## 4 Discussion  
The failure process of brittle rock (such as granite) is characterized by several distinct deformation stages, which include crack initiation, crack propagation, and coalescence [39]. The microcrack evolution characteristics before the peak stress point (Figure 18) reveals three progressive stages: During the OA phase (prior to \(\sigma_{\mathrm{ci}}\)), rock deformation primarily manifests as recoverable elastic behavior dominated by the closure of pre-existing pores and fractures, with minimal micro-fracturing activity observed. As stress transitions into the AB phase (stable crack growth stage between \(\sigma_{\mathrm{ci}}\) and \(\sigma_{\mathrm{cd}}\)), newly generated microcracks preferentially propagate along the boundaries of inherent defects, exhibiting stochastic spatial distribution patterns that demonstrate a negligible influence on peak strength. Upon exceeding \(\sigma_{\mathrm{cd}}\) (BC phase, the unstable crack propagation stage), the microcrack network achieves self-organized criticality, where branching-coalescence mechanisms govern the formation of dominant failure pathways, ultimately leading to a through-going macroscopic fracture surface at peak stress. By comparing the evolution characteristics of microcracks (Figure 18) with strain rate effects on characteristic stresses (Figure 10b) during rock de formation, it is found that a significant correlation exists between them. This correlation suggests that the differences in strain rate effects on characteristic stresses can be explained through microcracking evolution. Under unconfined conditions (0 MPa), the CSDIF of \(\sigma_{\rm ci}\) is lower than that of \(\sigma_{\rm cd}\) or \(\sigma_{\rm p}\). This discrepancy arises because \(\sigma_{\rm ci}\) marks the terminal point of elastic deformation in brittle rocks, during which micro-fracturing processes are limited and material behavior remains quasi-homogeneous. Conversely, a substantial quantity of microcracks is generated during the AC phase (Figure 18), which progressively interconnect and coalesce into continuous fracture networks. The intensity of microcrack activity within the rock demonstrates a positive correlation with the associated strain rate effect [40]. Notably, the deformation phases corresponding to \(\sigma_{\rm cd}\) or \(\sigma_{\rm p}\) exhibit significantly heightened microcrack activity levels compared to those associated with \(\sigma_{\rm ci}\). Consequently, the CSDIF of \(\sigma_{\rm ci}\) remains less pronounced than those of \(\sigma_{\rm cd}\) or \(\sigma_{\rm p}\). The strain rate effects on \(\sigma_{\rm cd}\) and \(\sigma_{\rm p}\) exhibit systematic attenuation with increasing confining pressure. The underlying physics involves confining pressure suppressing lateral dilatancy, thereby reducing tensile microcrack proportion while enhancing shear-dominated micro-fracturing [41]. Given the lower strain rate sensitivity of fracture toughness in shear failure compared to tensile failure [42; 43], this transition mechanism establishes a pronounced negative correlation between confining pressure and strain rate effects on \(\sigma_{\rm cd}\) and \(\sigma_{\rm p}\). The disparity in the strain rate sensitivity of fracture toughness between tensile and shear failure modes constitutes an important mechanism governing the differential dynamic responses of rock strength parameters. This mechanism is corroborated by the distinct strain rate effects observed in rock tensile versus compressive strengths. Figure 19 presents comparative data on the dynamic strength factors of granite under tensile and compressive loading. The dynamic increase factor (DIF) is defined as the ratio of the strength at a given strain rate to the reference value at a strain rate of \(10^{-5}\) s\({}^{-1}\). The DIF of tensile strength reaches 1.61 at the strain rate of \(10^{-1}\) s\({}^{-1}\). Both semi-circular bend tests and Brazilian disc tests demonstrate dynamic tensile strength factors exceeding 1.3.