# Tensile Properties of Granite Under Cyclic Thermal Shock and Loading
## 4 Failure Characteristics Analysis
### Microscopic Damage Mechanism Analysis  
Figure 15 presents scanning electron microscope (SEM) images of granite specimens subjected to 400 \({}^{\circ}\)C cyclic water-cooling shocks, tested under both conventional Brazilian splitting and cyclic loading-unloading Brazilian splitting conditions. In the natural state (zero water-cooling shocks, Figure 15a), the microstructure of granite is relatively intact, and the fracture surfaces are fairly neat, with cracks predominantly following the intergranular paths. After undergoing cyclic loading-unloading (Figure 15b), the fracture surface of granite becomes progressively rougher, although its overall microstructure remains relatively sound; correspondingly, the tensile strength remains high and the increase in peak displacement is limited under cyclic loading-unloading. With the increase in the number of water-cooling shocks, after one and three shocks (Figure 15c,e), due to the evaporation of pore water and the effects of water-cooling shocks, the internal structure of granite becomes relatively loose. The microcrack morphology still is mainly intergranular, accompanied by a small number of intragranular cracks. The damage in the crystals primarily occurs in the peripheral regions of the mineral grains, which is attributed to the non-uniform thermal expansion of mineral particles at high temperatures that causes stress concentrations at the edges of intergranular cracks, leading to failure in those regions [47]. Following cyclic loading-unloading, the number of microcracks within the granite increases significantly (Figure 15b,d), with micro-fracture zones appearing at the grain boundaries, and the microcrack patterns becoming complex and tortuous. This is because cyclic loading-unloading promotes the gradual propagation and extension of microcracks until they eventually  
Figure 14: Macroscopic failure morphology of granite.  
coalesce. At this stage, the increased crack density leads to a reduction in tensile strength and an increase in the increment of peak displacement. When the number of water-cooling shocks reaches five and seven (Figure 15g,i), the effect of thermal damage on the granite becomes markedly more significant, with the intergranular gaps enlarging once again. Under the electron microscope, the formation of micropores can be clearly observed. Moreover, compared with the conditions of fewer water-cooling shocks, the microcracks in granite are predominantly transgranular, with both their lengths and widths increasing significantly [44]. The cyclic loading-unloading process further promotes the propagation of cracks, and the intersection of cracks forms a complex network of fissures. This leads to the detachment of relatively large debris from the fracture surface, rendering the micro-fracture surface of the granite much rougher. At the same time, in the cyclic loading-unloading Brazilian splitting tests, the density of the hysteresis loops in the load-displacement curves of granite is significantly reduced, while the peak displacement required to induce failure increases considerably.  
Figure 15: SEM image of granite specimen after 400 \({}^{\circ}\)C cyclic water-cooling shock. (**a**) conventional test and 0 cycles; (**b**) 80% load upper limit and 0 cycles; (**c**) conventional test and 1 cycle; (**d**) 80% load upper limit and 1 cycle; (**e**) conventional test and 3 cycles; (**f**) 80% load upper limit and 3 cycles; (**g**) conventional test and 5 cycles; (**h**) 80% load upper limit and 5 cycles; (**i**) conventional test and 7 cycles; (**j**) 80% load upper limit and 7 cycles.