## 3 Results and Discussion
### Attenuation of Coal Strength under Discontinuous Cyclic Loading  
The final uniaxial compression test was performed on all damaged samples that were subjected to a force loading, and the stress-strain curve is shown in Figure 9. The UCS of all curves is extracted to Table 4. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{0 min} & \multicolumn{2}{c}{A-1} & \multicolumn{2}{c}{A-2} & \multicolumn{2}{c}{A-3} \\ \cline{2-7}  & Before & After & Before & After & Before & After \\ \hline Micropores & 85.31 & 77.6 & 80.29 & 75.06 & 79.43 & 75.84 \\ Mesopores & 14.32 & 18.25 & 19.05 & 20.44 & 19.22 & 18.47 \\ Macropores & 0.37 & 4.15 & 0.66 & 4.5 & 1.34 & 5.69 \\ \hline \multirow{2}{*}{5 min} & \multicolumn{2}{c}{B-1} & \multicolumn{2}{c}{B-2} & \multicolumn{2}{c}{B-3} \\ \cline{2-7}  & Before & After & Before & After & Before & After \\ \hline Micropores & 83.20 & 75.66 & 82.32 & 75.25 & 85.66 & 76.37 \\ Mesopores & 15.66 & 18.66 & 17.29 & 19.74 & 13.7 & 18.94 \\ Macropores & 1.14 & 5.68 & 0.4 & 5 & 0.63 & 4.69 \\ \hline \multirow{2}{*}{10 min} & \multicolumn{2}{c}{C-1} & \multicolumn{2}{c}{C-2} & \multicolumn{2}{c}{C-3} \\ \cline{2-7}  & Before & After & Before & After & Before & After \\ \hline Micropores & 86.05 & 74.11 & 84.69 & 72.52 & 83.41 & 72.51 \\ Mesopores & 13.58 & 20.88 & 14.23 & 21.85 & 15.72 & 22.12 \\ Macropores & 0.37 & 5.01 & 1.08 & 5.63 & 0.87 & 5.36 \\ \hline \multirow{2}{*}{15 min} & \multicolumn{2}{c}{D-1} & \multicolumn{2}{c}{D-2} & \multicolumn{2}{c}{D-3} \\ \cline{2-7}  & Before & After & Before & After & Before & After \\ \hline Micropores & 82.46 & 68.9 & 85.24 & 69.46 & 82.43 & 70.11 \\ Mesopores & 16.36 & 22.86 & 14.02 & 24.67 & 16.48 & 25.15 \\ Macropores & 1.18 & 8.24 & 0.74 & 5.87 & 1.09 & 4.74 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Proportional variations of pore sizes in core (%). Figure 8: Proportion change characteristics of pores of different sizes: (**a**) proportion of micropores; (**b**) proportion of mesopores; (**c**) proportion of macropores (cracks); (**d**) changes in the gap of the proportion of pores before and after coal failure. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Holding Time (min)** & **Sample** & **UCS (MPa)** & **Average UCS (MPa)** \\ \hline \multirow{3}{*}{0 min} & A-1 & 17.6 & \multirow{3}{*}{18.7} \\  & A-2 & 19 & \\  & A-3 & 19.5 & \\ \hline \multirow{3}{*}{5 min} & B-1 & 13.6 & \multirow{3}{*}{14.2} \\  & B-2 & 13.8 & \\  & B-3 & 15.2 & \\ \hline \multirow{3}{*}{10 min} & C-1 & 12.5 & \multirow{3}{*}{11.9} \\  & C-2 & 11.2 & \\ \cline{1-1}  & C-3 & 11.9 & \\ \hline \multirow{3}{*}{15 min} & D-1 & 7.69 & \multirow{3}{*}{8.79} \\ \cline{1-1}  & D-2 & 10.3 & \\ \cline{1-1}  & D-3 & 8.38 & \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Uniaxial compressive strength (UCS) of coal samples under different holding times. After the sample underwent discontinuous cyclic loading, the load-bearing capacity of the sample decreased significantly, and the UCS decreased with an increase in holding time. When the holding time was 0 min, the average UCS was 18.7 MPa, and the strength attenuation rate was 18.7%. When the holding time was 15 min, the average UCS was 8.79 MPa, and the strength attenuation rate was 61.8%. The increase in holding time increased the total pore volume in the rock mass, especially the increase in volume of macropores (cracks) and mesopores, which aggravated internal damage of the rock mass and gradually reduced the UCS. These results indicate that the destructive effect of stress on coal was reflected in the size and duration of the force action. The rock mass produced a mechanical behavior that was similar to metal material creep. It is embodied in the initiation-expansion-assembly of cracks of various sizes inside the rock mass. The number and size of microcracks accumulated to a certain extent and large cracks formed. The accumulation of large cracks eventually caused macroscopic cracks in the rock mass and macroscopic damage to the coal mass.