Ultrasonic Characterization of Surfaces and Interphases

Abstract

Ultrasonic waves have been used extensively for material characterization and for sensing in material process control. The waves produce small-amplitude mechanical vibrations and, depending on the mode being used, may induce both longitudinal and shear stresses in the solid. Information on the structural properties of a substance can be obtained by measuring both the velocity and the attenuation of the ultrasonic wave. The phase velocity of the wave depends on the elastic constants and density of the body while attenuation depends on microstructure and crystalline defects.In an isotropic solid medium, which has only two independent elastic moduli, there exist two elastic waves: the longitudinal and the shear. Three kinds of bulk elastic waves may propagate in an anisotropic solid: a quasilongitudinal and two quasitransverse waves, differing in polarization and velocity. To determine the set of elastic constants, one must measure the phase velocity in several different directions relative to the crystallographic axes.The attenuation of an ultrasonic wave is associated with absorption of elastic waves (inelastic effect) and the scattering of elastic waves by structural inhomogeneities. Scattering may be the governing attenuation mechanism in polycrystalline, composite, and ceramic materials. As a result of scattering, elastic energy is lost by the prime ultrasonic beam in the form of a stochastically scattered field, which is gradually absorbed in the material. The latter is associated with conversion of elastic into thermal energy as a result of various inelastic effects termed internal friction.