Ultrasonic energy flux deviation and off-diagonal elastic constants of wood

Wood is a natural, inhomogeneous, anisotropic composite material whose elastic behavior can successfully be described as that of an orthotropic continuum. Christoffel´s equation relates ultrasonic propagation velocities to the elastic constants. Six of the nine independent constants can readily be determined from velocity measurements in the directions of the symmetry axes. Determination of the remaining three constants can proceed by measuring the propagation velocities in a set of propagation directions in each of the three material symmetry planes. For all propagation directions away from the three material symmetry axes, the energy flux of the ultrasonic beam will deviate from the propagation direction. The energy flux deviation is sensitive to changes in the off-diagonal elastic constants. Acknowledging this and measuring the energy flux deviation will allow more accurate determination of off-diagonal elastic constants