ASA 125th Meeting Ottawa 1993 May

5pPA3. Increased backscattering from an inhomogeneous liquid--solid interface at the Rayleigh angle.

Peter B. Nagy

Laszlo Adler

Dept. of Welding Eng., Ohio State Univ., Columbus, OH 43210

It is shown that the experimentally observed peak in the incoherent backscattering from a liquid--solid interface at the Rayleigh angle is caused by the sharp maxima of the longitudinal and shear transmission coefficients. Of course, both refracted waves are evanescent in this region, i.e., they do not carry energy away from the interface but rather keep the vibration within approximately one wavelength of the surface. Nevertheless, the resulting vibration in the solid is much stronger than at other angles of incidence. The incoherent scattering can be caused by either geometrical irregularity or material inhomogeneity (e.g., surface roughness or polycrystalline grain structure). This paper presents analytical and experimental results showing that, regardless of the physical nature of the scattering inhomogeneity, there is a distinct maximum in the backscattered intensity around the Rayleigh angle. The width of the peak is determined by the density ratio between the solid and the liquid while the maximum backscattering at the peak is essentially independent of the average material properties and is determined solely by the scattering inhomogeneity.