Experimental study on acoustic emission and multifractal characteristics of fractured sandstone during loading-induced failure

• The uniaxial compressive strength of fractured sandstone first decreases and then increases with the increase of crack dip angle, reaching its minimum value at 60°. • The acoustic emission ring-down counts and their cumulative values show distinct stage evolution characteristics, including a “quiet period” during the plastic softening stage and a sharp rise in the post-peak stage, which can serve as precursor signals for rock mass instability. • The multifractal spectrum of acoustic emission signals transforms from a “left-hooked” shape to a “right-hooked” shape with increasing stress, revealing the evolution of the fracture mechanism from uniform micro-fracturing to localized high-energy fracturing. • The multifractal parameters Δ α and Δ f ( α ) can serve as short-term and imminent warning indicators for rock mass instability and failure. • The combined threshold of Δ α > 0.50 and Δ f ( α ) > 0.30 is proposed as a joint early warning criterion for catastrophic failure modes (0°, 15°, 30°, 90°), corresponding to a 10% peak strength stress margin, while a rate-of-change indicator is recommended for monitoring progressive failure modes (45°, 60°, 75°) under our experimental conditions. To investigate the mechanical behavior, evolutionary characteristics of acoustic emission (AE) and multifractal features of fractured sandstone during loading and failure, uniaxial compression tests were conducted on sandstone specimens with prefabricated crack dip angles ranging from 0° to 90° and hole diameters ranging from 2 mm to 20 mm. The results show that the uniaxial compressive strength decreases first and then increases with the increase of crack dip angle, reaching the minimum value at 60°. AE counts can effectively characterize the damage evolution stages. A typical “quiet period” occurs during the plastic softening stage, and the counts increase sharply in the post-peak stage, which can be regarded as a precursor signal for rock instability. The abrupt increase in the multifractal parameters Δ α and Δ f ( α ) can be used as short-term and imminent warning indicators. It is recommended that Δ α > 0.50 and Δ f ( α ) > 0.30 be adopted as the quantitative threshold for joint early warning, corresponding to a stress margin of about 10% of the peak strength. The average values of Δ α and Δ f ( α ) at different dip angles effectively quantify the fracture heterogeneity. The specimen with a 60° dip angle presents the highest average Δ α , indicating that large-scale fractures dominate the failure process. The research results can provide a theoretical basis and quantitative reference for the monitoring and early warning of dynamic disasters in fractured rock masses.