Continuous wave transmission techniques for measuring ultrasonic phase and group velocities in dispersive materials and composites

Abstract

Phase and group velocities V and U, respectively, were measured in the 100:1 frequency range 50 kHz to 5 MHz using longitudinal and shear ultrasonic continuous waves, in Teflon and in a dispersive three-dimensional reinforced composite. A transmission approach was developed which utilized broadband transducers and a pair of wedges to obtain V by a differential path, fixed frequency technique, and to obtain U by a fixed path differential frequency technique. The path or frequency was incremented to fit additional whole wavelengths in the path, as indicated by 2π phase shifts in a Lissajous pattern. Cross checks between U and V confirmed the validity of the approach at least for the two materials investigated.