Evaluation method of rock brittleness based on post-peak energy conversion under monotonic and cyclic triaxial compression

Rock brittleness and brittle failure play significant roles in deep underground space stability, unconventional gas extraction drilling, hydraulic fracturing efficiency, and coal mining disaster prevention, and these rock engineering processes involve cyclic loading conditions. However, nearly all the existing rock brittleness indices (BIs) are conducted with monotonic loading, and the variation in brittleness with cyclic loading has not been comprehensively studied. This study rearranged energy evolution and conversion based on stress–strain curves to investigate the existing energy-based rock BIs. Monotonic and cyclic triaxial compression experiments were conducted on red sandstone specimens to investigate the differences in the mechanical properties. This paper proposes a BI based on post-peak energy conversion to evaluate rock brittleness under monotonic and cyclic loadings. This index is defined as the ratio of the extra inputted/released energy to the fracture energy. The results showed that the variations in mechanical parameters in the two types of experiments were consistent at the pre-peak stage and different at the post-peak stage. The value of the proposed BI revealed that rock brittleness under cyclic loading was lower, which conformed with the qualitative analysis of stress–strain curves and macroscopic failure plane. The proposed BI strongly correlated with the mechanical parameters under monotonic and cyclic loading. The applicability verification, compared with the existing energy-based BIs for different rock types, confining pressures, and inclination angles of bedding planes, showed that the novel BI exhibited an excellent linear correlation with the strength parameters and had broad applicability.