13th International Conference on Fracture June 16–21, 2013, Beijing, China -7- 15 20 25 30 35 40 45 50 55 60 0 1 2 3 4 5 6 KIC KID KI(t) V(t) time (μs) KI (MPa*m0,5) 0,0 0,2 0,4 0,6 0,8 1,0 crack speed (km/s) Figure 7. The dynamic stress intensity factor and the corresponding crack speed in dynamics tests. 3.3. Discussion The results obtained by a single method for recording of dynamics of a crack in plates of PMMA allow to mark out similarities and differences in stages of a fracture process in fragile and quasi-fragile materials under quasi-static and dynamic loads. The slit scan of the development process of a crack front allows to estimate the crack speed on different time intervals, and thus the spatial scale (either the section of a crack front or the all front). The experiments demonstrate the general property of spasmodic development of a crack front and qualitative agreement with the results obtained by other authors [3, 4]. The average speed of crack propagation in our experiments, both in the quasi-static and dynamic tests, did not exceed 0.5CR, but the instantaneous crack speed could approach the Rayleigh wave speed CR on the "jumps". The oscillations of the crack front speed correlate with changes of a profile of fracture surface. The observed dynamics of cracks may be related to a pre-fracture zone before the tip of a main crack [9-12], i.e. development of micro damages ensemble in the area of high stresses. The characteristic view of such pre-fracture zone, registered in the dynamic tests after stopping a crack, is shown in Fig. 8. Figure 8. Magnified view of the pre-fracture zone before a crack tip. The photo was made after stopping the crack in dynamic tests. The arrow indicates the direction of crack propagation. The principal difference between the two ways of loading is the behavior of the average speed of
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