Mode I fracture analysis of granite under rapid thermal disturbances using digital image correlation and 3D laser scanning

With the advancement of clean energy technologies, granite, a common host rock in deep engineering, faces increasing challenges from rapid thermal disturbances. This study conducted three-point bending tests on semi-circular bend (SCB) granite specimens subjected to rapid heating and cooling treatments (25 °C–15 °C, 200 °C–15 °C, 400 °C–15 °C, and 600 °C–15 °C). Integrating digital image correlation (DIC), acoustic emission (AE), and three-dimensional (3D) laser scanning enabled a systematic investigation of multi-scale responses covering two-dimensional (2D) fracture processes and 3D fracture morphology. Additionally, the fracture process zone (FPZ) and fracture surface features were analyzed using DIC displacement gradient, box-counting, least squares, and gray-level co-occurrence matrix (GLCM) methods. Results show that increasing thermal disturbances significantly degrade the mode I fracture toughness, with a reduction of up to 88.5% at 600 °C. This degradation is accompanied by markedly intensified AE activities and the expansion of both crack mouth opening displacement (CMOD) and crack tip opening displacement (CTOD). Thermal damage facilitates crack initiation and propagation at lower load levels and significantly alters the size and shape of the FPZ. Furthermore, fractal dimension, roughness, and textural statistics exhibit pronounced temperature dependence. Cross-scale analysis reveals that the dispersed microcrack network induced by thermal shock drives performance degradation, while quartz phase transition and feldspar softening dominate brittle damage and plastic dissipation, the degradation path of fracture toughness, which integrates thermal stress and damage evolution. Finally, quantitative correlations between fracture parameters and 3D morphological features confirm the potential of fracture surface morphology for reconstructing energy dissipation mechanisms. These findings provide a critical theoretical basis and risk-assessment benchmarks for deep rock engineering, enhancing predictive capability for structural instability under extreme thermal conditions.