Mechanism of Fracture Process Zone Development in Sandstone and Limestone Under Mode I Loading Wet-Dry Cycles and Temperature

This study investigates the influence of wet-dry cycling and temperature loading on the microcracking behavior and fracture process zone (FPZ) evolution of sandstone and limestone under Mode I loading, using notched semi-circular bending tests in combination with acoustic emission (AE), digital image correlation (DIC), and thin-section microscopy. Through frequency-domain analysis of AE data, four distinct stages in the development of the FPZ were identified: micro-crack formation, crack nucleation, macro-crack initiation, and propagation. The results show that within the low-temperature range (25-100 degrees C), the fracture toughness of the rocks increases with temperature; however, in the high-temperature range (300-500 degrees C), the fracture toughness significantly decreases, with a marked acceleration in the reduction after reaching the critical temperature. High-temperature treatments also led to a significant increase in AE activity and energy release, particularly at 300 degrees C and 500 degrees C, where the frequency of microcracks and energy release were notably higher. Microscopic analysis revealed a transition in fracture mode from tensile to shear failure, with a significant reduction in FPZ length as the temperature increased. The shortening of FPZ was more pronounced in limestone than in sandstone. At 500 degrees C, the FPZ lengths in sandstone and limestone were reduced by 39.26% and 49.2%, respectively. Furthermore, AE proved superior to DIC in capturing the geometric changes of the FPZ, particularly during the micro-crack formation stage. This study provides valuable experimental data and theoretical insights for rock mechanics and underground engineering, especially in the context of high-temperature environments combined with wet-dry cycling effects.HighlightsEffects of wet-dry cycling and temperature (25-500 degrees C) on FPZ evolution in sandstone and limestone.Frequency-domain analysis of acoustic emission identifies four FPZ development stages.Elevated temperatures enhance acoustic emission activity and energy release.