Acoustoelastic birefringence effect in wood I: Effect of applied stresses on the velocities of ultrasonic shear waves propagating transversely to the stress direction

The velocity changes of ultrasonic shear waves propagating transversely to the applied stress direction in wood were investigated. The wave oscillation directions were parallel and normal to the uniaxially applied stress direction. The velocities of the shear waves for both oscillations decreased as the compressive load increased, and increased as the tensile load increased. The velocity of the normally oscillated shear wave showed smaller change against the stress applied than that of the parallel oscillated wave. The initial birefringence due to the orthotropy of wood was observed without any stress. Velocity changes in the two principally oscillated shear waves were proportional to the stress within the stress range tested. The acoustoelastic birefringence effect was obtained from the velocity difference between the two shear waves. The relative difference between the two velocities (called acoustic anisotropy) was given as a function of the applied stress. The acoustoelastic birefringence constants were obtained from the relationships between the acoustic anisotropy and the applied stress.