# Tensile Properties of Granite Under Cyclic Thermal Shock and Loading
## 5 Discussion  
Figure 16 illustrates the schematic diagram of the microscopic failure mechanism of granite subjected to cyclic water-cooling shock at 400 \({}^{\circ}\)C combined with cyclic loading and unloading. Under natural conditions, the mineral grains within granite remain relatively intact, containing only a small number of pre-existing microcracks (Figure 16a). The cyclic thermal shock at 400 \({}^{\circ}\)C significantly alters the physical properties of the granite. The evaporation of internal pore water and the intense thermal shock induce heterogeneous expansion among mineral grains, leading to the widening of intergranular cracks and the initiation of numerous intragranular microcracks (Figure 16c). After five cycles of water-cooling shock (Figure 15g), newly formed microcracks not only emerge within the crystals but also begin to interconnect, evolving into transgranular cracks. This process increases the porosity of granite [48]. As the number of thermal shock cycles increases, the number and width of cracks gradually reach a stable state [11]. Consequently, granite undergoes degradation in quality, expansion in volume, reduction in density, and a decrease in P-wave velocity, with the degradation rate gradually tapering off as the number of cycles increases. The corresponding mechanical properties also deteriorate. In conventional Brazilian splitting tests, the tensile strength of granite decreases progressively with more thermal shock cycles. Compared with the natural state, the tensile strength of granite after 1, 3, 5, and 7 cycles of water-cooling shock decreases by 41%, 51%, 58%, and 62%, respectively, with the rate of reduction gradually diminishing. These findings confirm a direct correlation between the number of microcracks and the degradation of tensile strength in rock. In addition, previous studies have proposed automated methods for measuring the water absorption of materials [49; 50], which can be used to quantify microcrack density in granite and further explore the relationship between microcrack density and the deterioration of mechanical properties. Under natural conditions, when granite is subjected to cyclic loading and unloading, as shown in Figure 16b, the original pores between mineral crystals are compacted, generating interaction forces between the grains. The applied load induces new microcracks in the contact zones between grains, and the pre-existing cracks within the crystals gradually propagate and extend under stress. However, the mineral grains remain largely intact, and the resultant failure predominantly occurs along grain boundaries where the particles are cemented (Figure 15b). Throughout the cyclic loading process, the load-displacement curves of granite under various preset upper load limits are densely clustered, and no significant variation in elastic stiffness is observed. Due to the strong cementation between mineral grains, granite specimens subjected to 65% and 70% of the preset peak load did not experience failure. This suggests that, under natural conditions, the tensile fatigue failure threshold of granite exceeds 70% of its peak strength, which aligns with the findings of [PERSON] and [PERSON] regarding tensile failure thresholds in rock [21; 31]. When granite is subjected to both cyclic water-cooling shock and cyclic loading, substantial thermal damage is already present prior to mechanical loading. Cyclic loading induces a compaction effect on the granite, enhancing its elastic stiffness (Figure 13), and this compaction effect becomes more pronounced with higher upper load limits [24], resulting in more significant increases in elastic stiffness under cyclic loading. A competitive mechanism exists between thermal damage induced by cyclic thermal shocks and the compaction effect during the early stages of cyclic loading [51; 52]. Before the fifth cycle of water-cooling shock, damage accumulation dominates.