induced by adiabatic heating [11]. In rubber modified epoxy, R2, R6 and R10, although thermal blunting owing to localized adiabatic heating at the crack-tip may happen, its dependence at this high loading rate could be weak. Instead, the toughening mechanisms of rubber cavitation and matrix plastic deformation [2] are dominant and favored by the low loading rates, leading to the results displayed in Figure 7, especially when the rubber content is high. Again, further studies on the effects of loading rates on the relative amount of thermal blunting, cavitation and matrix plastic flow in these materials are essential to confirm the toughness results obtained. Figure 8 Normalized nano-rubber toughening efficiency of composites A filler-toughening ratio parameter, η, can be used to evaluate the toughness increase associated with the nano-rubber particles. Thus, η is calculated from: IC ICm ICm K K K (9) where KIC, KICm are the toughness values of nano-rubber filled epoxies and neat epoxy matrix, respectively. Hence, η is a ratio of the net toughness caused by the nano-rubber presence over the net toughness of the epoxy matrix. Figure 8 plots the toughening efficiency parameter η of nano-rubber based on equation (9). In Figure 8, the toughening ratio of R2, R6 and R10 at each loading rate was best fitted, and their slopes could be found to have a distinct trend: it drops with increasing loading rates. As explained in the above section, this may be due to the reduced rubber cavitation and matrix plastic flow under the high strain rate. 5. Summary and Conclusions The fracture performance of nano-rubber filled epoxies under a wide range of loading rates from
RkJQdWJsaXNoZXIy MjM0NDE=