Effects of stress lower limit during cyclic loading and unloading on deformation characteristics of sandstones

The aim of this research was to study the influences of the stress lower limit during cyclic loading and unloading on the deformation characteristics and laws of sandstones. For this purpose, a conventional triaxial compression test, triaxial cyclic loading and unloading test under an increased stress lower limit, and triaxial cyclic loading and unloading test under a constant stress lower limit were conducted. Based on the evolution laws of the deformation parameters in different stress paths, the deformation characteristics of sandstones under different stress paths were elaborated. The test results demonstrate that the stress lower limit can significantly affect the evolution of irreversible deformation. When the stress lower limit remained unchanged, the overall deformation development process was gentle and the failure was mild. However, when the stress lower limit increased stepwise, the elastic modulus and Poisson's ratio increased significantly, and the accumulation of axial irreversible deformation mainly occurred in a short time before failure. However, this also occurred in the overall process when the stress lower limit did not change. Moreover, the increased stress lower limit could greatly reduce the time of the volume compaction of samples, so that the samples entered the volume expansion stage early, and finally exhibited bluff-type brittle fracture in failure. According to the evolution characteristics of the deformation parameters, the entire deviatoric stress-strain curve could be divided into three stages: the crack closure stage, stable crack growth stage, and unstable crack growth stage. The increased stress lower limit could reduce the thresholds of each stage. Furthermore, the most sensitive parameter for stage identification was the volumetric strain. The increase in the stress lower limit hardened the sandstone during the early stages, which inhibited the deformation of the first two stages, but caused more rapid and stronger damage in the third stage. Therefore, the stress lower limit significantly controlled the intensity of the deformation development and accumulation in the samples. The research results provide important scientific bases for the design and construction of underground rock engineering. (C) 2019 Elsevier Ltd. All rights reserved.