Acoustoelastic effect of wood .1. Effect of compressive stress on the velocity of ultrasonic longitudinal waves parallel to the longitudinal direction of the wood

This paper reports the results of an experimental investigation of the changes in the velocities of ultrasonic waves propagated parallel to the direction of the applied stress in wood. The ultrasonic mode is the longitudinal wave along the direction of the applied stress, and the compressive load is applied parallel to the longitudinal direction of the wood. The ultrasonic velocities were measured by making use of the sing-around method. The experimental results indicated the existence of an acoustoelastic phenomenon in wood. The percentage changes in the ultrasonic velocities were given as functions of the applied stresses. At an initial stress level, the ultrasonic velocity increased with increases of the compressive stress. Then the ultrasonic wave changed its speed from an increase to a decrease gradually with additional increases of the stress. This phenomenon of the changing speed from an increase to a decrease, is not found in metal materials, for example. This is a unique phenomenon for wood. The acoustoelastic constants were obtained from the linear relationships between the relative changes in velocity and the applied stresses at an initial stress level. They showed larger Values than those of metals. The velocity propagated through wood was very sensitive to the applied stress. This suggests that the acoustoelastic effect of wood is possible to be applied to the determination of the stress conditions of wood materials. To make it possible, systematic experiments on the acoustoelasticity of wood should be made according to the ultrasonic mode, propagating direction, and stress direction.