Inspection of Ceramics Incorporating Size Estimation Methods Using Conventional Ultrasonics

One of the principal characteristics of the inspection of ceramic materials is the small size of the critical defects involved. In order to meet this challenge, either very high frequency techniques must be developed or conventional (low frequency) techniques must be used in special ways. This paper addresses the latter approach by investigating the use of commercial transducers combined with focusing techniques in a water bath at frequencies in the range of 5 to 20 MHz as well as signal analysis techniques using plane waves in the 30 to 40 MHz range using special order commercial transducers. Although the high velocity of sound in the ceramics used (silicon nitride) put some restrictions on the numerical aperture which could be obtained with acoustic lenses, a focused beam ultrasonic system for use in a water bath was designed and used to produce maps which showed the location and reflecting power of defects in flat ceramic specimens. Subsequent analysis of the reflectors discovered by this focused beam system was carried out by a hand-held ultrasonic probe which irradiated the sample with plane waves. By performing Fourier analysis of the echo signals from the defects and comparing the frequency spectrum observed with calculated spectra, it was possible to estimate the effective spherical size of the scattering objects. Analysis of the frequency dependence of the scattered energy in the long wave length limit also provides a measure of an effective spherical volume of the scatterer but this method was found to require additional signal processing methods which will be developed at a later date