13th International Conference on Fracture June 16–21, 2013, Beijing, China -8- cracks. Under quasi-static uniform extension of the plates, the average speed over smoothly reaches a maximum value. This fact is consistent with the results of [1]. In the case of pulse pressure on the notch edges, the average speed takes a maximum value almost instantly and then can be considered by a constant. That was observed in [4]. 1,0x10-5 2,0x10-5 3,0x10-5 4,0x10-5 5,0x10-5 6,0x10-5 7,0x10-5 0 200 400 600 800 1000 1200 1400 crack speed (m/s) time (s) time step 0.15 μs (375 points) time step 0.25 μs (225 points) time step 0.5 μs (113 points) time step 1 μs (57 points) time step 2 μs (29 points) time step 3 μs (19 points) Figure 9. Speed of crack front extension depending on a time interval of registration of the crack front position. Figure 10. Average crack speed versus instantaneous stress intensity factor in plates of PMMA under different loading conditions. The difference of experimental results for the same character of loading (see e.g. 1 and 2, 3 and 4) can be explained by resolution of recording equipment. Fig. 9 shows the crack speed in the quasi-static tests at different time intervals of the "fixing" moments of the crack locations. In [2, 4] "continuous" registration of a crack front position was applied (the electric signal from a resistive bridge or the slit scan of an image), while in [1, 3] single shooting with a given frame rate was used. Moreover, in [2, 4] the position of the crack front was registered, while in [1, 3] caustics or
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