Preparatory Notes for ASNT NDT Level III Examination - Ultrasonic Testing, UT

The real limitation of this technique is that in many materials the ultrasonic
pulse becomes distorted, which can reduce the accuracy of the measurement.
One way to avoid this problem is to measure the phase difference between
two-tone bursts by changing the frequency to keep the phase difference
constant (Ref 5). Small specimens are used in a water bath, and the pulses
received from the front and back surfaces overlap. The presence of stress
also rotates the plane of polarization of polarized shear waves, and there is
some correlation between the angle of rotation and the magnitude of the
stress. Measurement of this rotation can be used to measure the internal
stress averaged over the volume of material traversed by the ultrasonic beam.