Thermo-induced damage evolution and multi-mode fracture toughness degradation in sandstone: Integrated microstructural quantification and mesoscopic modeling

Abstract

Thermo-mechanical degradation in brittle geomaterials is governed by progressive microstructural damage that fundamentally alters fracture resistance. This study presents an integrated experimental–numerical investigation of temperature-dependent damage evolution and multi-mode fracture toughness degradation in quartz-rich sandstone. Specimens were thermally conditioned up to 700 °C and subsequently subjected to uniaxial compression, Brazilian tensile, and semi-circular bending (SCB) tests under Mode I, Mode II, and mixed-mode loading. Microstructural alterations were quantified using high-resolution scanning electron microscopy, enabling direct measurement of microcrack density as an internal damage descriptor. Results reveal a distinct critical thermal threshold within the 400 to 500 °C interval, beyond which microcrack density increases nonlinearly and fracture toughness degrades rapidly across all loading modes. A regression-based damage–fracture coupling model is proposed to relate normalized fracture toughness to the experimentally quantified damage variable. The model exhibits a high coefficient of determination and low prediction error, demonstrating that microcrack density governs fracture resistance degradation in a physically consistent manner. A grain-based particle flow model calibrated against intact mechanical properties was subsequently implemented with damage-informed bond strength reduction. The simulations reproduce crack initiation, propagation patterns, and temperature-dependent toughness decline with strong quantitative agreement. The proposed framework establishes a mechanistically grounded linkage between mineral-scale thermo-induced damage and macroscopic fracture behavior, providing a predictive basis for fracture-controlled stability assessment in thermally disturbed rock masses.