Laser-Ultrasonic Non-Destructive Testing:hniques and Applications in Nuclear Industry
公開日:
カテゴリ: 第2回
Introduction
er-ultrasonics has brought practical solutions to a ety of nondestructive evaluation problems that cannot solved by using conventional ultrasonic techniques ed on piezoelectric transduction [1-4]. er-ultrasonics uses two lasers, one with a short pulse the generation of ultrasound and another one, long se or continuous, coupled to an optical interferometer detection. Laser-ultrasonics allows for testing at a long doff distance and inspection of parts without any pling liquid. The technique features also a large ection bandwidth, which is important for numerous lications, particularly involving small crack detection, ng and material characterization. aser-ultrasonics, a pulsed power laser is usually used to erate ultrasonic waves. When a laser pulse is irradiatedDC: Makoto OCHIAI, Sinsugita-cho 8, Isogo-ku, okohama, 235-8523, Power and Industrial Systems &D Center, Toshiba Corporation, Tel:045-770-2307, mail: mak.ochiai@toshiba.co.jpn Member n Member n Member n Membera sample surface, an acoustic pulse is generated due hermoelastic or ablative interaction between laser and material. Ablation process achieved by the irradiation a high power laser pulse is more suitable to obtain ense ultrasonic signals. This method of excitation ultaneously generates a various ultrasonic modes; face-skimming longitudinal waves (P), surface acoustic es (SAW), bulk longitudinal waves (L) and bulk sheares (S). These ultrasonic waves are detected by another er combined with an optical interferometer as a micro placement of the surface. e obvious application of laser-ultrasonics is ndestructive testing (NDT) of surface-breaking cracksburied defects [5]. Crack detection using SAW, signal plitude of which is the largest among the excited waves,initially achieved success. Cooper et al. [6] showedsmall slits having a depth of the order of 100 mm are ectable with laser-induced SAW in the ultrasonic se-echo measurements. In order to measure a depth of slit, one possible technique based on ultrasonic modeversion at an edge of the slit has been suggested; this hnique however requires rather complicated ultrasonic opagation analysis including mode-conversion and fairy nsitive detection of weak mode-converted ultrasounds.this paper, we will review recent developments in er-ultrasonics for crack detection and small crack depth easurement on materials used in nuclear industry. equency analysis of SAW is used to measure the crack pth [7,8]. Experimental results obtained on a stainless el plate with stress corrosion cracking (SCC) are monstrated. Also, developed laser-ultrasonic NDT stem for inner surface of bottom-mounted strumentation (BMI) in pressurized water reactor (PWR) Eintroduced.Setup for laser-ultrasonic testingbasic experimental setup for laser-ultrasonic NDT is hematically shown in Fig.1. Laser pulses from a switched Nd: YAG laser with a maximum energy of out 30 mJ/pulse in 6-10 ns pulse duration and a avelength of 532 nm, was launched onto the surface of e test piece. These energies are quite intense but are still low the threshold for optical fiber delivery [9,10). The neration laser pulses were focused by the optical head to a diameter of about 1 mm to generate SAW. The -nerated SAW was detected as micro surface splacements using a confocal Fabry-Perot interferometer CFPI) [11,12]. The detection system had a broadband equency response extending from about 1 MHz to 100FCOptical Fiber (generation laser)Generation Laser (Q-switched Nd:YAG)BS**Scanning HechanismFCDetection LaserOptical Fiber Optical Head 1/(detection laser) (generation laser)Optical Interferometer(CFPI***)Signal Acquisition (A/D converter)Trigger Signal Processing DETECTIONUnit SYSTEMNater TankOptical Head (detection laser)? FC: Fiber Coupling opticsBS: Beam Splitter *** CFPI: Confocal Fabry-Perot InterferometerFig.1 Experimental setup for laser-ultrasonic testingHz. The signal from the interferometer was converted to digital waveform. Each waveform, representing the urface displacement, was stored into external memory for ter signal processing. Trigger signal synchronized withe generation laser irradiation was also fed in order to Lentify the accurate time of ultrasounds generation. It is oted that the existence of water is not essential for this measurement: it is used solely to imitate inspectionvironment for reactor internal components.neasurementusingmeasurementusing. Crack depth aser-induced SAWis well known that SAW travels only through the surface nyer which is as thin as one wavelength of itself. As hown in Fig.2, most energies of SAW having higher requency (shorter wavelength) is reflected, delayed and node-converted according to the geometry of a small rack. On the other hand, the lower frequency SAW longer wavelength) penetrating deeper layer is not so ensitive to the geometry; it therefore easily travels over racks to the other side. Since the laser-ultrasonic technique allows generation and letection of wide frequency band SAW, it should be a -uitable tool for analyzing frequency response of cracks by comparing incident and transmitted SAW.(a) Reflection Mode (crack detection)GenerationLaserDetectionLaserReflected(Echo)crackSignal amplitudeIncident SAWIncident SAWReflected(Echo)(b) Transmission Mode (crack depth measurement)GenerationLaserDetectionLasercrackSignal AmplitudeansmittedIncident SAWassunseTransmittedTimeFig.2 Schematic illustrations of (a) crack detection by using reflection of higher frequency SAW and (b) crack depth measurement by using transmission of lower frequency SAW9Slit depth: 1.5mmSlit depth: 1.0mmSlit depth: 0.5mmAMSlit depth: 0.2mm???????4??No slitTime (1 #s/div) Fig.3 Typical waveforms of transmitted SAW through slits having depths of 0.2mm-1.5mm on stainless steelTypical waveforms transmitted through cracks having depths of from 0.2 mm to 1.5 mm machined on stainless steel plates is shown in Fig.3. The pulse-width of Transmitted SAW becomes wider with an increase of crack depth. This result shows that crack behaves as a low pass filter (LPF) to the broadband SAW. The cut-off frequency of this LPF should be related to the slit depth. A signal analysis process in frequency-domain shown in Fig.4 is developed to obtain the absolute crack depth. In this process, firstly, the frequency spectrum, palt), is calculated from the time-domain signal waveform of the transmitted SAW through a crack having a depth of d. The estimation index value (EIV) is then obtained through properWaveform of transmitted SAWFrequency spectrumNo siitNormalized power spectrum PWith slit2108 Frequency f (MHz)Calibration curve || Estimation Index valueCrack depthFig.4 Signal processing flow to obtain crack depthweighting and integration as shown innormalization, weighting and integration as shown in equation (1).fHfr polf)dfEIVI 11.-1f““ Po(f)dfHere, f““ is a weighting function, polf) is a reference frequency spectrum and fH and fL are the highest and lowest frequency of interest, respectively. A previously and properly prepared calibration curve, which indicates a relationship between crack depth and EIV, is referred and finally the crack depth is obtained. To confirm the performance of this crack sizing method, a series of experiments is performed on 8 machined test pieces, each of which includes 3 slits having depth of 0.5, 1.0 and 1.5mm, made of stainless steel, welded stainless steel, nickel alloy and welded nickel alloy and other 7 cracking test pieces including 14 SCCs, as shown in Table1. The result of depth measurement is shown in Fig.5. The actual crack depth is measured by the destructive cross-section observation after the experiments. In both cases on EDM slits and SCCs, good agreement with a standard deviation of less than 0.2 mm is achievedcrack sizingTable 1 Test pieces for laser-ultrasonic crack sizing experimentT/P# Material Shape Slit/CrackSLUT1PlateStainless steelLUT2ConcaveLUT3| WeldedPlateLUT4stainless steelConcave3 EDM slits (depths of 0.5, 1.0 and 1.5mm)LUT5PlateNickel alloyLUT6ConcaveLUTZWelded nickel PlateLUT8alloyConcaveT19T23T24 LiStainless steelPlateSCCsL2SCC06SCC03SCCs310Measured crack depth (mm)OSCCEDM slit0 0.5 1 1.5 2 2.5Actual crack depth (mm) Fig.5 Crack depth measurement by laser-ultrasonicsbetween measured and actual crack depth. It is confirmed that this method of crack sizing is capable to detect and to measure the depth of micro cracks.4. Application to actual reactor internalsThe bottom-mounted instrumentation (BMI) in PWR consists of dozens of tube-shape structures to guide in-core neutron detectors. The tubes are made of Alloy 600 and are welded at the bottom of the reactor vessel. Since inner surface of the each welded part has the potential of SCC initiation, proper inspection, preventive maintenance and countermeasure techniques are expected. A new laser-based preventive maintenance system, which includes the laser-ultrasonic NDT system and the laser peening system [13,14] is developed to perform both inspection and stress improvement on the inner surface of BMI tubes. As shown in Fig.6, the laser-based preventive maintenance system is composed of laser system, beamWork platformMonitor/cootrolsystemLaser system9.5mmOptical fiberBMIJ-weldRemote handling equipmentArea to be testedOptical head18mReactor vesselFig.6 Laser-based preventive maintenance system for-3maintenance systemGeneration laser 4 Detection laser(laser peening)Optical HeadOptical fiberBMI inner surfaceDichroic mirror reflect and collect detection laserDetect SAWSAWMirror reflect generation laserand for laser peeningGenerate SAW (stress improvement)9.5mmFig. 7 Concept of optical head used for laser-based maintenance on inner surface of BMIedtodelivery system with an optical fiber, an optical head, a remote handling equipment and a monitor/control system. The laser system and the monitor/control system are placed on the operation floor. The laser beams are delivered by the optical fiber having a length of about 40 m. The remote handling equipment is hanged under the work platform and is fixed on the top of the BMI tube. The optical head is inserted in the BMI tube and is rotated and traversed vertically with irradiating inner surface helically. Since the inner diameter of the BMI is very narrow, f9.5mm for example, a small optical head based on a new concept is required. The developed optical head equips with two mirrors in one housing; one is used to irradiate generation laser to the tested surface and another reflects and collects detection laser to detect ultrasonic signals, as shown in Fig. 7. A prototype of the laser-based preventive maintenance system is produced and tested its performance in full-scale mock-up facility as shown in Fig.8. As a result, it isReactor VessolRemote handling equipmentthe testom below thebit platformXWork platformforFig.8 Full-scale mock-up experiment for laser-based - 311 -Fig.8confirmed that the laser-ultrasonic NDT system detects micro cracking on the inner surface of BMI tube and the depth of the cracks are successfully measured using the suggested signal processing on the transmitted SAW.5. ConclusionWe have reported that the laser-ultrasonic NDT technique coupled with signal processing based on the frequency response analysis is capable of providing very accurate depth of micro cracks including actual SCCs. Also, the laser-ultrasonic NDT system for the inner surface of BMI tubes is developed and its performance is verified through full-scale mock-up experiments. It should be noted that these excellent results were led by several features of laser-ultrasonics. Laser-ultrasonics is not only a technique of interest for the non-contacting remote inspection without any coupling liquid but also offers many other attractive features, such as:1) wide bandwidth ultrasound can be used, 2) small laser spots allow inspection on the contoursurface located in limited space, and 3) combination uses with other laser-basedmaintenance technologies are easily achieved. The crack depth measurement technique based on the frequency analysis of laser-induced SAW provides its best performance on the micro cracking having a depth of a few mm. From a point of the penetration depth, very low frequency, e.g. the order of 100 kHz, should be used to measure deeper depth.References[1] Scruby, C.B. and Drain, L. E., Laser-ultrasonics: techniques and applications, Adam Hilger, Bristol, UK (1990). [2] Monchalin, J. -P., Progress towards the application of laser-ultrasonics in industry, Review of Progress in Quantitative Nondestructive Evaluation, 12, 495-506, Plenum Press, NY (1993). [3] Choquet, M., Heon, R., Padioleau, C., Bouchard, P., Neron, C. and Monchalin, J.-P., Laser-ultrasonic inspection of the composite structure of an aircraft in a maintenance hanger, Review of Progress in QuantitativeNondestructive Evaluation, 14, 545-552, Plenum Press, NY (1995). [4] Monchalin, J.-P., Neron, C., Bussiere, J.-F., Bouchard, P., Padioleau, C., Heon, R. and Choquet, M., Laser-ultrasonics: From the laboratory to the shop floor, CSNDT journal, Sept/Oct, 5-26, 1997 [5] Ochiai, M., Levesque, D., Talbot, R., Blouin, A., Fukumoto, A. and Monchalin, J.-P., Detection and characterization of discontinuities in stainless steel by the laser ultrasonic synthetic aperture focusing technique, Mater. Eval., 62, No.4, 450-459 [6] Cooper, J. A.., Crosbie, R. A., Dewhurst, R. J., McKie A. D. W. and Palmer, S. B., Surface acoustic wave interactions with cracks and slots: a noncontacting study using lasers, IEEE Trans. UFFC, UFFC-33, No.5, 462-470 (1986) [7] Scala, C. M. and Bowles, S. J., Laser ultrasoniccs for surface-crack depth measurement using transmitted near-field Rayleigh waves, Review of Progress in Quantitative Nondestructive Evaluation, 19, 327-334, Plenum Press, NY (2000). [8] Ochiai, M., Butsuen, T., Miura, T., Kuroda, H., Soramoto, S. and Kanemoto, S., Sizing of micro cracks using laser-induced broad-band surface waves, J. At. Energy Soc. Japan, 43, No.3, 275-281(2001) (in Japanese) [9] Schmidt-Uhlig, T., Karlitschek, P., Yoda, M., Sano, Y. and Marowsky, G., Laser shock processing with 20 MW laser pulses delivered by optical fibers, Eur. Phys. J. AP., 9, No.3, 235-238 (2000) [10] Yoda, M., Sano, Y., Schmidt-Uhlig, T. and Marowsky, G., Fiber delivery of 20 MW laser pulses and its applications, Review of laser engineering, 28, No.5, 309-313 (2000) (in Japanese) [11] Monchalin, J. -P. and Heon, R., Laser ultrasonic generation and optical detection with a confocal Fabry-Perot interfereometer, Mater. Eval., 44, No.9, 1231-1237 [12] Shan, Q., Bradford, A. S. and Dewhurst, R. J., New field formulas for the Fabry-Perot interferometer and their application to ultrasound detection, Meas. Sci. Technol., 9, 24-37 (1998) [13] Sano, Y., and Kimura, M., Sato, K., Obata, M., Sudo, A., Hamamoto, Y., Shima, S., Ichikawa, Y., Yamazaki, H.,312Naruse, M., Hida, S., Watanabe, T. and Oono, Y., Development and application of laser peening system to prevent stress corrosion cracking of reactor core shroud, Proceedings of 8th International Conference on Nuclear Engineering (ICONE8), ICONE-8441, April, 2000, Baltimore[14] Chida, I., Yoda, M., Mukai, N., Sano, Y., Ochiai, M., Miura, T. and Saeki, R., Laser based maintenance technology for PWR power plants, Proceedings of 13th International Conference on Nuclear Engineering (ICONE13), ICONE13-50334, May, 2005, Beijing, China313“ “Laser-Ultrasonic Non-Destructive Testing:hniques and Applications in Nuclear Industry“ “Makoto OCHIAI,Takahiro MIURA,Satoshi YAMAMOTO,Toru ONODERA
er-ultrasonics has brought practical solutions to a ety of nondestructive evaluation problems that cannot solved by using conventional ultrasonic techniques ed on piezoelectric transduction [1-4]. er-ultrasonics uses two lasers, one with a short pulse the generation of ultrasound and another one, long se or continuous, coupled to an optical interferometer detection. Laser-ultrasonics allows for testing at a long doff distance and inspection of parts without any pling liquid. The technique features also a large ection bandwidth, which is important for numerous lications, particularly involving small crack detection, ng and material characterization. aser-ultrasonics, a pulsed power laser is usually used to erate ultrasonic waves. When a laser pulse is irradiatedDC: Makoto OCHIAI, Sinsugita-cho 8, Isogo-ku, okohama, 235-8523, Power and Industrial Systems &D Center, Toshiba Corporation, Tel:045-770-2307, mail: mak.ochiai@toshiba.co.jpn Member n Member n Member n Membera sample surface, an acoustic pulse is generated due hermoelastic or ablative interaction between laser and material. Ablation process achieved by the irradiation a high power laser pulse is more suitable to obtain ense ultrasonic signals. This method of excitation ultaneously generates a various ultrasonic modes; face-skimming longitudinal waves (P), surface acoustic es (SAW), bulk longitudinal waves (L) and bulk sheares (S). These ultrasonic waves are detected by another er combined with an optical interferometer as a micro placement of the surface. e obvious application of laser-ultrasonics is ndestructive testing (NDT) of surface-breaking cracksburied defects [5]. Crack detection using SAW, signal plitude of which is the largest among the excited waves,initially achieved success. Cooper et al. [6] showedsmall slits having a depth of the order of 100 mm are ectable with laser-induced SAW in the ultrasonic se-echo measurements. In order to measure a depth of slit, one possible technique based on ultrasonic modeversion at an edge of the slit has been suggested; this hnique however requires rather complicated ultrasonic opagation analysis including mode-conversion and fairy nsitive detection of weak mode-converted ultrasounds.this paper, we will review recent developments in er-ultrasonics for crack detection and small crack depth easurement on materials used in nuclear industry. equency analysis of SAW is used to measure the crack pth [7,8]. Experimental results obtained on a stainless el plate with stress corrosion cracking (SCC) are monstrated. Also, developed laser-ultrasonic NDT stem for inner surface of bottom-mounted strumentation (BMI) in pressurized water reactor (PWR) Eintroduced.Setup for laser-ultrasonic testingbasic experimental setup for laser-ultrasonic NDT is hematically shown in Fig.1. Laser pulses from a switched Nd: YAG laser with a maximum energy of out 30 mJ/pulse in 6-10 ns pulse duration and a avelength of 532 nm, was launched onto the surface of e test piece. These energies are quite intense but are still low the threshold for optical fiber delivery [9,10). The neration laser pulses were focused by the optical head to a diameter of about 1 mm to generate SAW. The -nerated SAW was detected as micro surface splacements using a confocal Fabry-Perot interferometer CFPI) [11,12]. The detection system had a broadband equency response extending from about 1 MHz to 100FCOptical Fiber (generation laser)Generation Laser (Q-switched Nd:YAG)BS**Scanning HechanismFCDetection LaserOptical Fiber Optical Head 1/(detection laser) (generation laser)Optical Interferometer(CFPI***)Signal Acquisition (A/D converter)Trigger Signal Processing DETECTIONUnit SYSTEMNater TankOptical Head (detection laser)? FC: Fiber Coupling opticsBS: Beam Splitter *** CFPI: Confocal Fabry-Perot InterferometerFig.1 Experimental setup for laser-ultrasonic testingHz. The signal from the interferometer was converted to digital waveform. Each waveform, representing the urface displacement, was stored into external memory for ter signal processing. Trigger signal synchronized withe generation laser irradiation was also fed in order to Lentify the accurate time of ultrasounds generation. It is oted that the existence of water is not essential for this measurement: it is used solely to imitate inspectionvironment for reactor internal components.neasurementusingmeasurementusing. Crack depth aser-induced SAWis well known that SAW travels only through the surface nyer which is as thin as one wavelength of itself. As hown in Fig.2, most energies of SAW having higher requency (shorter wavelength) is reflected, delayed and node-converted according to the geometry of a small rack. On the other hand, the lower frequency SAW longer wavelength) penetrating deeper layer is not so ensitive to the geometry; it therefore easily travels over racks to the other side. Since the laser-ultrasonic technique allows generation and letection of wide frequency band SAW, it should be a -uitable tool for analyzing frequency response of cracks by comparing incident and transmitted SAW.(a) Reflection Mode (crack detection)GenerationLaserDetectionLaserReflected(Echo)crackSignal amplitudeIncident SAWIncident SAWReflected(Echo)(b) Transmission Mode (crack depth measurement)GenerationLaserDetectionLasercrackSignal AmplitudeansmittedIncident SAWassunseTransmittedTimeFig.2 Schematic illustrations of (a) crack detection by using reflection of higher frequency SAW and (b) crack depth measurement by using transmission of lower frequency SAW9Slit depth: 1.5mmSlit depth: 1.0mmSlit depth: 0.5mmAMSlit depth: 0.2mm???????4??No slitTime (1 #s/div) Fig.3 Typical waveforms of transmitted SAW through slits having depths of 0.2mm-1.5mm on stainless steelTypical waveforms transmitted through cracks having depths of from 0.2 mm to 1.5 mm machined on stainless steel plates is shown in Fig.3. The pulse-width of Transmitted SAW becomes wider with an increase of crack depth. This result shows that crack behaves as a low pass filter (LPF) to the broadband SAW. The cut-off frequency of this LPF should be related to the slit depth. A signal analysis process in frequency-domain shown in Fig.4 is developed to obtain the absolute crack depth. In this process, firstly, the frequency spectrum, palt), is calculated from the time-domain signal waveform of the transmitted SAW through a crack having a depth of d. The estimation index value (EIV) is then obtained through properWaveform of transmitted SAWFrequency spectrumNo siitNormalized power spectrum PWith slit2108 Frequency f (MHz)Calibration curve || Estimation Index valueCrack depthFig.4 Signal processing flow to obtain crack depthweighting and integration as shown innormalization, weighting and integration as shown in equation (1).fHfr polf)dfEIVI 11.-1f““ Po(f)dfHere, f““ is a weighting function, polf) is a reference frequency spectrum and fH and fL are the highest and lowest frequency of interest, respectively. A previously and properly prepared calibration curve, which indicates a relationship between crack depth and EIV, is referred and finally the crack depth is obtained. To confirm the performance of this crack sizing method, a series of experiments is performed on 8 machined test pieces, each of which includes 3 slits having depth of 0.5, 1.0 and 1.5mm, made of stainless steel, welded stainless steel, nickel alloy and welded nickel alloy and other 7 cracking test pieces including 14 SCCs, as shown in Table1. The result of depth measurement is shown in Fig.5. The actual crack depth is measured by the destructive cross-section observation after the experiments. In both cases on EDM slits and SCCs, good agreement with a standard deviation of less than 0.2 mm is achievedcrack sizingTable 1 Test pieces for laser-ultrasonic crack sizing experimentT/P# Material Shape Slit/CrackSLUT1PlateStainless steelLUT2ConcaveLUT3| WeldedPlateLUT4stainless steelConcave3 EDM slits (depths of 0.5, 1.0 and 1.5mm)LUT5PlateNickel alloyLUT6ConcaveLUTZWelded nickel PlateLUT8alloyConcaveT19T23T24 LiStainless steelPlateSCCsL2SCC06SCC03SCCs310Measured crack depth (mm)OSCCEDM slit0 0.5 1 1.5 2 2.5Actual crack depth (mm) Fig.5 Crack depth measurement by laser-ultrasonicsbetween measured and actual crack depth. It is confirmed that this method of crack sizing is capable to detect and to measure the depth of micro cracks.4. Application to actual reactor internalsThe bottom-mounted instrumentation (BMI) in PWR consists of dozens of tube-shape structures to guide in-core neutron detectors. The tubes are made of Alloy 600 and are welded at the bottom of the reactor vessel. Since inner surface of the each welded part has the potential of SCC initiation, proper inspection, preventive maintenance and countermeasure techniques are expected. A new laser-based preventive maintenance system, which includes the laser-ultrasonic NDT system and the laser peening system [13,14] is developed to perform both inspection and stress improvement on the inner surface of BMI tubes. As shown in Fig.6, the laser-based preventive maintenance system is composed of laser system, beamWork platformMonitor/cootrolsystemLaser system9.5mmOptical fiberBMIJ-weldRemote handling equipmentArea to be testedOptical head18mReactor vesselFig.6 Laser-based preventive maintenance system for-3maintenance systemGeneration laser 4 Detection laser(laser peening)Optical HeadOptical fiberBMI inner surfaceDichroic mirror reflect and collect detection laserDetect SAWSAWMirror reflect generation laserand for laser peeningGenerate SAW (stress improvement)9.5mmFig. 7 Concept of optical head used for laser-based maintenance on inner surface of BMIedtodelivery system with an optical fiber, an optical head, a remote handling equipment and a monitor/control system. The laser system and the monitor/control system are placed on the operation floor. The laser beams are delivered by the optical fiber having a length of about 40 m. The remote handling equipment is hanged under the work platform and is fixed on the top of the BMI tube. The optical head is inserted in the BMI tube and is rotated and traversed vertically with irradiating inner surface helically. Since the inner diameter of the BMI is very narrow, f9.5mm for example, a small optical head based on a new concept is required. The developed optical head equips with two mirrors in one housing; one is used to irradiate generation laser to the tested surface and another reflects and collects detection laser to detect ultrasonic signals, as shown in Fig. 7. A prototype of the laser-based preventive maintenance system is produced and tested its performance in full-scale mock-up facility as shown in Fig.8. As a result, it isReactor VessolRemote handling equipmentthe testom below thebit platformXWork platformforFig.8 Full-scale mock-up experiment for laser-based - 311 -Fig.8confirmed that the laser-ultrasonic NDT system detects micro cracking on the inner surface of BMI tube and the depth of the cracks are successfully measured using the suggested signal processing on the transmitted SAW.5. ConclusionWe have reported that the laser-ultrasonic NDT technique coupled with signal processing based on the frequency response analysis is capable of providing very accurate depth of micro cracks including actual SCCs. Also, the laser-ultrasonic NDT system for the inner surface of BMI tubes is developed and its performance is verified through full-scale mock-up experiments. It should be noted that these excellent results were led by several features of laser-ultrasonics. Laser-ultrasonics is not only a technique of interest for the non-contacting remote inspection without any coupling liquid but also offers many other attractive features, such as:1) wide bandwidth ultrasound can be used, 2) small laser spots allow inspection on the contoursurface located in limited space, and 3) combination uses with other laser-basedmaintenance technologies are easily achieved. The crack depth measurement technique based on the frequency analysis of laser-induced SAW provides its best performance on the micro cracking having a depth of a few mm. From a point of the penetration depth, very low frequency, e.g. the order of 100 kHz, should be used to measure deeper depth.References[1] Scruby, C.B. and Drain, L. E., Laser-ultrasonics: techniques and applications, Adam Hilger, Bristol, UK (1990). [2] Monchalin, J. -P., Progress towards the application of laser-ultrasonics in industry, Review of Progress in Quantitative Nondestructive Evaluation, 12, 495-506, Plenum Press, NY (1993). [3] Choquet, M., Heon, R., Padioleau, C., Bouchard, P., Neron, C. and Monchalin, J.-P., Laser-ultrasonic inspection of the composite structure of an aircraft in a maintenance hanger, Review of Progress in QuantitativeNondestructive Evaluation, 14, 545-552, Plenum Press, NY (1995). [4] Monchalin, J.-P., Neron, C., Bussiere, J.-F., Bouchard, P., Padioleau, C., Heon, R. and Choquet, M., Laser-ultrasonics: From the laboratory to the shop floor, CSNDT journal, Sept/Oct, 5-26, 1997 [5] Ochiai, M., Levesque, D., Talbot, R., Blouin, A., Fukumoto, A. and Monchalin, J.-P., Detection and characterization of discontinuities in stainless steel by the laser ultrasonic synthetic aperture focusing technique, Mater. Eval., 62, No.4, 450-459 [6] Cooper, J. A.., Crosbie, R. A., Dewhurst, R. J., McKie A. D. W. and Palmer, S. B., Surface acoustic wave interactions with cracks and slots: a noncontacting study using lasers, IEEE Trans. UFFC, UFFC-33, No.5, 462-470 (1986) [7] Scala, C. M. and Bowles, S. J., Laser ultrasoniccs for surface-crack depth measurement using transmitted near-field Rayleigh waves, Review of Progress in Quantitative Nondestructive Evaluation, 19, 327-334, Plenum Press, NY (2000). [8] Ochiai, M., Butsuen, T., Miura, T., Kuroda, H., Soramoto, S. and Kanemoto, S., Sizing of micro cracks using laser-induced broad-band surface waves, J. At. Energy Soc. Japan, 43, No.3, 275-281(2001) (in Japanese) [9] Schmidt-Uhlig, T., Karlitschek, P., Yoda, M., Sano, Y. and Marowsky, G., Laser shock processing with 20 MW laser pulses delivered by optical fibers, Eur. Phys. J. AP., 9, No.3, 235-238 (2000) [10] Yoda, M., Sano, Y., Schmidt-Uhlig, T. and Marowsky, G., Fiber delivery of 20 MW laser pulses and its applications, Review of laser engineering, 28, No.5, 309-313 (2000) (in Japanese) [11] Monchalin, J. -P. and Heon, R., Laser ultrasonic generation and optical detection with a confocal Fabry-Perot interfereometer, Mater. Eval., 44, No.9, 1231-1237 [12] Shan, Q., Bradford, A. S. and Dewhurst, R. J., New field formulas for the Fabry-Perot interferometer and their application to ultrasound detection, Meas. Sci. Technol., 9, 24-37 (1998) [13] Sano, Y., and Kimura, M., Sato, K., Obata, M., Sudo, A., Hamamoto, Y., Shima, S., Ichikawa, Y., Yamazaki, H.,312Naruse, M., Hida, S., Watanabe, T. and Oono, Y., Development and application of laser peening system to prevent stress corrosion cracking of reactor core shroud, Proceedings of 8th International Conference on Nuclear Engineering (ICONE8), ICONE-8441, April, 2000, Baltimore[14] Chida, I., Yoda, M., Mukai, N., Sano, Y., Ochiai, M., Miura, T. and Saeki, R., Laser based maintenance technology for PWR power plants, Proceedings of 13th International Conference on Nuclear Engineering (ICONE13), ICONE13-50334, May, 2005, Beijing, China313“ “Laser-Ultrasonic Non-Destructive Testing:hniques and Applications in Nuclear Industry“ “Makoto OCHIAI,Takahiro MIURA,Satoshi YAMAMOTO,Toru ONODERA