## 5 Discussion  
### Fracture Analysis  
The fracture morphology in rock failure is related to the rock type and the conditions the rock is under (e.g., temperature, humidity, loading rate, and confining pressure). The fracture morphology records the irreversible shape of the rock fracture and provides information about crack initiation, propagation, penetration, and failure. Therefore, the properties of rock failure can be inferred by examining and analyzing the fracture morphology. The results of this analysis can then be fed back to engineering as a valuable reference for practical production. Scanning electron microscopy (SEM) can reveal the shape of the fracture clearly and is commonly applied for research into rock fractures [4; 27]. After rock failure had occurred in the present study, the fracture was examined by SEM; the photographs obtained are shown in Figure 9.  
The SEM image of the sample exposed only to room temperature (20 \({}^{\circ}\)C) shows a relatively flat fracture surface with almost no discrete cracks (Figure 9a). As samples are exposed to increasingly high temperatures, the Brazilian splitting and SCB fracture surfaces gradually become rougher, as shown in Figure 9b-f. A river-like pattern can be observed clearly in both BD and SCB samples, as shown in Figure 9b,c. The river-like pattern originates from the grain boundaries, and as cracks develop, it spreads throughout the mineral particles. As the exposure temperature increases, the range of the pattern is larger and the surface is rougher. A tear-like fracture pattern is caused by temperature and load, and the crack changes direction and tears in a relatively weak layer. The tear-like fracture pattern was observed in both the BD and SCB samples, as shown in Figure 9e,f. River-like and tear-like fractures are mainly formed by tensile failure. Where the main crack surface is perpendicular, breakage occurs due to uncoordinated deformation of weak mineral particles or cement. The fractured areas become interconnected and penetrate to form a fractured structure, as shown in Figure 9d. The action of high temperature causes significant uncoordinated deformation between the mineral particles in the rock and between the particles and the cement. Large quantities of cracks are generated, and the structure seriously deteriorates. The failure of rocks is a process in which the various sub-parts of the rock break through and form large macroscopic cracks.  
It can be seen from the above analysis that the fracture morphology in the \(K_{IC}\) and \(\sigma_{t}\) tests is very similar, and the failure mechanism is the same. Temperature has a significant effect on the morphology of the rock fracture. This is because, as the temperature increases, the thermal motion of mineral particles, crystals, and atoms in the sandstone gradually increases. The number of weak crystal faces of  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \(\mathbf{T}\) (\({}^{\circ}\)C) & **FS Marble** & **FS Gabbro** & **SP Granite** & **BS Granite** & **SC Sandstone** \\ \hline
25 & 0.00299 & 0.00382 & 0.00310 & 0.00181 & 0.00292 \\
100 & 0.00299 & 0.00343 & 0.00270 & - & 0.00306 \\
200 & 0.00094 & 0.00336 & 0.00262 & 0.00150 & 0.00303 \\
300 & 0.00069 & 0.00314 & 0.00140 & 0.00183 & 0.00262 \\
400 & 0.00030 & 0.00330 & - & 0.00069 & 0.00234 \\
500 & 0.00021 & 0.00266 & - & 0.00043 & 0.00238 \\
600 & - & 0.00177 & 0.00131 & 0.00012 & 0.00251 \\ \hline \hline \end{tabular}
\end{table}
Table 4: The \(r_{IC}\) value (m) for different processing temperatures.  
the minerals inside the rock increases, and the potential for local plastic deformation increases. These factors result in the formation of river-like and tear-like fracture patterns. Moreover, as the temperature increases, the internal structure of the rock gradually deteriorates, and the numbers of pores and cracks gradually increase. This is an important reason for the decline in the mechanical properties of the rock.