Figure 3 shows good agreement between the data measured by Marsh for polyurethane [13] and
that for both types of PRR for UP ≥ 0.4 mm/µs. Despite nano-reinforcement, the 1 wt.% PG1190
Hugoniot broadly agrees with that for pure PRR. This is consistent with work by Munson and May [16] in which the high strain rate dynamic response of three epoxy resin systems which exhibited different degrees of cross-linking was shown to be experimentally indistinguishable, particularly at higher particle velocities. This result was taken as an indication that structural and equilibrium molecular spacing changes due to different epoxy cross-linking densities had only a small effect on high-pressure compressibility. Any small differences in the response of the two different forms of PRR would probably be further reduced by the evidence of two phases within the 1 wt.% PG1190 indicated by the shear wave velocities in Table 2. For particle velocities < 0.4 mm/µs the two experimental data sets are observed to trend below that of polyurethane [13] – although, it should be noted that Marsh’s data [13] only deals with UP ≥ 0.66 mm/µs. While, to the authors’ knowledge, no low UP high strain rate investigations of polyurethane have been undertaken, similar non-linear Hugoniots have previously been observed for other polymers. A complex non-linear UP-US relationship was noted for UP < 0.6 mm/µs in the thermoplastic PMMA. While tentatively linked to elastic-plastic behaviour due to the breakdown of polymer bonds over a short timescale, insufficient data was available to determine the underlying mechanism [18]. For the elastomer polychloroprene Bourne and Millett [19] observed a change in the gradient of the Hugoniot at lower particle velocities, noting different linear responses above/below UP = 1 mm/µs. However, given that only four UP < 1 mm/µs UP-US data points were measured it is reasonable to assume that, as with PMMA, the behaviour in this regime was likely non-linear as well. Again, insufficient information was available to isolate the underlying mechanism. Munson and May [16] studied epoxy resins and suggested that
non-linear behaviour should occur at low stress levels in such systems. Differences in low strain rate stress/strain curves were observed and contrasted with similar high strain rate dynamic responses for different epoxy resins. As all resins studied were below the glass transition temperature it was determined that changes in behaviour could only be attributed to a variation in equilibrium molecular separation within the epoxies at different compressive stresses. The similar high stress responses of these resins implied that this mechanism only produced small differences in compressibility, which in turn were only detectable at lower stresses/particle velocities.