Energy dissipation and damage mechanism of rocks under true triaxial graded perturbations

Mineral resources into the deep mining will encounter a series of problems, due to the different mining methods lead to the roadway, quarry perimeter rock in the different unloading, loading perturbation, the perimeter rock energy dissipation and damage mode has significant differences. This study investigated sandstone's mechanical properties and macrofine damage characteristics under true triaxial graded loading-unloading conditions with different loading rates using a self-developed TAWZ-5000/3000 rock true triaxial test system. A Soft Island DS5 Acoustic Emission (AE) system was used for simultaneous monitoring. The performed tests clarified the effects of loading rate on mechanical behavior, energy dissipation, damage characteristics, fractal dimension, and AE characteristics of sandstone under true triaxial graded loading-unloading conditions. Higher loading rates resulted in higher peak intensities, fractal dimensions, and fragmentation degrees after sandstone damage. With the increased loading rate, the maximum principal stress-strain curve shape gradually changes from a convex and concave trumpet-type to a slender curved moon-type. The total dissipated energy of sandstone showed an exponential growth trend with increased loading-unloading cycles. The damage variables showed an S-type trend of 'slow increase -> accelerated increase -> slow increase'. The damage mode was dominated by tensile shearing and composite damage. With the increased loading rate, the number of macroscopic cracks in the rock samples grew, the ratio of large-to-small sandstone broken particles gradually decreased, the AE fluctuation amplitude sharply dropped, while the AE average frequency in the high-density core area sharply grew. The phenomenon of 'sudden increase in AF and sudden decrease in b-value' can be regarded as a precursor characteristic of sandstone instability under true triaxial graded loading and unloading.