NDTnet - August 1996, Vol.1 No.08

STROBOSCOPIC VISUALIZATION OF ULTRASONIC FIELDS ON SOLIDS BY SAMPLING

Author Bernd Köhler

German

1. Abstract


2. METHODS OF VISUALISATION OF SOUND FIELDS

3. PRINCIPLE OF STROBOSCOPIC MEASUREMENTS

    The basic idea is indicated in Fig. 6. A probe (not shown) sets up pulses of ultrasonic waves. As they travel to the surface (or over it) the (say) surface displacement u is measured by a point transducer, digitised and stored. From this time signal only the value for the time f0 is taken and stored together with the position co-ordinates x1, y1. For a next position with co-ordinates x2, y2 we repeat the measurement, storing again displacement for the time t0 and so on. So a complete map of u(x,y,t0) can be sampled e.a. a snapshot. This is rather simple and it can be done using existing scanners and modifying only the software. In fact we used in our experiments the scanning system shown in Fig. 1.Of course, the displacement data for various time points can be stored in parallel to generate a series of snapshots out of the same scan. In this way a movie of the wave prapagation can be generated in the same time which is necessary for conventional scanning to produce intensity images.

    The type of point transducers can be varied. Piezoelectric pin transducers, electrodynamic probes and laser interferometer are possible receivers. A necessary condition to apply a transducer is aperture is small compared to the trace wavelength. The measurement e.g. the sensor contact should not affect the sound propagation. Of course, different sensors measure different physical quantities.The above discussion the surface displacement u should be replaced e.g. by particle velocity in normal direction or in some tangential direction (in plane motion) for various types of electrodynamic probes.

Fig. 6 Principle of collecting snapshots by sampling

4. EXAMPLES OF APPLICATIONS

5. SUMMARY

6. LITERATUR

  1. B. Köhler, Sound Fields of Ultrasonic Circumferential Arrays - Case of an Array for Steam Generator Heat Exchanger Tubes, 21st International Symposium on Acoustical Imaging, 28.-30. March, Laguna Beach, CA, USA
  2. F. Fellinger, K.J. Langenberg, Numerical Techniques for Elastic Wave Propagation and Scattering, in: Elastic Waves and Ultrasonic Nondestructive Evaluation, Ed. by S.K. Datta et al.; North Holland 1990, p. 81-86
  3. R.D.Andrews, Study of Waveforms in Acoustic Diffraction Patterns Using a Stroboscopic Schlieren Technique, SPIE Vol. 248, International Congress on High Speed Photography and Photonics, San Diego, 1982
  4. K. J. Pohl, H. A. Crostack, E. H. Meyer, Holographic Visualization of Laser Induced Ultrasonic Rayleigh Waves, International Conference on Hologram Interferometrie and Specle Metrology, Nov. 5.-8. 1990 Baltimore USA,
  5. E. Neumann, u.a., Ultraschallprüfung von austenitischen Plattierungen und austenitischen Schweißnähten, Berlin 1995


Author

Bernd Köhler Fraunhofer-EADQ, Krügerstraße 22, D-01326 Dresden, Germany
email: koehler@eadq.izfp.fhg.de

| UT-Front Page | |Top to this page|

Rolf Diederichs 1.August 1996, info@ndt.net
/DB:Article /DT:active /AU:Koehler_B /IN:FHG /CN:DE /CT:UT /CT:optic /ED:1996-08