Scattering of a Broadband Ultrasonic Pulse by Discontinuities

Abstract

In oder to obtain quantitative information from ultrasonic waves scattered from hidden defects, a series of scattering experiments were carried out using broadband ultrasonics. The scattering ampli- tudes were measured as a function of frequency and scattering angle from various reflectors in water and from flat-bottomed holes in metals. A geometrical theory of diffraction was applied to obtain analytical expressions for scattering of both longitudinal and shear waves in crack- like flaws. The agreement between theoretical predictions and experi- mental results is good. Scattering experiments were also carried out from various shaped cavities embedded in diffusion-bonded titanium samples. The wave is mode converted and scattered as both shear and longitudinal waves. These two waves are time separated, gated out elec- tronically, and the signal spectrum analyzed separately. The resultant spectrum is a characteristic of the cavity shape. HE USE OF broadband ultrasonics pulse and the analysis of the frequency spectrum from a void or discontinuity was suggested first by Gericke (l) in 1963 as a possible method to characterize flaw geometry. Experimental systems were designed by Whaley and Cook (2) in 1969 and used in systematic experimental work by Whaley and Adler (3) in 1971 to show that there is a quantitative relationship between the frequency spectra of the echo and size and orientation of the ends of flat-ended rods in water. A simple model was sug- gested by Adler and Whaley (4) which was based on inter- ference of the wavelets scattered from edges of the target. The spacing between these extreme edges was related to the fre- quency distribution by this model. A Fourier analysis treat- ment of the two signals scattered back from the two extreme edges of the discontinuity was given by Simpson (5) in 1974 and has provided an explanation of some of the anomalies of the spectrum. Discontinuities of more complicated geometries may be treated by diffraction theory as suggested by Adler and Lewis (6) in 1975. They adopted Keller's (7) geometri- cal theory of diffraction. The work presented here is the ex- tension of both experimental and theoretical treatment of broadband ultrasonic waves scattered from various discontinu- ities in an attempt to develop a model for quantitative flaw characterization. Theory