The anisotropy of biological composites studied with ultrasonic technique

One of the most common biological composites is wood material. This natural orthotropic like material is characterized by a high anisotropy determined by the special disposition of the microstructure elements. The anisotropy of wood can be described in various ways using the values of ultrasonic velocities of bulk waves (longitudinal and shear) observed on the velocity surface deduced from the theoretical relationships given by the Christofel's equation. The simultaneous view into the three symmetry planes of the anisotropic behavior of wood is presented on the velocity surface. The spatial filtering action of wood structure is easily connected with longitudinal and shear velocities. The first step in examining the anisotropy of wood is to relate the velocities to the symmetry axes. The simplest way to describe the anisotropy of wood is to express the ratios of velocities. These ratios can be calculated separately for longitudinal or shear waves or for a combination of both. The birefringence of shear waves have a particular interest for the fine definition of anisotropy. A more global appreciation of wood anisotropy than the values of individual velocities is given with acoustic invariants. The stability of calculation of acoustic invariants versus different propagation angles confirms the validity of the chosen model for the tested material. Wood species having high density and any important organized structure in the millimeter scale exhibit a high ratio of invariants. The acoustic behavior of tropical wood species is less anisotropic than that of species from a temperate zone having low density.