Experimental investigation on crack propagation and fracture process zone evolution in flame-treated granite

To investigate the mechanisms of crack propagation in thermally treated granite, rapid thermal disturbance was simulated using flame heating at 25–600 °C, followed by three-point bending tests. Crack propagation paths and crack tip responses were dynamically analyzed using digital image correlation (DIC). The results demonstrate that thermal treatment markedly weakens the structural integrity of granite: Fracture toughness decreases by 64.5 %(from 49.17 MPa·mm^0.5 To 17.46 MPa·mm^0.5), crack tip opening displacement increases to 33.5 μm, and the fracture process zone (FPZ) extends to 17.33 mm. Crack propagation mode evolves from brittle failure at ambient temperature to stable and asymmetric growth at elevated temperatures. Scanning electron microscopy (SEM) reveals that microcracks initiate along grain boundaries and progressively extend into grains, forming a connected damage network. With increasing temperature, stress concentrations induced by heterogeneous mineral thermal expansion accumulate, and near 573 °C, the α–β phase transition of quartz causes abrupt unit cell volume change and local stress redistribution, serving as a critical driver of high-temperature structural degradation