toughness of polyester/TiO2 nanocomposites with three-point bend specimens, which were higher than their quasi-static fracture toughness. However, there is little known on the dynamic fracture behaviors of epoxies filled with soft nano-rubber particles at high loading rates. But, such results are necessary to understand their performance and reliability as structural materials under those extreme conditions. In this study, ~100 nm diameter nano-rubber particles were used to process epoxy composites and prepare single-edge notched specimens for dynamic toughness measurements with a split Hopkinson tensile bar (SHTB) at a maximum loading rate of 3×104 MPa·√m/s. Toughness tests at lower quasi- static rates were also conducted on an Instron machine and the results were analyzed. 2. Materials and specimen The resin selected was standard diglycidyl ether of bisphenol A(DGEBA) with an epoxide equivalent weight (EEW) of 185 g/mol and Araldite-F was supplied by Sigma-Aldrich in Australia. Spherical rubber particles ~100 nm with excellent dispersion were supplied with 25 wt.% (weight percent) concentration in bisphenol A resin by Kaneka Corporation, Japan. The curing agent was a cycloaliphatic secondary amine, Piperidine, obtained from Sigma-Aldrich. Material formulations were prepared by mixing plain DGEBA resin with required amounts of nano-rubber master batch. Three weight fractions of nano-rubber particles were made, 2 wt.%, 6 wt.% and 10 wt.% nano-rubber particles in epoxy (denoted by R2, R6 and R10 hereafter), respectively. Samples with neat epoxy were also made to study the effect of nano- rubber. After adding the curing agent to the mixture, it was poured into two pre-heated moulds for curing at 120℃ for 16 h. One mould was fabricated for Compact Tension (CT) specimens following ASTM D5045-99 [5]; the other was for making bulk materials, which were cut into beams 4.5×14×65 mm3 for dynamic fracture tests. Figure 1 shows the TEM microstructures of R10, which indicates clearly nanoparticles dispersed uniformly in the epoxy matrix. Figure 1. Nano-rubber dispersion of R10 All samples were first heated for 2 h at 80℃to remove any residual stress owing to surface polishing. A natural crack was then obtained by tapping a fresh razor blade at a specimen notch tip. The length a of the CT specimens was between 0.45W and 0.55W [5].
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