13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- with a notch. The speed of the crack front is shown in Fig. 1b. The speed was obtained by differentiating of the crack trajectory according to the formula of the central difference derivative. Note that the applied scheme synchronization does not allow you to capture the initial part of the crack trajectory. Comparison of the crack trajectory and the fracture surface shows that the beginning of an unstable behavior of the crack corresponds to the beginning of the "ribbed" surface structure [3,7] with the distance between the ribs about 1 mm. The most pronounced oscillation frequency of the crack front speed is in range of 400-700 kHz. The similar dependence of crack propagation speed was also obtained in [3]. In this work, the speed of a crack was determined by change in electrical resistance of an aluminum layer deposited on the sample surface. It was shown that the crack front oscillations correlated with the profile of the fracture surface, and the critical speed of the transition to the unstable regime Vc = 0.34CR (CR is the Rayleigh wave speed) does not depend on the geometry and thickness of a sample and the tensile rate. 2,0x10-5 3,0x10-5 4,0x10-5 5,0x10-5 6,0x10-5 7,0x10-5 0 3 6 9 12 15 18 21 24 27 30 33 crack length (mm) time (s) Figure 2. Slit scan of the stress-wave pattern at a crack tip under quasi-static tension of plane samples of PMMA. 2,0x10-5 3,0x10-5 4,0x10-5 5,0x10-5 6,0x10-5 7,0x10-5 4,0 4,5 5,0 5,5 6,0 6,5 caustic diameter (mm) time (s) Figure 3. The caustic diameter at crack propagation in quasi-static tests.
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