increased by the light collection. The size of the region of the measurement site is about 20 μm square when the crossing angle is 6 degrees. Since the measurement site is limited to a region of several tens of μm that intersects and collects two lasers, in a transparent solid such as a glass material, a sound wave at a specific location in the depth direction can be controlled by controlling the measurement site to be collected. It is possible to measure the propagation speed and thermal conductivity of However, since a substance that is not transparent to visible light such as a metal sample cannot be condensed inside the sample, only the surface can be measured.

  The detection light for observing the generated elastic wave is incident at an angle that satisfies the Bragg condition when the elastic wave formed by the interference fringes of the light is regarded as a diffraction grating. Then, the intensity of the signal light or reflected light is increased, and the straightness is improved, so that it is possible to easily remove scattered light that does not contribute to the signal.

  The time response of the photodetector for detecting the signal light is assumed to have a performance having a time resolution equal to or less than the vibration period of the elastic wave, and the vibration of the elastic wave can be sufficiently observed.

  The time response of the digital oscillograph for recording the time change of the electrical signal converted by the photo detector has a performance with a time resolution equal to or less than the vibration period of the elastic wave, and is sufficient to record a long time change. It is assumed to have performance with storage capacity.

  An electric signal representing a time change of diffracted light or reflected light recorded by a digital oscillograph is transferred to a personal computer and recorded. Analysis is performed from the recorded electrical signal using a nonlinear least square method. 2 and 3 are diagrams showing temporal changes in diffracted light measured with an ethanol sample that is a liquid. The two measurements are made with exactly the same equipment arrangement and differ only in the time domain for recording.

  Analysis is performed using the nonlinear least square method from the recorded time-change signal of the diffracted light. When analyzing the vibration period of the damped vibration of elastic waves, measurement data in the fast region of 10 microseconds or less is used. The acoustic velocity of the elastic wave is calculated from the wavelength of the elastic wave and the vibration period. Under the above conditions, when the wavelength of the elastic wave is 19.2 μm and the vibration period calculated from the experimental result of FIG. 2 is 16 nanoseconds, the sound velocity of the measurement object is calculated to be 1200 m / second.

  When analyzing thermal conductivity, measurement data in a slow region of about 100 microseconds is used. From the experimental results of FIG. 3, the time constant of attenuation of the diffracted light intensity is calculated as 120 microseconds.

  As described above, sound velocity (acoustic wave propagation velocity) measurement and thermal conductivity measurement can be performed with the same data that differs only in the time domain by the same optical system.

  In this method, when the focused light of the pulse laser divided into two is scanned along the irradiation position on the metal surface, the sound velocity and thermal conductivity are affected by the strain induced in the metal crystal according to the degree of cure of the work hardening layer. Changes. By detecting this change and estimating the position and / or thickness or depth of the work hardening layer, the number of pulse lasers necessary for evaporating and removing the work hardening layer causing stress corrosion cracking can be obtained. . This enables rapid ultrashort pulse laser removal on site. That is, for example, when there is a work hardened layer on the metal surface, the propagation speed of the sound wave is increased. This is clarified by the decrease in the period of the damped oscillation in FIG. At the same time, the damping time of the damped vibration is reduced. This corresponds to the existence time of the damped oscillation in FIG.

  Conventionally, the work hardened layer has been detected by destructive inspection such as a Vickers hardness tester or contact inspection such as a thermocouple or an ultrasonic vibrator. In addition, synchrotron radiation X-ray diffraction and thermographs are not compact, quick, and simple because they are large in size, slow in time response, and cannot be used in high-temperature environments. It was unsuitable for integrating with technology. The present invention is a new work-hardened layer detection technology that can radically innovate this point.

It is a figure showing the reflected light measurement of the pulse laser induced elastic wave attenuation | damping process based on this invention. Measurement example of diffracted light in an ethanol sample. It is a figure in which the period of a damped vibration represents the vibration of a sound wave. In this example, the vibration cycle is 16 nsec and the wavelength of the sound wave is 19.2 μm, so that a sound velocity value of 1200 m / sec is obtained. Measurement example of diffracted light in an ethanol sample. It is a figure which shows that the time change of a non-vibration component originates in attenuation | damping of the signal strength by heat conduction.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Short pulse laser 2 Continuous light laser 3 Long focus condensing lens 4 Sample to be measured 5 Photo detector 6 Digital oscillograph 7 Personal computer

Claims (4)

  In a measurement method for non-contact detection of the presence of surface-modified parts that cause stress corrosion cracking on the material surface, interference fringes generated by irradiating the surface of the material with an ultrashort pulse laser divided into two at the same time A method of detecting the presence of a surface-modified part by simultaneously measuring the velocity and thermal conductivity of an elastic wave by measuring the decay process of the elastic wave to be produced as the time variation of the reflection intensity of another laser beam.   The method according to claim 1, wherein the substance is a plate sample having a residual stress made from a stainless steel material of a reactor pressure vessel wall or a shroud or recirculation piping in the reactor pressure vessel.   The method according to claim 1, wherein the substance is a stainless steel material of a target mercury container inner wall of a spallation neutron source that is exposed to a high-intensity proton beam and a fast neutron beam for a long period of time and whose surface is deteriorated. The velocity of the elastic wave induced based on the strain induced on the material surface according to the degree of cure of the work hardening layer on the material surface when the ultrashort pulse laser divided into two is focused on the material surface and scanned. 4. The method according to claim 1, wherein the change is detected and the presence of the work hardened layer is detected in a non-contact manner by changing the thermal conductivity.