Study on the damage evolution characteristics of prefabricated double-fracture sandstone based on acoustic-optical combined tests

To investigate the influence of rock bridge length and water content on the mechanical properties and damage evolution patterns of sandstone, uniaxial compression tests were conducted on prefabricated double-fracture sandstone samples with varying rock bridge lengths under both dry and saturated conditions. Acoustic emission(AE) and digital image correlation(DIC) techniques were used to monitor the internal damage and surface crack propagation of the sandstone in real-time. The results indicate that the strength of the fractured samples is lower than that of the intact samples, and the longer the rock bridge length, the greater the uniaxial compressive strength of the samples. Saturated samples exhibited lower strength compared to dry samples. Fractured samples will show stress drops during the failure process, which corresponded to peaks in AE counts. As the rock bridge length increased, the first stress drop gradually approached the stress peak. The AE count patterns of the saturated samples were consistent with those of the dry samples. The AE localization events and DIC strain anomaly regions in both saturated and dry samples initially concentrated at the rock bridge area and then gradually developed along the prefabricated fractures towards the sample boundaries. These findings contribute to the monitoring and early warning of rock instability and failure in water-rich conditions.