Anomalous ultrasonic dispersion in fluid-coupled, fibrous composite plates

The ultrasonic dispersion characteristics of vertically polarized plate waves in fluid-coupled plates of unidirectional graphite-fiber reinforced epoxy have been found experimentally to display anomalous dispersion. This anomaly consists of a mixing of the two fundamental plate modes at phase velocities near the bulk composite transverse wavespeed. A calculation of ultrasonic reflection from a fluid-coupled composite plate accounts very well for the previously unreported experimental observations. It is demonstrated that the anomalous dispersion is a consequence of the presence of the fluid and is strongly enhanced by the highly anisotropic elastic nature of the composite. It is further shown that a much weaker manifestation of the effect also exists in fluid-coupled plates of elastically isotropic materials.