13th International Conference on Fracture June 16–21, 2013, Beijing, China -9- isochromatics, which are integral characteristics of stress fields at the crack tip, were captured. Another one important difference between the results at the different loading conditions is the values of the fracture characteristics. The stress intensity factor at the moment of the crack start in the dynamic tests significantly exceeds its analog in the quasi-static tests. Figure 10 shows dependence of the average speed of the crack on the dynamic stress intensity factor. It is clear that the shape of the curve depends on the method of obtaining mean values of the crack speed and the SIF. In this case, the average was obtained by means of polynomial approximation. The dash curves were received in the dynamic tests at the same charge of the condenser, but at different thickness of the samples. It can be seen that the crack speed depends on the state of stress at the crack tip. Reduction of the thickness of the sample leads to reduction of the crack speed. In contrast to the quasi-static tests, the stress intensity factor and the average crack speed in the dynamic tests decrease with crack growth that well correlates with change of a roughness of the fracture surface. This fact can be explained by the quantity of input elastic energy in the crack tip. In case of the quasi-static tests there is a constant supply of mechanical energy in the system "grips-sample". However, in the case of a dynamic load on the crack (such source of loading as a pulse laser, an explosion of explosives or a conductor, etc.), only the final quantity of energy injects in a sample; and the fracture characteristics are defined by the pulse energy and the possible rate of absorption of this energy in the area of the crack tip. 4. Summary The study of the dynamics of crack propagation in plates of PMMA under quasi-static and dynamic loading was carried out. It is shown that under both the quasi-static and dynamic testing conditions the change in the instantaneous crack front speed has a stepwise character. However the behavior of the mean speed depends on the type of loading. In the case of quasi-static loading, the crack accelerates to its maximum value smoothly. In the case of pulse loading, the crack accelerates to its maximum value almost immediately. At the same time the limiting value of the stress intensity factor under impact loading of a crack significantly exceeds its limit value for quasi-static loading. The analysis shows that the differences observed in experimental results on study of crack propagation can be explained by the different time resolution of recording equipment. Moreover, results significantly depend on duration and intensity of loading, as well as on the stress state in the area of fracture. References [1] J.W. Dally, Dynamic photoelastic studies of fracture. Experimental Mechanics, (1979) 349-361. [2] K. Ravi-Chandar, W.G. Knauss, An experimental investigation into dynamic fracture: 1 On steady-state crack propagation and crack branching. Int. J. of Fract. 26 (1984) 141-154.
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