Mechanical responses of sandstone exposed to triaxial differential cyclic loading with distinct unloading rates of confining stress: A lab scale investigation

Abstract This article investigates the mechanical responses and acoustic emission (AE) characteristics of sandstone under the triaxial differential cyclic loading (DCL) at different unloading rates of confining stress. The test results indicate that strength of rock specimens under different stress paths of triaxial unloading confining stress-differential cyclic loading (TUCS-DCL) can be fitted by the Mohr–Coulomb, Hoek–Brown, and Bieniawski criteria. The confining stress unloading rate can dominate the radial strain rate, while the axial DCL pattern has an unpronounced effect. The confining stress unloading rate affects the energy evolution in radial and axial directions of specimens, with the ratio of radially released energy to axially consumed energy fluctuating more significantly during the fast unloading of confining stress, the valley value of the ratio can serve as a precursor for failure. The confining stress unloading rate has no significant effect on stress–strain phase shift, while axial rapid-loading-slow-unloading can correspond to a larger magnitude of phase shift. AE signals begin to significantly increase after the confining stress is unloaded to zero, and a notable Kaiser effect is observed during cyclic loading preceding the failure.