Sandstone damage and acoustic emission characteristics under the cyclic loading and unloading of the intermediate principal stress of the true three-axis

The work aimed to study the damage and acoustic emission (AE) characteristics of sandstone under the cyclic disturbance stress during underground engineering excavation. Graded cyclic intermediate principal stress \sigma _2 was tested using the rock-unloading test system of the true three-axis disturbance and the AE system. The mechanical properties of sandstone were studied under \sigma _2 to prove its damage law and AE characteristics, and the effect of intermediate principal stress under true triaxial cyclic process were revealed. The results showed that the graded cyclic loading and unloading of \sigma _2 had a greater influence on ultimate intermediate principal stress \varepsilon _\mathrm2max and the smallest influence on ultimate minimum principal stress \varepsilon _\mathrm3max . \sigma _2 caused \varepsilon _\mathrm2max and loading and unloading moduli to increase first and then decrease with increased minimum principal stress \sigma _3 . Cyclically loading or unloading \sigma _2 caused unstable failure to sandstone. When minimum principal stress \sigma _3 was small, sandstone unloaded at \sigma _2 and damaged as it approached \sigma _3 . When \sigma _3 was large, sandstone loaded at \sigma _2 and damaged as it approached maximum principal stress \sigma _1 . Growth rate \beta of corresponding rock-octahedron stress \tau _8 was similar during failure ( \beta =1.1260). Characteristic parameter RA-AF of AE signals was analyzed. The failure mode of the sandstone samples gradually shifted from tensile shear failure to shear failure with increased \sigma _3 . Besides, the loading damage variable of sandstone increased first and then stabilized as \sigma _2 increased. The unloading damage variable showed a significant S-shaped trend of deceleration, acceleration, and re-deceleration.