Fracture Characteristics and Precursor Signals of Flawed Sandstone Based on Acoustic Emission and Digital Image Correlation Techniques

To reveal the deformation and fracture characteristics and mechanisms of flawed rock masses, this study conducted uniaxial compression tests on sandstone specimens containing prefabricated flaws with four inclination angles (30°, 45°, 60°, and 75°). The failure process of the specimens was monitored throughout using acoustic emission (AE) and digital image correlation (DIC) techniques. AE was employed to detect microfracture signals within the specimens, while DIC captured the evolution of microstrain on the surface. The results indicate that the sandstone samples with double flaws exhibit stress drop behavior prior to failure, with the timing and magnitude of these drops varying according to the flaw angle. Analyses of acoustic emission count rate and energy indicate that acoustic emission activity begins to fluctuate before 70% of peak stress is reached, with the count rate being particularly sensitive to the internal changes within the flawed rock mass. Five types of crack development were observed: wing cracks, oblique secondary cracks, coplanar secondary cracks, out-of-plane shear cracks, and out-of-plane tensile cracks. An image coefficient of variation (GCV), based on image gray characteristics, was introduced using the characteristic parameter of the image gray histogram. The GCV curve demonstrates a sudden change prior to the instability of flawed sandstones. For sandstone samples with flaw inclination angles of 30°, 45°, 60°, and 75°, the time differences between the abrupt change point of the GCV curve and the final failure is 43, 36, 28, and 4 s, respectively. This characteristic behavior can be used for effective short-term prediction of rock fracture instability.