Influence of temperature on statistical characteristics of thermal cracks in granite rock: A macroscopic-scale visualization study

• Developed advanced fluorescence imaging and segmentation methods for thermal crack analysis. • Demonstrated a “coalescence-rupture” cyclic mechanism in thermal crack evolution. • Identified log-normal crack length distributions and limited temperature-dependent orientation shifts. • Enhanced micromechanical numerical modeling with accurate statistical and geometry parameters. Thermal cracks significantly affect the physical and mechanical properties of granite, with implications for underground engineering and energy exploitation. This study combines macroscopic fluorescence visualization with image processing, innovatively integrating Otsu and Bradley threshold segmentation to achieve large-scale, accurate extraction and parameterization of cracks. Using the extensive database of thermal crack parameters obtained, we analyzed the statistical characteristics of thermal cracks at different temperatures (25–500 ℃). The results reveal that thermal cracks undergo a “coalescence-rupture” cycle as temperature increases, reflecting the accumulation and release dynamics of thermal stress. Crack length distributions exhibit log-normal behavior, highlighting nonlinear evolution and the emergence of long-tail features at intermediate temperatures. Crack orientation shows limited temperature dependence and is mainly controlled by the intrinsic fabric and mineralogy of the granite. The orientation distributions are broadly consistent across temperatures with only slight peak fluctuations, and are well described by statistically robust multimodal normal fits. These findings provide valuable insights into the thermal behavior of granite and offer robust statistical parameters for numerical modeling and engineering applications. These results provide statistically parameterized inputs that help bridge laboratory observations and engineering-scale thermo-mechanical assessments, supporting safer and more efficient design and operation of high-temperature underground energy systems and subsurface infrastructure.