3. RESULTS AND DISCUSSION
3.1 High frequency elastic properties
Measured sound velocities, together with measured densities and calculated elastic properties and associated errors [9], are presented in Table 1 for both forms of PRR investigated. No difference in longitudinal wave velocity was observed for the two resins. Given the similar densities this was not
unexpected (similar results for two epoxy resins with similar densities was observed in [16]. However, unlike pure PRR, the 1 wt.% PG1190 shear wave exhibited two distinct regions, representing shear wave velocities of 0.68 and 1.48 mm/µs. The second signal occurred at a time interval ~2.13 times that to the initial pulse – insufficient time in the pulse-receive mode for a reflection to be detected. Consequently, it was apparent that the (slower) second signal was distinct from the first. Elastic properties for both elements of the 1 wt.% PG1190 shear wave are therefore included in Table 1 (although, for cS = 1.48 mm/µs, a physically un-realistic negative Poisson’s ratio was calculated). The velocity of the second element of the 1 wt.% PG1190 shear wave was observed to be similar to that in pure PRR. It is therefore postulated that this two-phase shear was indicative of the structure of the as-cured resin (the existence of a substantial PRR matrix surrounding an interconnected nano-reinforced region with a slower shear wave in the matrix seems a plausible scenario). It should be noted that because longitudinal waves were attenuated to a substantially lower degree then shear waves by both resins, the strong pure PRR longitudinal wave may have obscured any signal unique to the postulated interconnected region.