Theoretical and Experimental Studies on the Dynamic Fracture Behavior of Sandstone With Surface Cracks After High‐Temperature Exposure

The dynamic fracture of three‐dimensional (3D) cracks in deep rock masses under high‐temperature exposure is critical for underground engineering safety under impact loading, yet the underlying mechanisms remain unclear. In this study, red sandstone specimens containing 3D surface cracks were investigated using a split Hopkinson pressure bar (SHPB) and ultra‐high‐speed imaging after high‐temperature exposure. Microscopic evolution, analyzed via X‐ray diffraction (XRD) and scanning electron microscopy (SEM), was correlated with macroscopic fracture responses to clarify the effects of temperature, loading rate, and crack depth. A novel three‐dimensional modified maximum principal stress (3D‐MMPS) criterion was established by incorporating thermal damage and incubation time effects. Results show that high‐temperature exposure promotes tortuous, energy‐dissipating crack paths along thermally induced micro‐cracks while attenuating the rate‐strengthening effect. Increased crack depth shifts crack initiation toward the free surface and raises the surface initiation angle. The proposed 3D‐MMPS model accurately predicts these crack‐front transitions.