JP4386709B2 - Material nondestructive inspection method and apparatus by laser ultrasonic wave - Google Patents

Description

  The present invention relates to a material nondestructive inspection method and apparatus using laser ultrasonic waves for quickly, accurately and objectively performing a nondestructive inspection of an internal structure of a material having a rough surface such as a concrete structure.

  The most common method of inspecting a concrete structure is a sounding method in which internal defects are checked by visual recording of surface cracks and hammering by an inspector hammering. This method requires the skill of an inspector, has low reliability and reproducibility, and cannot handle a large number of inspection objects. In addition, since it depends on human senses, it is insufficient as a method for inspecting a concrete structure that requires long-term monitoring such as the absoluteness of judgment and the objectivity of data recorded with time.

  The above inspection method was carried out on the premise that the concrete structure maintains a semi-permanent strength, and the necessity of periodic inspection of the concrete structure revealed by the recent concrete wall peeling accident etc. It is not a method that can cope with. A surface crack detection device combining a laser and a photodetector has been put to practical use as one of objective and rapid inspection methods for concrete anomalies.

  In addition, internal defect detection technology based on the principle of thermography using the heat of sunshine and infrared lamps is under development, but measurement becomes unstable due to the external thermal environment. Therefore, in order to predict the risk of accidents such as peeling, an objective and reliable detection system that directly detects internal defects is required. Therefore, in order to deal with such problems, Patent Document 1 examines an elastic wave exploration method (shock elastic wave method), an electromagnetic wave exploration method, and an ultrasonic exploration method, and further exceeds the respective exploration methods. We have proposed a diagnostic method and apparatus for concrete structures using laser ultrasonics.

  The diagnostic method according to Patent Document 1 irradiates a surface of a concrete structure with a pulsed laser beam to generate an elastic wave due to thermal expansion, and the surface wave at the time of irradiation is generated with a laser interferometer that collimates the surface. Waves are detected over time, and the presence or absence of internal defects or buried objects in the concrete structure at the diagnosis site is diagnosed from the waveform change from the detection of the surface wave to the detection of the elastic wave.

  In this diagnostic method, rapid thermal expansion is generated on or near the surface of the object to be probed by irradiation with a pulse laser, and the thermal expansion distortion due to the thermoelastic effect is propagated into the object as an elastic wave (ultrasound). Diagnosis is performed by receiving laser reflected light affected by the waves with a laser interferometer. The laser reflected light affected by the elastic wave is not specified, but the change in amplitude (intensity) due to the surface wave and the change in amplitude due to the elastic wave propagated thereafter are detected in the interferometer over time, It is considered that the presence of an internal defect or an embedded object is detected by a change in the intensity of an optical signal representing surface waves and elastic waves.

  On the other hand, a phase velocity scan capable of evaluating a material of an object, that is, detecting a defect in a wide range, by applying a laser ultrasonic wave generation method using a thermoelastic effect used in a non-destructive inspection described in Non-Patent Document 1. Patent Document 2 discloses an invention of a non-contact nondestructive material evaluation method and apparatus by the method. The purpose of this evaluation method is to evaluate the material of the subject over a wide range by operating the probe light and the coherent energy beam as a set and relatively operating the subject.

  In this material evaluation method, interference fringes are generated by irradiating two slightly different frequencies of coherent energy beams (laser beams) while scanning the subject in a crossing manner. The interference fringes act on the surface of the subject. By forming a strain distribution with the same interval as the interference fringe interval, an elastic wave is emitted in a direction determined by the ratio of the acoustic velocity of the elastic wave propagating inside the subject and the scanning speed of the interference fringe, and is irradiated on the surface of the subject. The bulk ultrasonic wave emitted from the surface by the probe light is detected in a non-contact manner, and one of the subject, laser light, and probe light is scanned to move the non-contact detection point.

  In the above-described material evaluation method according to Patent Document 2, two ultrasonic pulses having slightly different frequencies are irradiated in a predetermined direction with an incident angle θ, and bulk ultrasonic waves are propagated to a subject, thereby causing internal defects. When the bulk ultrasonic wave reflected and diffracted by the light comes out on the surface of the object, it is detected as an optical signal by the bulk ultrasonic wave detection means such as the engineering knife edge method, and the data for evaluating the presence of the internal defect and the material, etc. Is detected.

  However, in the material evaluation method of Patent Document 2 described above, a pulse laser beam that generates bulk ultrasonic waves is made incident in a predetermined direction with an incident angle θ, but this is because the material surface is in a mirror state. And cannot be applied to a method for detecting an internal defect of a material having a large number of uneven surfaces with a rough surface such as a concrete structure.

  On the other hand, the diagnostic method of Patent Document 1 aims at diagnosing the presence of internal defects or embedded objects in concrete structures. The laser device and the laser interferometer are set by the irradiation direction control device so that the reflected light of the pulse laser beam emitted toward each direction of the plurality of diagnosis parts is received by the laser interferometer. The reflected light is first affected by the surface wave, and after that, the signal light received by the laser interferometer is detected over time due to the influence of the elastic wave later.

  However, irradiation with high-energy pulsed light that has a clear effect on the reflected light due to surface waves and elastic waves generated in the diagnostic material causes a rise in the temperature of the specimen surface, and acoustic and plasma emission. Phenomena such as plasma generation and plasma generation due to atmospheric breakdown occur. Therefore, the reflected light of this pulsed light is affected not only by surface waves and elastic waves, but also by the effects of sound and plasma emission, and it becomes noise to the laser interferometer, and accurate and reliable data can be obtained. I can't.

  On the other hand, when pulsed light that does not produce sound, plasma emission, etc. is irradiated by lowering the energy level of the pulsed light, the reflected light becomes scattered light when the surface of the material to be diagnosed is rough, and is reflected by the laser interferometer. Since the light is weak, it is unsuitable as an accurate and reliable signal light with high accuracy. In addition, the generation and detection of ultrasonic waves are performed with a single pulse laser, but the pulse width of the pulse laser is about nanoseconds, and the elastic wave reflected inside the device under test with a single pulse is again on the surface. It is impossible to capture where it came back (typically tens of microseconds), and this evaluation method is not physically valid.

The cause of the inconvenience as described above is that one laser pulse light simultaneously obtains the functions of both elastic wave generation and signal light affected by the elastic wave. That is, all the processing with one laser pulse light is to be performed even though the conditions for the laser light required to generate the elastic wave and the conditions of the laser light that the signal light should have are greatly different. It comes from. Accordingly, the diagnosis result by the diagnosis method of Patent Document 1 is naturally very inaccurate and unreliable, and in theory, practical data can hardly be used, and internal defects of the material to be measured can be used. And the buried object cannot be diagnosed accurately.
JP 2002-296244 A JP 9-33490 A DAHutchins: Physical Acoustics, eds. WPMason and RNThurston (Academic, San Diego, 1998) Vol. XVIII, P21

  In consideration of the conventional problems described above, the present invention generates laser ultrasonic waves most efficiently and is influenced by laser ultrasonic waves when performing nondestructive inspection of materials using laser ultrasonic waves. Using two laser beams with different properties so that signal light can be obtained most efficiently, the surface of the material is not only a mirror surface but also a rough surface such as a concrete structure with extremely high accuracy, accuracy and reliability. It is an object of the present invention to provide a material nondestructive inspection method and apparatus using laser ultrasonic waves that enables a reliable inspection.

As a means for solving the above-mentioned problems, the present invention irradiates a predetermined region on the surface of a material to be measured with the strongest level of pulsed laser light without acoustic, plasma emission, and atmospheric plasma, and generates an elastic wave based on the surface thermal stress. A laser beam having a converging spot diameter smaller than the condensing spot diameter of the pulse laser beam is generated as a probe light and is generated in the material to be measured and is coaxial with the pulse laser beam and within the predetermined region. The irradiation area of the probe light becomes an area where the influence of the surface wave accompanying the irradiation of the pulse laser light is small due to the smaller spot diameter of the probe light than the pulse laser light. the was the probe beam, spatially become changed wavefronts in accordance with the surface state when reflected from the material surface, and is frequency modulated by the acoustic wave Is reflected as issue light, by the laser ultrasonic waves to the signal light detecting a change in the frequency component of the signal light incident on the laser interferometer using a phase conjugate mirror, thereby detecting the internal defect of the material This is a material nondestructive inspection method.

In addition, as an inspection apparatus for performing the above material nondestructive inspection method, a predetermined region of the surface of the material to be measured is irradiated with pulsed laser light at the strongest level without acoustic, plasma emission and atmospheric plasma, and elasticity based on the thermal stress of the surface An elastic wave excitation means for generating a wave in a material to be measured, and a polarization beam splitter for branching a continuous-output laser beam into two beams and transmitting one as a probe beam and the other as a reference beam in different directions The probe light having a diameter smaller than the condensing spot diameter of the pulsed laser light is transmitted to irradiate the predetermined region coaxially with the pulsed laser light, and the irradiated region of the probe light is larger than the pulsed laser light. become a region influence of surface waves due to the irradiation of the pulse laser light is small by focusing spot diameter of the probe light is small, the probe light irradiation, A signal light transmitting unit for transferring the frequency modulated signal light wavefront is changed, and the acoustic wave in accordance with the surface state when reflected from the charge surface, to reflect the pulsed laser beam, the probe beam A common mirror having a polarization characteristic to be transmitted and provided with a rotation mechanism; telescope means for allowing the probe light and a material surface to be irradiated with a predetermined spot diameter within a predetermined region; and the signal light and the reference And a laser interferometer that crosses each other and enters the phase conjugate mirror to generate interference light, and the interference light output from the interferometer is detected by a photodetector to detect the signal light. A material nondestructive inspection apparatus using a laser ultrasonic wave configured to detect an internal defect of a material by changing a frequency component can be provided.

  The above-described material nondestructive inspection method is a method capable of obtaining a highly accurate and accurate signal indicating the presence / absence of a defect inside the material in the inspection apparatus. In this method, pulsed laser light is mainly used to generate elastic waves inside the material, and is irradiated under appropriate laser conditions. The appropriate laser condition is a laser intensity that gives a quiet impact that does not cause other phenomena that are noise sources other than elastic waves, and is the strongest level laser light that does not accompany acoustics, plasma emission, and atmospheric plasma.

  On the other hand, a continuous wave laser beam is irradiated as a probe beam with a predetermined spot diameter within a predetermined region of the material surface. The light is scattered according to the degree of reflection, reflected as scattered light with a distorted wavefront, and transferred as signal light subjected to frequency modulation by the elastic wave. If there is no defect inside the material, the elastic wave is reflected from the back side to the front side of the material thickness, and if there is a defect, it is reflected from a position corresponding to the defect.

  For this reason, the signal light is influenced by elastic waves in different states depending on the presence or absence of defects, and is transferred as signal light that has been subjected to frequency modulation according to each state and has a distorted wavefront. This signal light is incident on a laser interferometer using a phase conjugate mirror (photorefractive crystal), where it interferes with the reference light incident at a predetermined angle, and a diffraction grating is formed and diffracted by this interference. The reference light is output as strong interference light having the same wavefront shape as the signal light and subjected to the same phase modulation. Therefore, the presence or absence of a defect in the material is detected by detecting this interference light with a photodetector.

  The material nondestructive inspection method and apparatus according to the present invention irradiates a material surface with pulsed laser light and continuous wave laser light having different properties, and generates the elastic wave most effectively by the pulsed laser light, and reflects this elastic wave. Since signal light including phase modulation is obtained from continuous-wave laser light and the inside of the material is inspected in a non-contact manner using a high-contrast interference signal, the presence or absence of defects inside the material is detected with extremely high accuracy, accuracy, and speed. The remarkable effect that it can test | inspect with reliable data is acquired.

  Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is an overall schematic configuration diagram of an apparatus for carrying out the material nondestructive inspection method of the present invention. As shown in the figure, the inspection apparatus includes a pulse laser 20 that generates pulsed laser light and a signal laser 1 that generates continuous-wave laser light. As the signal laser 1, an Nd: YAG laser (double harmonic) that continuously oscillates laser light having a wavelength of 532 mm is used.

The pulse laser 20, in the illustrated example, Nd outputs power E = 0.5 J, a pulse laser beam having a pulse period F = 10 pps: YAG laser (fundamental wave) is used, mirrors M ₀ and the movable mirror 7 Then, the material surface of the object W to be measured is irradiated with pulsed laser light. This pulsed laser beam is a suitable laser that generates short-time pulses, spatially concentrated in a narrow spot diameter, and generates elastic waves by instantaneous stress when focused on the material surface of the object W to be measured. Irradiation is performed to satisfy the conditions.

This appropriate laser condition is a laser irradiation condition suitable for selectively generating only the elastic wave necessary for measurement without generating other phenomena (acoustic and plasma emission, etc.) that become noise sources during measurement. This is because it is necessary to give a quiet impact. As the appropriate laser conditions, in the illustrated example, the pulse width, the pulse energy, and the focused spot size are adjusted so that the intensity of the laser beam focused on the surface of the workpiece W is 100 MW / cm ² .

However, an optical system (including the mirror M ₀ and the movable mirror 7) having a sufficiently long focal length F value (F value of 100 or more) so that the focused spot diameter does not change depending on the distance between the laser emission port and the measurement target surface. Used. In addition, the spot diameter of the irradiation surface by the condensing optical system can be transferred to the laser interferometer with a sufficient amount of signal light from the surface to be measured so that damage is not caused even by the power of the impact pulse laser and as will be described later. A sufficiently large aperture (diameter D = 5 cm in the illustrated example) is set.

  On the other hand, the laser light from the signal laser 1 is used to generate probe light that is an original light source of signal light and reference light that is a reference in a laser interferometer described later. The laser light from the signal laser 1 passes through the variable density filter 2 and is tilted by 45 degrees with respect to the paper surface by the λ / 2 wavelength plate 3, and a part of the light is reflected by the polarization beam splitter 4 to be converted into two laser lights. The branched and reflected light is sent as probe light, and the transmitted light is sent as reference light.

  In order to change the power of the signal light, which will be described later, the density variable filter 2 is lightly shaded so that the transmittance changes in the rotation direction. For example, chromium or aluminum is applied to the glass vapor deposition film on the glass substrate. The thickness is attached with a gradient, and the transmittance is changed by rotating the transmission position of the filter by a rotation mechanism (not shown). This variable density filter 2 is provided in order to keep the intensity ratio of signal light / reference light in a laser interferometer, which will be described later, appropriately, and the intensity of the signal light is a signal light monitor that monitors the signal light level behind the laser interferometer. 16 is always monitored.

The laser beam reflected by the polarization beam splitter 4 passes through mirrors M ₁ and M ₂ as probe light and is converted from linearly polarized light to circularly polarized light by a λ / 4 wavelength plate 5 and formed by a magnifying lens 6a and an objective lens 6b. The telescope is adjusted so as to collect light with an appropriate spot diameter (d = 2 to 3 mm) on the surface of the object W to be measured, and is transmitted through the movable mirror 7 coaxially with the above-described pulse laser beam and irradiated onto the material surface. The The optical system from the mirror M ₁ to the movable mirror 7 forms an image transfer optical system.

  The movable mirror 7 is a kind of polarizing mirror that reflects the pulsed light from the pulsed laser 20 and transmits the probe light from the polarizing beam splitter 4, and is configured to vary the irradiation direction in various ways by a rotating mechanism (not shown). In the state shown in the figure, this rotating mechanism is incorporated with a mechanism similar to the mechanism that rotates the astronomical telescope so that it can measure up to a large angle change up to 180 °, but is not directly related to the gist of the present invention. Therefore, details are omitted.

  The irradiated probe light is reflected as scattered light on the scattering surface of the material surface, and is reflected as signal light that has undergone phase modulation due to the influence of vibration caused by elastic waves generated on the scattering surface by irradiation of pulsed laser light. . Details of the operation will be described later. The reflected signal light passes through the movable mirror 7, is collected by the objective lens 6b, travels through the image transfer optical system in the reverse direction, passes through the λ / 4 wavelength plate 5 again, and is returned to the original linearly polarized light.

However, since the polarization direction is transmitted through the λ / 4 wavelength plate 5 twice, the 90-degree polarized light rotates with respect to the incident light. For this reason, the signal sent from the mirror M ₁ passes through without being reflected by the polarization beam splitter 4, further tilts the polarization direction by the λ / 2 wavelength plate 8, and passes through the condenser lens L ₁ and the mirror M ₄ to cause laser interference. The light is incident on the photorefractive crystal 9 (phase conjugate mirror).

On the other hand, the laser beam transmitted through the splitter 4 out of the laser beam sent to the polarization beam splitter 4 through the λ / 2 wavelength plate 3 is generated and sent as a laser beam having a phasing surface with a well-defined phase. M ₃ and the condensing lens L ₂ are incident on the photorefractive crystal 9 at a predetermined crossing angle as reference light with respect to the signal light.

As the photorefractive crystal 9, for example, a barium titanate BaTiO ₃ crystal can be used. When two light waves of reference light and signal light are incident and mixed, an interference fringe of two light waves is formed in the crystal, and according to the interference fringe. A (refractive) diffraction grating is written, in which the two light waves are diffracted by the diffraction grating and combined with each other. For this reason, the wavefront of one light wave (reference light) is the same (phase conjugate wave) as the wavefront of the other light wave (signal light), which amplifies (moves energy) the other light wave with one light wave. It will have the same action. The photorefractive crystal 9 eliminates the mismatch (phase modulation) between the wavefronts of the reference light and signal light by the principle of the dynamic hologram using the diffraction grating.

In the photorefractive crystal 9, a copy of the wavefront shape from the signal light to the reference light occurs, and the reference light diffracted in the same direction as the signal light by diffraction strongly interferes with the signal light, and as a result, the same frequency modulation as the signal light Is output from the photorefractive crystal 9, and the interference light passes through the condenser lens L ₃ , the λ / 4 wavelength plate 10, and the λ / 2 wavelength plate 11 into two polarized waves by the beam splitter 12. To be separated.

  The separated interference lights are received by photodetectors (pin photodiodes) 13a and 13b, converted into electrical signals, input to the oscilloscope 14, and recorded. The signal light output in the same direction as the reference light passes through the polarizer 15 and the signal light monitor 16 detects the intensity level of the signal light. The signal light is fed back to the rotating mechanism of the density variable filter 2 to provide the signal light. The amount of light is adjusted.

  The presence / absence and depth of the internal defect of the material are detected as follows by the material nondestructive inspection apparatus of the embodiment configured as described above. In this inspection apparatus, as described above, the pulse laser beam and the continuous output (CW) laser beam from the two lasers of the pulse laser 20 and the signal laser 1 are respectively movable mirrors on the surface of the object W to be measured. 7 is irradiated on the same axis. The pulsed laser beam is irradiated to generate a local temperature rise by irradiating the surface of the object to be measured as an impact pulse, and to generate an elastic wave by the instantaneous stress.

  As described above, the irradiation with the pulsed laser light is performed so as to meet the appropriate laser conditions. In general, when a powerful laser beam is focused on a material surface, phenomena such as temperature rise on the material surface, generation of plasma accompanied by sound and plasma emission, and generation of atmospheric plasma due to atmospheric breakdown occur. Sound and plasma emission disturb the laser interferometer, making it difficult to measure. For example, when the acoustic wave overlaps with the signal laser beam or the optical path of the signal beam, the refractive index of the atmosphere is changed and the shape of the light wavefront is affected. This is a noise for the laser interferometer.

  Plasma light emission also affects the photodetector through an optical system for detecting signal light, a photorefractive crystal, and the like. In particular, since the impact laser pulse light has high intensity, there is a possibility that the laser light becomes strong enough to saturate the photodetector. Therefore, it is necessary to irradiate the impact pulse laser so that these noise sources are not generated as much as possible. Of course, it is also necessary to take measures against noise removal of the signal light detection system, which will be described later.

The appropriate laser conditions are mainly determined by the intensity of the laser beam focused on the surface of the workpiece W. The intensity of the laser beam focused on the surface depends on the pulse energy, the pulse width, and the focused spot diameter. However, since the pulse energy and the pulse width are fixed by the laser generation unit used, the laser light intensity on the surface is ultimately determined by the focused spot diameter. Therefore, as described above, the focused spot is such that the laser beam intensity on the material surface is about 1 MW / cm ² (when noise reduction is important) or 100 MW / cm ² (when elastic wave intensity is important). The diameter was 4-5 cm or several mm (1-2 mm).

  Even if the surface of the object to be measured is irradiated with the pulse laser beam so as to meet the above-mentioned appropriate laser conditions, it is necessary to further eliminate the influence of the surface wave. When the temperature of the material surface suddenly rises due to irradiation with pulsed laser light, if the intensity of the laser light is sufficiently high, the surface temperature exceeds the plasma generation threshold value, and ablation (material ejection) occurs. Stress is generated on the surface due to the generation of momentum due to temperature expansion or ablation, and an elastic wave (P wave) propagating inside the material is generated.

  On the other hand, the surface stress also generates surface waves (R waves) that appear when pebbles are thrown on the water surface. In general, the R wave has a propagation velocity of about 60% of the P wave, so that the reflection timing inside the P wave material and the timing at which the R wave propagates on the surface match depending on the installation distance between the transmitter and the receiver. May be covered with P wave and become noise.

  Therefore, in order to avoid such inconvenience, the impact pulse laser beam and the signal laser beam are incident on the same axis, and the surface wave generated by the impact pulse laser beam is dissipated to the periphery (several 20 microseconds). It is only necessary to capture only the elastic wave reflected from the inside of the material in the surface state. In order to realize this, an elastic wave excitation laser and a signal laser are arranged coaxially, and measurement with less noise is made possible by detecting the elastic wave after the R wave has passed.

  The continuous output laser light from the signal laser 1 is separated into the probe light and the reference light by the polarization beam splitter 4 as described above. The probe light is condensed to a predetermined spot diameter (d = 2 to 3 mm in the illustrated example) by the objective lens 6b of the telescope, passes through the movable mirror 7, and is irradiated on the surface of the object W to be measured. The object to be measured W is concrete in the illustrated example, and the surface thereof is a scattering surface having a large number of irregularities (see FIG. 2A).

  Therefore, as shown in FIG. 2B, the reflected light reflected from the surface is reflected as scattered light, and phase modulation is caused by the influence of the elastic wave propagating in the material by the above-described impact pulse laser light. The signal light travels in the opposite direction via the image transfer optical system and enters the photorefractive crystal 9 via the polarization beam splitter 4. At the same time, the reference light transmitted through the polarization beam splitter 4 is also transmitted with the signal light. Are incident so as to intersect at an angle of.

  For this reason, in the photorefractive crystal 9, as described above, the signal light and the reference light that have been subjected to the frequency modulation described above by the diffraction grating based on the principle of the dynamic hologram interfere with each other, and a strong interference light signal (1 of the reference light). Next light) is output (see FIGS. 3 and 4). In this case, a dynamic hologram is given the principle name as described above because the diffraction grating is also dynamically recorded corresponding to the wavefront variation that changes temporally due to the influence of the elastic wave on the material surface. .

  Note that the hologram generation efficiency in the photorefractive crystal 9 strongly depends on the intensity ratio between the signal light and the reference light. On the other hand, when the material has a rough surface with irregularities such as concrete, the reflectivity of the probe light irradiated on the material surface varies greatly depending on the surface state, so the intensity of the signal light also varies. Therefore, when scanning and irradiating the probe light, it is necessary to appropriately maintain the intensity ratio of the signal light and the reference light in the photorefractive crystal 9 corresponding to the intensity level of the signal light that varies in various ways.

  For this reason, the density position of the density variable filter 2 is adjusted by feedback control based on the detection signal from the signal light monitor 16 so that the intensity of the probe light can be changed to an optimum state corresponding to each fluctuation state. Is done. As a result, even if the intensity of the signal light fluctuates on the scattering surface at each location where the measurement position has moved one after another, an appropriate intensity ratio between the signal light and the reference light is maintained, and even a signal from the scattering surface has a high contrast interference signal. Can be obtained.

  The interference light is separated into two polarized waves by the beam splitter 12 using the two wave plates 10 and 11 of FIG. 1, and the interference light of each polarization is received by the two photodetectors 13a and 13b. In this case, the intensity difference (wavefront distortion) of the noise source existing in the signal light and the phase modulation as the signal component are separated from each other by using the phase difference between the two to improve the S / N ratio. Also, when each signal is input to the recorder of the oscilloscope 14, it is recorded in two types of signal processing units, high speed (microwave) and low speed (millimeter wave). Then, frequency analysis of elastic waves is performed, and the presence or absence of internal defects is detected by frequency analysis, and the depth of internal defects is detected by time analysis.

The material nondestructive inspection apparatus is actually installed on, for example, the movable carriage Vh, and can be used to detect the internal state of the concrete layer in the tunnel, for example, as shown in FIG. On the movable carriage Vh, the laser beam from the signal laser 1 and the pulse laser 20 is irradiated to the concrete surface layer in the tunnel through the movable mirror 7 which is a common mirror, and P _{11 to} P _1n and P ₂₁ of the concrete surface layer are irradiated. Irradiation while moving to a large number of measurement points P _2n ,..., And the internal state of the material at each measurement point can be detected quickly in a short time.

  The material nondestructive inspection method and apparatus of the present invention can obtain interference signals with high contrast even when the surface is a rough surface such as a concrete structure as well as a mirror surface material. It can be applied to the detection of internal defects in all kinds of materials such as concrete structures such as high-rise buildings or general materials.

Overall schematic configuration diagram of material nondestructive inspection apparatus of embodiment Diagram explaining the relationship between elastic waves and signal light on the material surface Illustration of wavefront compensation function by laser interferometer Illustration of wavefront compensation function in phase conjugate mirror Illustration of nondestructive inspection status of multi-point position in tunnel

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Signal laser 2 Variable density filter 3 λ / 2 wavelength plate 4 Polarizing beam splitter 5 λ / 4 wavelength plate 6 Telescope unit 6a Magnifying lens 6b Objective lens 7 Movable mirror 8 λ / 2 wavelength plate 9 Photorefractive crystal 10 λ / 4 Wavelength plate 11 λ / 2 wavelength plates 13a, 13b Photodetector 14 Oscilloscope 15 Polarizer 16 Signal light monitor 20 Pulse laser M _{0 to} M ₅ mirror W Device under test

Claims (4)

Acoustic a predetermined region of the measured material surface, the plasma emission and irradiated with a pulse laser beam having the strongest level without atmospheric plasma, the elastic wave based on the thermal stress of the surface with resulting in material being measured, the pulse laser beam of transmitting the laser beam of the continuous output of small focused spot diameter than the focusing spot diameter as the probe light is irradiated to the predetermined area on the same axis and the pulsed laser beam irradiation area of the probe light, the When the focused spot diameter of the probe light is smaller than that of the pulse laser light, the influence of the surface wave accompanying the irradiation of the pulse laser light is small, and the irradiated probe light is reflected from the material surface. spatially become changed wavefronts depending on the surface condition, and is reflected as a signal light frequency modulated by the acoustic wave, the phase conjugate mirror the signal light Laser interferometer incident on detecting a change in the frequency component of the signal light, thereby the material non-destructive inspection method according to a laser ultrasonic waves to detect the internal defects of the materials used. Based on the signal from the signal light monitor that detects the intensity of the reference light emitted from the laser interferometer, the density variable filter is operated to maintain the intensity ratio of the signal light and the reference light within a predetermined range . The material nondestructive inspection method using laser ultrasonic waves according to claim 1. An acoustic wave excitation means for irradiating a predetermined region of the surface of the material to be measured with the pulse laser beam at the strongest level without acoustic, plasma emission and atmospheric plasma, and generating an elastic wave in the material to be measured based on the thermal stress of the surface; A polarization beam splitter that splits continuous output laser light into two beams, one of which is used as probe light and the other as reference light, which is transmitted in different directions; and a diameter smaller than the focused spot diameter of the pulsed laser light Transmitting the probe light and irradiating the predetermined area coaxially with the pulse laser light, and the irradiation area of the probe light has a smaller condensing spot diameter of the probe light than the pulse laser light. becomes region is less affected surface waves caused by the irradiation of light, the probe light irradiation, the wavefront in accordance with the surface state when reflected from the material surface However, and a signal optical transmission means for transferring a signal light frequency modulated by acoustic waves, to reflect the pulsed laser beam has a polarization property of transmitting the probe light, and a common mirror with a rotation mechanism A telescope means for irradiating the surface of the material with a predetermined spot diameter within a predetermined area and a phase conjugate mirror by crossing the signal light and a reference light from a continuous output laser as a reference. Laser interferometer that causes interference light to be incident on the laser, and a laser configured to detect the internal defect of the material by detecting the interference light output from the interferometer with a photodetector and changing the frequency component of the signal light Ultrasonic material nondestructive inspection equipment.   A density variable filter is provided in the signal light transmission means system, and the intensity ratio of the signal light and the reference light is within a predetermined range based on a signal from a signal light monitor that detects the intensity of the signal light from the laser interferometer. The material nondestructive inspection apparatus using laser ultrasonic waves according to claim 3, wherein the apparatus is configured to hold.

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