Dispersion of acoustic waves by fiber-reinforced viscoelastic materials

Abstract

Acoustic waves are dispersed by two distinct mechanisms as they propagate through a fiber-reinforced viscoelastic material. The first mechanism, viscoelastic dispersion, is characterized by an increase in phase velocity with an increase in frequency, and the second mechanism, geometric dispersion, is characterized as a wave-filtering phenomenon in which periodic waves are selectively transmitted and reflected. To study the interplay between these two mechanisms, the dispersion characteristics of two fiber-reinforced viscoelastic materials were obtained at various temperatures using a water-bath technique. The first composite studied was a cloth-laminate quartz phenolic (2D-QP). Its dispersion characteristics (both phase velocity and attenuation versus frequency) were obtained at seven temperatures, ranging from 4 ° to 40 °C. The second material studied was stainless steel wires embedded in an Epon 828-Z matrix (Epon–SST). Its dispersion characteristics (phase velocity versus frequency) were determined at five temperatures, ranging from 5 ° to 42 °C. The characteristics of these two materials at their low-frequency limits are similar; namely, the phase velocity decreases with increasing temperature. There, the similarity ends. The relatively fine structure of the 2D-QP is weakly dispersive below 4 MHz, and viscoelastic dispersion dominates; whereas the relatively coarse structure of the Epon–SST is highly dispersive below 2 MHz (the cutoff frequency of the first passband was found to be about 1 MHz and this composite behaves as a wave filter. Subject Classification: 35.26, 35.50.