Determination of Mechanical Stress by Polarized Shear Waves

The determination of mechanical stress by ultrasonic techniques is based on the measurement of sound velocity which is influenced by stress. The physical background of this phenomenon is known since a long time, but the practical realization until now failed due to two reasons: The influence of stress on the ultrasonic velocity is rather small, typically of the order of 10–3. Therefore, in order to determine the velocity, the pathlength and the time-of-flight have to be measured accurately. Whereas time-of-flight measurements with high precision can be easily achieved, it is difficult and time-consuming to obtain the same precision in measuring the pathlength.Texture and microstructure also influence the sound velocity and hence these effects must be separated for the determination of stress. The first mentioned problem is solved by measuring the times-of-flight of two linearly polarized shear waves relative to each other. These two waves propagate along the same pathlength and from the theory of elasticity it follows that a stress determination is possible without accurate pathlength measurements.