ABSOLUTE MEASUREMENTS OF ELASTIC-WAVE PHASE AND GROUP VELOCITIES IN LOSSY MATERIALS

Traditional methods of determining phase and group velocities are often inadequate for many thick-section materials that exhibit greater than 30 dB, frequency-dependent propagation losses across the passband of the transducer. This article describes a measurement method that addresses this problem. Our method is mechanized as a pulsed, swept-frequency interferometer. The method's accuracy and reliability are enhanced by a combination of circuit-design improvements, which increase the signal-to-noise ratio and linearity, and signal-processing methods, which remove circuit-related measurement errors and compensate for diffraction. First we describe the foundations of our measurement method and its mechanization. Then we describe the signal-processing procedures, used to calibrate the instrumentation and to determine the absolute phase and group velocities. To illustrate the method, we determine the phase velocities in a very lossy, 50-mm-thick, glass/epoxy specimen in the 0.3-1.2-MHz region.