Study on the energy evolution mechanism of fissured granite under different intermediate principal stress conditions

In order to investigate the influence of the intermediate principal stress on the energy evolution of fissured granite under true triaxial stress, true triaxial mechanical tests are conducted on fissured granite under various intermediate principal stress （σ₂） conditions. Based on the test results, the mechanical response characteristics of fissured granite are analyzed, and the strain energy evolution mechanism is discussed according to the principle of energy dissipation. The results reveal that with the increase of σ₂, the crack initiation stress （σ_ci）, damage stress （σ_cd）, peak stress （σ_p） of the rock sample first increase and then decrease, while the crack initiation strain （ε_ci）, damage strain （ε_cd）, peak strain （ε_p） all decrease, indicating a distinct brittle failure characteristic of the rock sample. Using the three characteristic stress values （σ_ci, σ_cd, and σ_p） of the rock samples, the energy evolution curves of granite under different intermediate principal stress conditions are divided into four stages. The variation pattern of strain energy in different directions can be used to explain the failure behavior characteristics of the rock samples. When σ₂ ≤ 20 MPa, the dilatancy （k） value of the rock samples is relatively small, mostly less than 0, indicating that the rock samples tend to expand more along σ₂ direction during crack damage, thus a large number of cracks appear at the pre-existing fissure. When σ₂ > 20 MPa, the dilatancy （k） value of the rock samples is relatively large and mostly less than 0, indicating that the rock samples tend to expand more along σ₃ direction during crack damage, thus fewer cracks occur at the pre-existing fissure. The findings can deepen the understanding of the energy evolution mechanism of fissured rock mass under different intermediate principal stress conditions and have significant implications for related rock engineering with defects.