The changes in ultrasonic wave velocity propagating in wood normal to the direction of applied stress were investigated experimentally. On the basis of the experimental results, bending stress distributions in wood specimens were estimated acoustoelastically by carrying out measurements of ultrasonic wave velocity. The investigation showed that ultrasonic waves changed speeds only a little, due to the applied stress. Acoustoelastic constants determined from the results of the experiment were used to express the percentage change in ultrasonic wave velocity as a function of the applied stress. The velocity change was seen to also depend on the mode of the ultrasonic wave. Bending stress distribution estimated acoustoelastically agreed well with stress measurements using the strain gauge method and results of numerical calculations. The results of the study show conclusively that the acoustoelastic phenomenon can be applied to determine stress conditions of wood, and it is expected that the method presented here will in the near future be developed into a useful, practical technique for stress measurement of wood.
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