H.K. Wickramasinghe

Elastic Constants Measurement in the Acoustic Microscope

The Quate scanning acoustic microscope {l; is uniquely c apable of high resolution imaging with contrast derived from mechanical variations in the specimen. The extraction of quantitztive density and elasticity data from a conventional micrograph is complicated because the a coustic beam is tightly foe-ussed. The image is an integral over the complete angular s pectrim of waves present - longitudinal and shear wave properties are both relevant to the image. We rep>x-t on a technique in which the scattering spectrum of the object is recovered. Nith a suitable choice of couplinz fluid, t his spectrum reveals the critical angles and the nornal inpedance of the material. This is sufficient to characterise the b ulk and shear elastic moduli, as well as the density of isotropi~c materials.

NDE of Solids with a Mechanically B—Scanned Acoustic Microscope

Acoustic microscopy of the interior of solids is hampered by spherical aberration introduced at the sample surface. The incorporation of a spherical coupling piece in front of a conventional acoustic lens radically reduces the aberration. This is so not only when the coupling piece is concentric with the lens, but when the lens is axially displaced. This means that diffraction limited focussing can be achieved over a wide range of depths, typically up to several hundred wavelengths. Thus a B-scan imaging system with mechanical scanning of the lens focus can be used to image a slice through a solid specimen. Alternatively, the lens may be focussed at a constant depth and transversely C-scanned in the normal way. This paper reports on theory and experiments using this system for non-destructive imaging of cylindrical specimens.