Abstract

Laser-ultrasonics is an emerging nondestructive technique using lasers for the generation and detection of ultrasound which presents numerous advantages for industrial inspection. In this paper, the problem of detection by laser-ultrasonics of small defects within a material is addressed. Experimental results obtained with laser-ultrasonics are processed using the Synthetic Aperture Focusing Technique (SAFT), yielding improved flaw detectability and spatial resolution. Experiments have been performed on an aluminum sample with a contoured back surface and two flat-bottom holes. Practical interest of coupling SAFT to laser-ultrasonics is also discussed.

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References

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  1. J. Krautkramer and E. Krautkramer, Ultrasonic Testing of Materials, (Springer Verlag, New York, 1983).
  2. C. B. Scruby, L. E. Drain, Laser-ultrasonics: techniques and applications, (Adam Hilger, Bristol, UK,1990).
  3. J.-P. Monchalin, "Progress towards the application of laser-ultrasonics in industry," in Review of Progress in Quantitative Nondestructive Evaluation, Vol. 12A, (Plenum, New York, 1993) p. 495.
  4. J.-P. Monchalin, "Optical detection of ultrasound," IEEE Trans. Ultrason. Ferroelectr. Freq. Control, 33, 485 (1986).
    [CrossRef] [PubMed]
  5. J.-P. Monchalin and R. H‚on, "Laser ultrasonic generation and optical detection with a confocal Fabry-Perot interferometer," Mater. Evaluation, 44, 1231 (1986).
  6. P. Delaye, A. Blouin, D. Drolet, L.-A. de Montmorillon, G. Roosen, J.-P. Monchalin, "Detection of ultrasonic motion of a scattering surface by photorefractive InP:Fe under an applied field," J. Opt. Soc. Am. B, 14, 1723 (1997).
    [CrossRef]
  7. P. Delaye, A. Blouin, D. Drolet, J.-P. Monchalin, "Heterodyne detection of ultrasound from rough surfaces using a double phase conjugate mirror," Appl. Phys. Lett., 67, 3251 (1995).
    [CrossRef]
  8. P. V. Mitchell, G. J. Dunning, S. W. McCahon, M. B. Klein, T. R. O'Meara, D. M. Pepper, "Compensated High-bandwidth laser ultrasonic detector based on photo-induced emf in GaAs," in Review of Progress in Quantitative Nondestructive Evaluation, Vol. 15, (Plenum, New York, 1996) p. 2149.
  9. P. W. Lorraine, R. A. Hewes and D. Drolet, "High resolution laser ultrasound detection of metal defects," in Review of Progress in Quantitative Nondestructive Evaluation, Vol. 16, (Plenum, New York, 1997) p. 555.
  10. K. Mayer, R. Marklein, K. J. Langenberg and T. Kreutter, "Three-dimensional imaging system based on Fourier transform synthetic aperture focusing technique," Ultrasonics, 28, 241 (1990).
    [CrossRef]
  11. L. J. Busse, "Three-dimensional imaging using a frequency-domain synthetic aperture focusing technique," IEEE Trans. Ultrason. Ferroelectr. Freq. Control, 39, 174 (1992).
    [CrossRef] [PubMed]
  12. T.-J. Teo and J. M. Reid, "Multifrequency holography using backpropagation," Ultrasonic Imaging, 8, 213 (1986).
    [CrossRef] [PubMed]

Other (12)

J. Krautkramer and E. Krautkramer, Ultrasonic Testing of Materials, (Springer Verlag, New York, 1983).

C. B. Scruby, L. E. Drain, Laser-ultrasonics: techniques and applications, (Adam Hilger, Bristol, UK,1990).

J.-P. Monchalin, "Progress towards the application of laser-ultrasonics in industry," in Review of Progress in Quantitative Nondestructive Evaluation, Vol. 12A, (Plenum, New York, 1993) p. 495.

J.-P. Monchalin, "Optical detection of ultrasound," IEEE Trans. Ultrason. Ferroelectr. Freq. Control, 33, 485 (1986).
[CrossRef] [PubMed]

J.-P. Monchalin and R. H‚on, "Laser ultrasonic generation and optical detection with a confocal Fabry-Perot interferometer," Mater. Evaluation, 44, 1231 (1986).

P. Delaye, A. Blouin, D. Drolet, L.-A. de Montmorillon, G. Roosen, J.-P. Monchalin, "Detection of ultrasonic motion of a scattering surface by photorefractive InP:Fe under an applied field," J. Opt. Soc. Am. B, 14, 1723 (1997).
[CrossRef]

P. Delaye, A. Blouin, D. Drolet, J.-P. Monchalin, "Heterodyne detection of ultrasound from rough surfaces using a double phase conjugate mirror," Appl. Phys. Lett., 67, 3251 (1995).
[CrossRef]

P. V. Mitchell, G. J. Dunning, S. W. McCahon, M. B. Klein, T. R. O'Meara, D. M. Pepper, "Compensated High-bandwidth laser ultrasonic detector based on photo-induced emf in GaAs," in Review of Progress in Quantitative Nondestructive Evaluation, Vol. 15, (Plenum, New York, 1996) p. 2149.

P. W. Lorraine, R. A. Hewes and D. Drolet, "High resolution laser ultrasound detection of metal defects," in Review of Progress in Quantitative Nondestructive Evaluation, Vol. 16, (Plenum, New York, 1997) p. 555.

K. Mayer, R. Marklein, K. J. Langenberg and T. Kreutter, "Three-dimensional imaging system based on Fourier transform synthetic aperture focusing technique," Ultrasonics, 28, 241 (1990).
[CrossRef]

L. J. Busse, "Three-dimensional imaging using a frequency-domain synthetic aperture focusing technique," IEEE Trans. Ultrason. Ferroelectr. Freq. Control, 39, 174 (1992).
[CrossRef] [PubMed]

T.-J. Teo and J. M. Reid, "Multifrequency holography using backpropagation," Ultrasonic Imaging, 8, 213 (1986).
[CrossRef] [PubMed]

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Figures (6)

Fig. 1.
Fig. 1.

Laser-ultrasonic inspection of a pipe. Generation is assumed in a regime which produces essentially normally propagating ultrasonic waves. The arrows indicate the generation and detection laser beams at two locations determined by the optical scanner. The inserts indicate schematically the ultrasonic echoes as observed on an oscilloscope.

Fig. 2.
Fig. 2.

Principle of SAFT.

Fig. 3.
Fig. 3.

Photos of the test specimen.

Fig. 4.
Fig. 4.

C-scans and profiles of the a) filtered and b) F-SAFT data.

Fig. 5.
Fig. 5.

B-scans of filtered (a,c) and F-SAFT processed data (b,d) from the aluminum test specimen. B-scans of figs. 5c and 5d were taken through the 0.5 mm dia. flat-bottom hole.

Fig. 6.
Fig. 6.

C-scans and profiles from a subset of data corresponding to a 0.4 mm step: F-SAFT processed data a) without interpolation and b) with interpolation.

Tables (1)

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Table 1. Comparison of SNR, apparent diameter and depth resolution (DR).

Equations (4)

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( C ) = M i mesh S ( M i , t = 2 d i v )
Δ x vΔt z a , Δz vΔt 2
S ¯ ( σ x , σ y , z , f ) = S ¯ ( σ x , σ y , 0 , f ) exp ( 2 πiz f 2 ( v 2 ) 2 σ x 2 σ y 2 )
¯ ( σ x , σ y , z ) = f Ω S ¯ ( σ x , σ y , z , f )

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