Inhomogeneous wave generation and propagation in lossy anisotropic solids. Application to the characterization of viscoelastic composite materials

This article develops a method for investigating some anisotropic media, such as composites, by the use of ultrasonic waves transmitted through a plate-shaped sample immersed in water. The discussion begins with Christoffel's equations for plane linear anelastic waves under the assumptions that for small angles of incidence the wave modes are plane and inhomogeneous and that the anisotropy is representable by hexagonal symmetry. The water-sample interface is treated using the law of Snell-Descartes for nonabsorbing media and takes into account mode conversion and the generation of acoustic surface waves. The method produces viscoelastic constants and relative attenuation coefficients as a function of the angle of refraction. The experimental measurement apparatus is described and data are given for the 25-layer unidirectional Gr/epoxy composite. Results are presented in terms of slowness, damping vector, and attenuation curves. The results are significant in that they demonstrate the anisotropy both for the elastic stiffness and the attenuation. The method appears to hold promise for characterizing some classes of anisotropic media, including 2-D composites, in terms of their anelastic behavior.