## 4 Results and Analysis
### Characterizing the Fragmertized Process of Typical Specimens  
With the opening of the main crack on the surface of the specimen, secondary cracks dominated by shear stress gradually occurred at the contact points between the specimen and elastic bars. Figure 8 shows the process of main crack opening and secondary crack initiation and propagation in the typical experimental results for the Breccia and Andesite specimens. The high-speed images were binarized to obtain the pixel information of the cracks. Then, the width of the main crack at different moments was measured by pixel information, as shown in Figure 8. According to the widths of the main crack at different moments, the average opening velocities of the main crack in 250 us shown in Figure 8 were 12.85 m/s for the Breccia specimens at 4.86 m/s striking velocity and 20.94 m/s for the Andesite specimens at 4.88 m/s striking velocity. Figure 6: Full-field strain of the Breccia specimen at 4.86 m/s striking velocity: (**a**) 0 us; (**b**) 80 us; (**c**) 90 us; (**d**) 100 us; (**e**) 110 us; (**f**) 120 us. Figure 7: Full-field strain of the Andesite specimen at 4.88 m/s striking velocity: (**a**) 0 us; (**b**) 210 us; (**c**) 220 us; (**d**) 230 us; (**e**) 240 us; (**f**) 250 us. According to the pixel information of the binary image, FracPaQ used the Hough transform method to find co-linear patterns of pixels in the image and produced fracture traces. The fracture trace is a line segment whose endpoints are distributed along the crack contour. In other words, the cracking morphology in the binary image was fulfilled with fracture traces. We affected the numbers and lengths of fracture traces by adjusting the values of the Hough peaks and the Hough threshold. Figures 9 and 10 are the analysis results of binary images of the cracks in Figure 8 by FracPaQ with the number of the Hough peaks setting at 3000 and the Hough threshold setting at 0.1. In the figures, a Cartesian coordinate system is established with the contact end between the incident bar and the specimen as the origin, the bar direction as the \(X\)-axis, and the \(Y\)-axis (north direction) perpendicular to the bar direction. Figure 8: Images of the fragmentized process and main crack width at different moments: (**a**) Breccia specimen at 4.86 m/s striking velocity, (**b**) Andesite specimen at 4.88 m/s striking velocity. Figure 9: The fracture traces of the Breccia specimen at 4.86 m/s striking velocity obtained by FracPaQ: (**a**) 150 \(\upmu\)s; (**b**) 200 \(\upmu\)s; (**c**) 250 \(\upmu\)s; (**d**) 300 \(\upmu\)s; (**e**) 350 \(\upmu\)s; (**f**) 400 \(\upmu\)s.  
Therefore, we made a quantitative characterization of the cracking morphology with the statistical length and orientation information of the fracture traces. We defined a ratio of the length of one fracture trace to the sum of all the fracture trace lengths at the current moment as the normalized length. The distribution of the normalized lengths of the fracture traces along the \(Y\)-axis deflection angle (the north is 0) was statistically analyzed and was plotted as the rose diagram shown in Figure 11. The \(Y\)-axis deflection angle was counted from both clockwise (0-180\({}^{\circ}\)) and counter clockwise (180\({}^{\circ}\)-360\({}^{\circ}\)) directions; thus, these rose diagrams are centrally symmetric. In the early stage of the fragmentized processes (150 \(\upmu\)s in Figure 11a and 280 \(\upmu\)s in Figure 11b), the normalized length of the fracture traces corresponding to the main crack in the horizontal direction (\(Y\)-axis deflection angles are 90\({}^{\circ}\) and 270\({}^{\circ}\)) is dominant. With the main crack opening width increasing and secondary cracks expanding, the fracture traces in the vertical direction gradually increased. At the same time, the normalized lengths with the \(Y\)-axis deflection angle at 0\({}^{\circ}\) and 180\({}^{\circ}\) gradually increased. Thus, the capability of FracPaQ to characterize the fragmentized processes was validated.