ELASTIC-WAVE SCATTERING BY SHELLED SPHERICAL SCATTERERS IN A FOCUSED FIELD

Embrittlement of many important metal alloys has been related to the a ccumulation of undesirable materials at grain boundaries, a condition which may be detectable through measurement of ultrasonic scattering f rom the material's microstructure, Grains with decorated grain boundar ies are modeled as shelled microspheres embedded in an isotropic elast ic host, and a practical means of predicting scattering from these par ticles is developed. The incident field often used for measuring backs cattered grain noise is focused; both plane and focused incident field s are treated, Theoretical predictions of scattering from isolated sca tterers are compared with experimental measurements on metal microsphe res embedded in plastic to validate the computational procedure, then predictions of scattering from similar spherical structures embedded i n a metal host are presented. In the former case theoretical predictio ns are found consistent with observations, although differences betwee n shelled and nonshelled scatterers are obscured by the great contrast between host and scatterer. In the latter case, where host and core a re quite similar, even thin shells can produce scattering readily dist inguishable from the weak scattering in polycrystals that may be due t o locally inhomogeneous properties. Results of this study can be used to calculate a backscattering coefficient for calculations of grain no ise in metals containing, or composed of, numerous shelled scatterers. (C) 1996 Acoustical Society of America.