## 3 Analysis of Test Results
### Process of Crack Propagation and Specim Failure
#### 3.3.2 Crack Quantification and Box Dimension Calculation  
Referring to the process shown in Figure 6, based on the binarised images identified from the cracks in the Ratsnake surface, the processed images were quantified using FractLac embedded in ImageJ to analyse the evolutionary pattern of each specimen from crack generation to rupture injection. The average values were found using linear regression and multiple calculations. Due to the low degree of failure of some rocks at a speed of 10 m/s, the screening results are not representative; a total of 40 groups of coal samples and 24 groups of rock samples were obtained after rounding off part of the data to obtain the dimensionality-change values. Due to space limitations, the partial curves of the dimensionality change of the coal and rock boxes were plotted, as shown in Figure 13.  
Figure 12: The whole process of image destruction. (**a**) Original crack images; (**b**) Ratsnake software extraction results; and (**c**) last frame with comparison and Ratsnake results with image. Figure 13: Box dimension evolutions of cracks in coal and rock specimens. (**a**) Different saturation levels and impact speeds for coal samples; (**b**) different saturation levels and impact speeds for rock samples; (**c**) different temperatures for coal samples; and (**d**) different temperatures for rock samples. Figure 13a shows the distribution of box dimensions for different saturation and impact velocities, all at a temperature of \(-10\)\({}^{\circ}\)C. The following can be observed from this figure: (1) For the individual curves, an overall trend from a rapid increase in the initial phase to a slow increase and then to localised oscillations in the later phase is shown. The increase is due to the entire process being characterised by an increase in stresses on the coal or rock body over time and a continuous increase in damage. The oscillations can be caused by the transient closure of the internal fissures due to the compression of the internal fissures after the impact on the coal or rock body. (2) For the coal simples at an impact velocity speed of 6 m/s, the dimension of the dried sample is higher than that in the saturated state, with a faster end time. This indicates that the number of cracks and the damage degree of the sample in the dry state are greater than those in the saturated state. (3) Under the same saturation conditions, comparing the crack evolution characteristics under different impact velocities, with an increase in the impact velocity, the value of the box dimension increases, but the duration is shortened. This shows that the change in velocity can cause a large change in surface cracks and presents a positive correlation. A faster impact velocity correlates to a faster damage process, a faster evolution of the cracks, and an earlier ejection of the number of coal and rock breakers. Figure 13b shows the evolution pattern of the box dimension of the surface cracks of the rock samples under different saturation and velocity conditions at \(0\)\({}^{\circ}\)C. Figure 13b exhibits a similar pattern to Figure 13a: it is shown that, under different speed and saturation conditions, the box dimension shows the typical characteristics of fast in the early stage, slow in the late stage, and an oscillating increase. In addition, the box dimension as a whole increases with the decrease in saturation and increases with the increase in speed. Notably, on the one hand, the overall holding time of the rock is shorter than that of the coal sample; on the other hand, the final value of the box dimension is smaller than that of the original coal. This reflects several factors: (1) By the nature of the material, the rock is somewhat more brittle, more resistant to deformation, and less destructive at the time of destruction. (2) Rocky materials have a high rebound energy. For raw coal, the vast majority of the energy from the incident rod is absorbed by the sample and used to increase its surface energy.