Scattering of elastic waves by a sphere with orthorhombic anisotropy and application to polycrystalline material characterization

Scattering of elastic waves by an anisotropic sphere with orthorhombic symmetry inside an isotropic medium is studied and applied to characterization of polycrystalline materials with anisotropic grains. For a single sphere the waves in the isotropic surrounding are expanded in the spherical vector wave functions. Inside the sphere, the elastodynamic equations are first transformed to spherical coordinates and the displacement field is expanded in terms of the vector spherical harmonics in the angular directions and power series in the radial direction. The governing equations inside the sphere give recursion relations among the expansion coefficients in the power series. The boundary conditions on the sphere then determine the relation among the scattered wave expansion coefficients and those of the incident wave, expressed as the transition (T) matrix. For low frequencies the elements of the T matrix are obtained in explicit form. According to the theory of Foldy the T matrix elements of a single sphere are used to study attenuation and phase velocity of polycrystalline materials, explicitly for low frequencies. Comparisons of the present method with previously published results and recent FEM results show a good correspondence for low frequencies. The present approach shows a better agreement with FEM for strongly anisotropic materials in comparison with other published methods.

Automated summary

This study investigates the scattering of elastic waves by an anisotropic sphere with orthorhombic symmetry inside an isotropic medium, and applies it to the characterization of polycrystalline materials with anisotropic grains. The expansion coefficients of the wave and displacement field are determined inside the sphere using recursion relations and boundary conditions. The T matrix elements are then obtained and used to analyze the attenuation and phase velocity of polycrystalline materials, particularly at low frequencies. The proposed approach shows good agreement with previous results and finite element method for strongly anisotropic materials.