## 4 Discussion  
The average values of strength with respect to the specimen geometry are presented in Figure 15. The strength was measured at a loading rate of 1 MPa/s. The average tensile strength, in-plane shear strength, and out-of-plane shear strength were 7.55, 22.75, and 13.6 MPa, respectively. The results indicate that the maximum strength was attained in the in-plane shear test. The tensile strength was shown to be lower than the other strengths. Furthermore, the out-of-plane shear strength was approximately 60% lower than the in-plane shear strength, whereas the values of the in-plane shear strength and cohesion (22.7 MPa) were very close. Cohesion is defined as the shear strength of the Mohr-Coulomb failure criteria without a confining stress. Fracture formation in rock structures typically involves in-plane shear, out-of-plane shear, and a mixed mode loading system. Although it is possible for a rock mass to undergo simple sliding loads due to the complex geometry and random crack orientation within the rock structure, crack growth typically takes place  
Figure 14: Out-of-plane shear strength versus loading rate: (**a**) torsion test of rectangular section specimens; (**b**) circumferentially notched cylindrical specimen test. Figure 13: Comparison of the out-of-plane shear strength between the torsion test of rectangular section specimens and the circumferentially notched cylindrical specimen test. in the presence of mixed-mode loading [27]. X-ray tomography can help to understand the crack formation under stress [28]. For instance, Figure 16 illustrates a penny-shaped crack placed under shear loads in an infinite medium [29]. In-plane shear loads are placed on the edge of the x-axis of the penny-shaped (point A), whereas the edge of the y-axis of the penny-shaped crack (point B) is subjected to out-of-plane shear loads. Figure 17 presents the variation in stress corrosion index for the different tests performed in this study. According to the experimental results, the stress corrosion indices (\(n\)) were highly similar irrespective of the loading configuration and the specimen geometry. It was not affected by the loading configuration, including tension, in-plane shearing, or out-of-plane shearing. The fact that subcritical crack growth, causing time-dependent behaviour of rocks, is environmentally induced rather than mechanically induced explains these results. The stress corrosion index (\(n\)) is affected by the chemical process with the strained silicate material and pore water at the crack tip in rocks. The chemical reaction rates are not significantly affected by the loading configuration. Additionally, under in-plane or out-of-plane shear loading, the prevailing cracking process at the crack tip is tensional; thus, the microstructural interactions at the crack tip are similar irrespective of the far-field loading configuration. Examples of the formation of shear cracks due to the interaction and coalescence of micro-tensile cracks are found in the relevant literature. [PERSON] et al. reported that the elastic stress around tensile microcracks promoted a mutual interaction that produced shear fractures in brittle rock [30]. Table 2 presents the stress corrosion indices (\(n\)) for different rock types, obtained from previous works [31]. The stress corrosion indices are similar irrespective of the loading configuration, but they vary with respect to the rock type. Figure 16: Penny-shaped crack in an infinite medium under shear stresses. Figure 15: Variation in strength for the different tests. Combining the results of this study with those of existing literature reveals that the stress corrosion indices (\(n\)) for rocks are similar irrespective of the loading configuration, but they are different for different rock types. Therefore, the stress corrosion index can be considered a material constant for rocks. Stress corrosion index can be used to evaluate the time-dependent behaviours of rocks, such as the degradation of rock strength and lifetime prediction in rock structures.