Comparative analysis of through-transmission ultrasonic bulk wave methods for phase velocity measurements in anisotropic materials

In this paper a critical evaluation of ultrasonic bulk wave velocity measurements by the through-transmission and the recently introduced self-reference methods is reported. The major difference in these two techniques is the reference acoustic path: through-water for the through-transmission method and through-sample at normal incidence for the self-reference method. The error introduced by geometrical imperfection of the sample is compared theoretically and experimentally for two methods. Composite materials with different anisotropies are used in the experiment, including ceramic matrix (SiC/Si3N4) and graphite/epoxy composites. Both analytical and experimental studies show that the self-reference method has advantages over the through-transmission method, which is susceptible to geometrical imperfections in the sample. The effect of temperature fluctuation on the accuracy of ultrasonic phase velocity measurements is discussed and different compensation techniques are proposed.