Acoustic emission spatiotemporal evolution and multifractal spectrum of sandstone under the coupled effects of wet–dry cycles and cyclic loading

Seasonal water level fluctuations and other environmental factors cause deterioration of sandstone on reservoir banks due to the coupled effects of wet–dry cycles (WDCs) and cyclic loading (CL). This study aims to reveal the degradation mechanism of sandstone under the coupled effects of WDCs and CL, and to provide research insight into reservoir bank stability and rock mass failure evolution. CL tests at multiple stress levels were conducted on sandstone specimens subjected to 0, 1, 3, 6, and 10 WDCs, and real-time three-dimensional acoustic emission (AE) localization was performed. The sandstone's damage evolution under the coupled effects of WDCs and CL was systematically analyzed. The results showed that peak strength, cumulative AE counts, and medium- to high-frequency AE signals decreased with increasing WDCs. The Felicity ratio and AE b-value declined significantly as WDCs or CL increased, with CL exerting a stronger influence on microcrack propagation. The Felicity ratio of specimens with ten WDCs remained low during CL, indicating that the initial damage caused by WDCs was non-negligible. The peak frequency distribution and rise angle-average frequency indicated a failure mode transition from tensile-shear to shear-dominated. The AE signals exhibited significant multifractal characteristics. The multifractal spectrum width Δφ exhibited opposite trends under WDCs and CL, reflecting their divergent influences on AE energy heterogeneity. The multifractal dimension difference Δf of shear cracks increased with WDCs or CL, whereas the spectral shape Δφ0 showed a decreasing trend. High-energy AE shear events increased under both effects, significantly contributing to sandstone deterioration.