Deformation and failure characteristics of sandstone subjected to true-triaxial unloading: An experimental and numerical study

In this research, we conducted both laboratory experiments and discrete element simulations to investigate the influence of maximum principal stress level on true-triaxial unloading behaviors of sandstone samples. The results show that as the level of sigma(1) at the unloading point increases, the ultimate bearing capacity of sandstone sample is increasingly strengthened, while the sample collapses more easily during the unloading process, and the failure mode of sample changes from mixed tensile-shear failure to shear failure. With the increase in the level of sigma(1), the accumulative micro-cracks at the unloading point and micro-crack generation rate during the unloading phase exhibit increasing trends, while the ratio between the number of tensile micro-cracks and shear micro-cracks generally shows a downward trend. The formation of macro fracture in sandstone sample is closely related to the material inhomogeneity and true-triaxial stress state.