Theoretical description of ultrasonic propagation and scattering phenomena in polycrystalline structures aiming for simulations on nondestructive materials characterization

In microscopically inhomogeneous media ultrasonic scattering at grain and/or phase boundaries causes sound velocity dispersion and attenuation. These effects as well as the amplitudes of the scattering waves can be used for materials characterization. Concomitant, scattering at the microstructure of materials hampers defect detection and evaluation because the so-called structural noise superposes defect signals, and velocity dispersion corrupts defect positioning. Hence, the simulation of ultrasonic propagation and nondestructive testing procedures must contain microstructural scattering phenomena. In a general approach, the scattering wave energy flux densities in microscopically inhomogeneous materials are derived from the formal infinite Born series presentation of ultrasonic displacement vectors, the solutions of the elasto dynamic equation of motion. The energy flux densities are ensemble averaged respective the microscopic inhomogeneity and evaluated for single phase polycrystals in lowest non-zero order. The resultant directional and frequency dependent scattering wave amplitudes are dis-cussed in the context of results from the literature.