oscillations during crack propagation, the mean value of the fracture energy < GID > was calculated for each specimen before branching, if any. In practice, < GID > concerns the steady state regime and does not take into account the first 1.5 mm of propagation in RT-PMMA. < GID > varies from 0.49 to 2.1 KJ/m² and for a single straight line crack with smooth crack surfaces is typically 0.57 ± 0.12 KJ/m². In the case of a single straight line crack with crack surfaces showing small aborted but visible branches of length less than about 1 mm (Photo1), < GID > is typically 0.8 ± 0.15 KJ/m². When macroscopic crack branching occurs, the crack surfaces are very rough before branching and < GID > is of the order of 1.45 ± 0.7 KJ/m². The experimental mean dynamic fracture surface energy < GID > for a crack propagating at several hundred metres per second is substantially lower than the fracture energy GIc at the onset of propagation, which is typically close to 10 kJ/m 2 at low or medium stress intensity loading rates. The crack speed is not correlated with < GID > and lies in the range 550-610 m/s at temperatures between 19 °C and 27 °C. 0 50 100 150 200 75 100 125 150 175 200 crack branching no crack branch. branching location a (mm) t (µs) Figure 2: Crack length vs. time in the presence of crack branching (double branch at a = 99 mm) and in the absence of macroscopic crack branching. Note the similarity of the crack speeds and the quasi absence of crack speed variation. 80 100 120 140 160 180 200 0.0 0.5 1.0 1.5 2.0 2.5 3.0 °a = 600 m/s GID (kJ/m²) a (mm) 0 20 40 60 80 100 120 t (µs) quasi-static analysis Figure 3: Quasi-static and transient dynamic finite element analyses: typical results for crack propagation without branching. Photo 1: Photos showing different sizes of the frustrated micro-branches, increasing from left to right and corresponding to a RCP at about 0.6 cr in an RT-PMMA. The cracks propagated from left to right. The entire specimen thickness, 2 mm, is visible and reflected on the metallic coating which acts as a mirror. The right hand photo shows a macroscopic crack branching (three branches) and a sudden change in the fracture surface roughnesses. 3
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