Mesoscopic fracture evolution of granite under different thermal disturbances

• The thermal fracture threshold for granite is identified at 300 °C, above which crack coalescence and propagation accelerate. • Thermal shock cooling (water) induces more extensive fracturing than natural air cooling, significantly altering mineral area changes. • Heating contributes more significantly to overall granite damage than the subsequent cooling stage. • Real-time high-temperature hot-stage microscopy enables continuous observation of mesoscopic fracture evolution, providing a theoretical basis for rock stability assessment in high-temperature engineering.