In Situ Microscale Shear Failure Mechanism in Hot Dry Rock Under Thermal Cycling

Abstract Induced shear failure is key for enhancing hot dry rock (HDR) resource exploitation, aiming to creating an extensive fracture network. Although the macroscopic shear behavior of HDR has been extensively investigated, its microscopic shear failure mechanisms remain unclear. In this study, microscale mode II shear fracture experiments were performed for the first time under various thermal cycling. Micro‐scale double‐edge notched cube specimens were fabricated from primary minerals and interfaces using micro‐machining and subjected to in situ shear tests under a scanning electron microscope. The microscopic shear failure mechanisms and fracture parameters of the minerals and interfaces were investigated. The results showed that multiple thermal cycles reduced macroscopic shear strength by initiating thermal cracks rather than by reducing the strength of the microscale minerals. All three minerals exhibited L‐shaped crack propagation below 300°C, with shear initiation and tensile failure. Biotite failed progressively, whereas feldspar and quartz failed catastrophically. High temperatures and multiple thermal cycles caused thermal voids and complex crack paths in feldspar and quartz, whereas biotite exhibited fiber‐like plastic slip bands without thermal cracking. The orientation and strength of the mineral interfaces affected crack deflection and branching. Energy was dissipated by interlayer plastic slip in biotite and by micro‐crack friction and slip in quartz and feldspar. The microscopic mode II fracture toughness and critical energy release rate were 0.5–3.3 MPa·m 0.5 and 0.01–0.11 kJ/m 2 , respectively. This research provides novel insights into the microscale shear failure of HDR and a critical micromechanical basis for multiscale fracture modeling and macroscopic shear failure analysis.