13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- very short rise time application of the high-pass filter to the original signal increase signal-to-noise ratio and makes possible to measure time intervals between events. The definition of the size of time interval between the impulses is shown at the Figure 5. The second step involves measuring the distance between the green bars showing a sharp rise in impulse. We consider only the impulses which are above some discrimination level (the red line). Discrimination level is determined by noise level in the way that not more than 1 false positive pulse was detected on the reference signal. Reference signal was taken before loading. Figure 5. Determination of the size of time interval between the impulses of the light reflected from the fracture surfaces. The cumulative distribution function of the time interval in the double logarithmic plot (Figure 6) is fitted by the straight line (total number of points 1073). Figure 6. Cumulative distribution function of time interval in the double logarithmic plot. At small sizes (77 points – 7.1761 % of the total number of points), the curve deviates from the straight line because the size of time interval is comparable with the oscilloscope sample rate (1 GHz). The falloff at the largest interval sizes (16 points – 1. 5% of the total number of points) is due to finite-size effects. In this case the time interval is comparable with the process time. The central interval
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