13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- part is the line covering 91.3% of the total number of points. The process of light reflection looks like the process of avalanche spreading (Fig. 4). The lower plot represents the event frequency. The events are distributed in blocks. We have analyzed the time interval distribution in avalanches and found that the distribution at the initial stage (marked in blue in Fig. 4) cannot be described by the power law. The left plot in Figure 7 illustrates the distribution at the initial stage in double logarithmic coordinates. In the right plot in Figure 7, only the vertical coordinate is logarithmic. The time interval distribution is subjected to the exponential law. a) b) Figure 7. a) Cumulative distribution function of time interval for initial stage in a double logarithmic plot. b) Cumulative distribution function of time interval for initial stage in a coordinate system Ln (N) –time t. An exponential functional form requires a characteristic length scale which can be defined as ch X tV = , (1) where Xch=2.6 mm is the characteristic size, t is the x-coordinate of point C (t= 457 ns), and V=5800 m/s is the sound velocity in quartz. We suppose that this length scale correlates with the impactor thickness. The initial stage statistics does not change the total statistics, because only 10% of the points belong to the initial stage. Investigation of the fragments by optical microscopy (Fig. 8.) reveals that surface of the fragments is not smooth so there is no simple relationship between pulse amplitude and crack size as in [3] where cracks are represented as smooth disk-shaped objects. C
RkJQdWJsaXNoZXIy MjM0NDE=