INDUSTRIAL ULTRASONIC-IMAGING AND MICROSCOPY

Ultrasonic imaging and scanned acoustic microscopy are terms used to d escribe similar imaging processes at different magnifications and freq uencies. Both processes form images by acquiring spatially correlated measurements of the interaction of high-frequency sound waves with mat erials. With the exception of the interference measurement, called V(z ), and the gigahertz frequencies used by the higher frequency scanning acoustic microscopes, it is difficult to establish operational differ ences between them. This is especially true since almost all commercia l ultrasonic imaging systems use transducers producing focused beams a nd can display magnified high-resolution images. Ultrasonic C-scan ima ging was developed largely by the ultrasonic nondestructive testing in dustry. The development was gradual and evolutionary. Over a 50-year p eriod, better and better broadband transducers, electronics and scanne rs were developed for operation at progressively higher frequencies, n ow ranging from 1.0 to 100 MHz. Conversely, scanning acoustic microsco pes made a relatively sudden appearance 20 years ago on the campus of Stanford University. The first scanning acoustic microscopes operated at gigahertz frequencies and used microwave electronics that produced acoustic tone bursts with many wavelengths per pulse. Three factors co ntrol resolution in an acoustic image: diameter of the acoustic beam o r its point spread function (PSF); size and spacing of the pixels maki ng up the image; signal-to-noise ratio (contrast) of the feature being resolved. The beam diameter, or PSF, is controlled by the frequency o f the ultrasonic pulse and the focal convergence of the beam (or focal length to diameter ratio Z/d). In the coupling fluid, the Z/d ratio i s determined by the transducer diameter and lens, but in the material, Z/d is established by the materials ultrasonic velocities. Pixels are the squares of colour or greyscale that make up computer displays of scanned images. Following Nyquist's criterion, the resolution of those images is twice the size and spacing of the pixels. It follows, there fore, that in order to support the resolution of an ultrasonic beam, t he pixels must be no larger than half that beam diameter. Finally, the contrast of the feature being studied must be (at least) a clear shad e of grey above the background produced by the image noise. The noise can be due to the material or the electronics. Written to support indu strial ultrasonic inspection of materials, this discussion will emphas ise the similarities between imaging and microscopy rather than the di fferences. The roles of the focusing lens, the pulse frequency, and th e material being imaged, with respect to the final resolution of an ac oustic image, will be considered in detail. It will be shown that addi tional improvements in resolution can be achieved with image processin g. Finally, applications studies in metals, ceramics, composites, atta chment methods, coatings, and electronic assemblies will be used to de monstrate specific roles for imaging/microscopy in nondestructive test ing.