Only a few experimental methods are known which generate these ultrasound wave images but they are either limited in its applicability or very expensive. The aim of this paper is therefore twofold: Firstly existing methods of experimental production of sound field images will be reviewed. Secondly new and rather inexpensive method for sampling of stroposcopic pictures is given. Our considerations are limeted to sound fields in solids and on their surfaces.
The article provides a demonstration of a movie-file "rayleigh.mpg".
The file can be viewed with any MPEG viewer.
The movie shows a 10 second length of a 2 Mhz Rayleigh wave hidden between two notches on a AL-plate. The movie file can be downloaded from the SWRI South West Research Institute server - URL: ftp://ftp.nde.swri.edu/pub/nde/ultrasonic_fields/rayleigh.mpg
You can download it now also => Download rayleigh.mpg movie 750KB
The simplest and most common way for tracing sound fields in solids is launching a sound wave into a solid and measuring the sound intensity on the opposite side of the specimen. In Fig. 1 an appropriate experimental arrangement is given. A probe is excited to transmit ultrasonic waves into a test specimen. That ultrasound propagates through the specimen and its intensity is scanned for by a small aperture probe at the opposite surface. The amplitude or intensity values are collected by a computer which also controls the manipulator. The corresponding plots are called C-scans.
Complete different methods are sound field mappings by optical means. One possible experimental arrangement is schematically given in Fig. 2 (see ). Here the light of a stroboscopic source travels via two mirrors through an optically transparent sample. Two additional polarisers are crossed so that elastic waves in the sample are mapped as a brightness picture. In this way wave phenomena can be studied as for example in Fig. 3 where a wave hitting a cylindrical inclusion in water is mapped. Here also a lot of caustics (one indicated by an arrow) are visible.
However, the method described above is only applicable for optically transparent solids. To visualise waves on surfaces of solids an other optical method, the double pulse laser interferometry  has been proposed (Fig. 4). Here the surface is illuminated by two pulses of coherent light shifted nearly 180 degree in phase. If the surface is motionless, the two images cancel out nearly. If they are taken at time points separated by halve a period of the ultrasonic wave, points of maximum deflection are mapped bright whereas the points of zero displacement difference appear dark due to the mentioned light cancellation. In Fig. 5 an example is given where a probe transmits Rayleigh waves into the surface of a coated sample containing bonding flaws. The primary waves are clearly visible together with the bonding flaws mapped as disturbances of that plane waves. The two flaws are of a size of 6 mm and 14 mm. These two optical methods have the advantage to map the wavefront of the ultrasound at a fixed time whereas the conventional intensity method does not provide this information. But they have the disadvantages of either being limited to transparent solids (polarizing optics or schlieren method) or being very expensive. So a further method is proposed based on conventional scanners, which are available in most ultrasonic laboratories, to visualise sound fields much less expensive but with all the necessary information to characterize materials and defect states at surfaces of opaque solids.
Fig. 1 System for conventional (intensity) sound field measurements
Fig. 2 Polarizing optics for mapping ultrasonic fields
in transparent solids
Fig. 4 Experimental arrangement for
double pulse laser holography
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 |
Flat bottom holes have been drilled in an AL-plate from the rear. They end 0.5 mm below the surface. Transient surface waves of 2 MHz where transmitted by a piezoelectric transducer and the sound field has been recorded as anintensity map and stroboscopic map. The two holes are clearly visible as centres of spherical waves in the snapshots (Fig. 7b). Contrary to this, in the intensity map (Fig. 7a) the incident wave covers up the structure of the scattered wave completely and the holes can only be sensed.
|Fig. 7 plate containing |
two flat bottom holes of
5 mm and 3 mm drilled from the rear
|a) intensity map b) stroboscopic map|
An other interesting example is shown in Fig. 8. Here out of a time series of snapshots three interesting images are selected. The localisation of the artificial flaw is indicated by the dark bar. It is a notch of width 2 mm and ending 1 mm below the surface. The "tip diffraction" appears as a spherical ringing and later a transmitted and a refracted waveis visible. There are some locations where the wavefronts are not continuous (arrow). We interpret this by a lower sound velocity of the wave in the 0.5 mm thick and 2 mm wide area above the notch where some kind of plate wave occurs.
| Fig. 8 Snapshots of the sound field|
on an Al-plate surface;
incident Rayleigh waves of 2 MHz;
a notch machined from the rear is aligned 45°
to the sound propagation direction
with a width of 2 mm, ending 1 mm below the surface
Fig. 9 Snapshot of a Rayleigh wave|
field prapagationg along the left border
of the plate striking a flat bottom hole of 1 mm diameter
Fig. 9. reveals an other interesting effect. Here we have a Rayleigh wave striking a flat bottom hole of 1 mm diameter. But the hole is near the side edge of the plate, so the bundle of Rayleigh wave reaches the plate edge. Apparently a wave pattern occurs starting from the edge. Because the boundary condition - the normal projection of the stress tensor is zero for a free surface - is not fulfilled for Rayleigh waves at the edge they cannot exist exactly. Nevertheless the true kind of wave coming from the edge is not clear. Images like the shown provide help to discover such waveforms and may give hints to explain them.