Ultrasonic imaging using spatio-temporal matched field (STMF) processing--applications to liquid and solid waveguides.

This paper is devoted to imaging defects in liquid and solid ultrasonic waveguides. A new ultrasonic imaging technique, based on the spatio-temporal Green functions computation and cross-correlation, is presented. This technique extends the concept of matched field processing (MFP) used in ocean acoustics. Results of experiments conducted in water and in a solid Duralumin bar show that a strong improvement of the spatial resolution is observed with this MFP.

Time-reversed Lamb waves.

Lamb waves are extensively involved in plate structure inspection because of their guided nature. However, their dispersive nature often limits their use in flaw detection. In this paper we show that the use of a time-reversal mirror (TRM) allows to automatically compensate for the dispersive nature of Lamb waves. Experiments showing the spatial and temporal behavior of time-reversed Lamb waves, demonstrate the ability of TRMs to self-focus and to recompress dispersive pulses. This is demonstrated in a set of experiments in which a broadband ultrasonic laser source is used to simulate a point Lamb wave source and an optical interferometer is used to map the time reversed elastic field. We also show that TRM may work in pulse echo mode and allows to detect and to focus along large 2-D plates on any flaws located in the inspected area.

Directivity patterns of a moving thermoelastic source in solid media.

Generation of ultrasonic displacements under the thermoelastic effect has the advantage of being contactless and nondestructive. Unfortunately, the directivity and energy of the sound field of such a source are limited. In order to change these parameters the authors use a moving thermoelastic source by moving the laser beam on the solid surface. Moving the laser beam allows control of the ultrasonic beam directivity and an increase in the energy of the sound field. For this purpose, a large impulse of dye laser deflected by an acoustooptic cell working under the Bragg mode is used. Experimental results, in an aluminum sample give good agreement with the theoretical prediction.