Acoustoelastic effect of wood II: Effect of compressive stress on the velocity of ultrasonic longitudinal waves parallel to the transverse direction of the wood

Abstract

The changes in the velocity of ultrasonic waves propagating in wood parallel to the direction of applied stress are discussed. The ultrasonic mode was longitudinal waves traveling along the direction of applied stress with the compressive load applied parallel to the transverse direction of the wood. The ultrasonic velocities were measured by the sing-around method. The experimental results indicated the existence of an acoustoelastic phenomenon in the transverse direction of the wood. The percent change in the ultrasonic velocity was given as a function of the applied stress. The change in the velocity depended on the species and structural direction of the wood. That is, in the radial direction of hardwood, the ultrasonic velocity increased with increases in compressive stress at the initial stress level of less than 2MPa; it then gradually decreased with increases in stress. A change in velocity from an increase to a decrease was considered a unique phenomenon for wood. In contrast, in the radial direction of softwood and the tangential direction of hardwood, the ultrasonic velocity decreased with increases in stress from the beginning of loading. This phenomenon is also generally observed in metallic materials. The relations between velocity and stress at the initial stress level and between velocity and strain in the range of large deformation are represented by essentially straight lines. The acoustoelastic constants of wood were obtained from these relations at the initial stress level. The absolute values of the constants in the transverse direction of wood were larger than those for metals and were larger than those for the longitudinal direction of wood reported in our previous paper.