13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- The temporary scan of the caustic image was made to track change of stress fields in the process of crack propagation, Fig. 2. There is a clear qualitative picture, namely that elastic waves travel from the crack tip with a certain frequency. This frequency falls into the frequency range obtained during the registration of the crack trajectory (Fig. 1). The black arrow indicates the beginning of a distinct emission of elastic waves from the crack tip. There are two sets of lines that correspond to the longitudinal (dash line) and transverse speeds (dotted line). Fig. 3 shows the change of the caustic diameter at the crack propagation. 3.2. Dynamic Tests 4,0x10-5 8,0x10-5 1,2x10-4 1,6x10-4 2,0x10-4 30 32 34 36 38 40 42 44 crack length (mm) time (s) Figure 4. Crack propagation in case of dynamic loading of plates with a notch. Fig. 4 shows the typical trajectory of crack extension obtained under pulsed pressure on the edges of the notch in the plates of PMMA. The stepped form of crack trajectory is associated with an arrival of elastic waves reflected from lateral faces of a sample. Therefore, the greatest interest is the first step of crack development without influence of reflected elastic waves. For this purpose the samples were made with such sizes that energy of reflected waves was not enough to re-start the crack. The typical trajectory of cracks obtained in these samples is shown in Fig. 5а; and the corresponding crack speed is shown in Fig. 5b. The maximum fluctuations of the crack speed, as well as in the quasi-static tests, fall on the rougher area of the fracture surface. Unlike the quasi-static [3, 8] or the dynamic [9, 10] tests, here the return order of formation of characteristic zones of fracture surface is observed: fragmentary (large pieces), scaly, parabolic and mirror. The scan of caustic in the dynamic tests is complicated by presence of elastic waves emanating from the point of application of pressure to the crack edges, and presence of "residual" caustic before and after crack propagation, Fig. 6. In Fig. 7 shows the characteristic change of the stress intensity factor at the crack tip and the corresponding change of the crack speed. It can be seen that the stress intensity factor, at which began the crack extension, substantially exceeds the static value KIc for a crack start.
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