Competitive crack evolution in flawed rocks under thermal–mechanical coupling: A tangential stress and displacement decomposition approach

: Investigating failure mechanisms of heat-damaged, flawed rock under external loading is crucial for underground rock engineering safety. To study crack evolution patterns and mechanisms under flaw inclination and thermal damage effects, cement-mortar-like rock materials were examined. Based on the tangential stress theory surrounding flaws, this study examines stress differences on the surfaces of double-flaw and the effects of thermal damage. The flaw interaction coefficient K 1 ( α ) and thermal damage interaction coefficient K 2 ( β ) were defined. A model for the tangential stress distribution on the internal surface ( σ I ) and external surface ( σ E ) of a double-flawed rock under thermal damage was proposed to predict crack initiation zones. To assess the model's applicability, comparison experiments were conducted using natural samples of heterogeneous granite and yellow sandstone. To overcome the limitations in identifying surface cracks, a relative displacement ratio coefficient γ and a crack pattern decomposition method were proposed to characterize crack propagation and classify patterns in defect-containing specimens. The results show that: (1) By increasing the flaw angle, wing cracks go from tensile-dominated hybrid mode I to tensile mode, whereas anti-wing cracks stay in shear-dominated hybrid mode III. High temperatures further reduce initiation stress. (2) In medium-high inclination specimens, coplanar secondary cracks dominate the upper areas, while wing cracks cause lower failure in low-medium inclination specimens. Failure paths get more complicated at high temperatures. (3) Peak strength and elastic modulus varied with flaw angle, and post-peak behavior became ductile. (4) Between 25 °C and 250 °C, the specimen exhibits increased hardness and enhanced brittleness. As the temperature rises to 1000 °C, the strength progressively diminishes, fracture connectivity improves, resulting in an extensive microcrack network and culminating in a ductile failure mode. The results clarify the mechanisms of heat degradation in flawed rock at the laboratory specimen scale.