![]() In rubber-modified epoxy, the distinctly different failure mechanisms are displayed in Fig. 3(d) and 3(f). However, there is little difference in the fracture surfaces shown in Fig. 3(a) and 3(c). In neat epoxy, both Young’s modulus and yield strength increase with loading rate, which explains why the toughness is increased (by ~ 30%) using a critical crack opening displacement fracture criterion. The high rate induced temperature rise is insufficient to cause crack-tip thermal blunting (e.g., 20 ☌ in E) and the rate effect becomes dominant. For E, the toughness increases, but for R6 it decreases, from 5 × 10 - 4 s - 1 to 120 s - 1. The opposite trends observed are interesting. The effect of two loading rates on toughness and relevant typical load-displacement curves of E and R6 at 20 ☌ are shown in Fig. 2(b) and Fig.1 respectively.
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