Effect of Unloading Rate on the Mechanical Behavior and Fracture Characteristics of Sandstones Under Complex Triaxial Stress Conditions

In recent years, many studies have shown that it is meaningful to place rocks under stress paths corresponding to various loading and unloading conditions. However, the deformation evolution of rock under cyclic loading with consideration of the mechanical behavior and characteristics has rarely been studied under triaxial cyclic unloading and loading conditions. In practical engineering, particularly in underground or mining engineering, the stress increase in the rock mass in areas affected by mining is mainly caused by crack initiation and development when the rock is subjected to the effects of cyclic unloading and loading. In this study, variations in the stress–strain curves, irreversible strain, elastic modulus, and Poisson’s ratio are discussed and explained. The test results demonstrate that in comparison with a lower initial confining stress, increasing the initial confining pressure restrains the radial deformation of sandstone samples, and the degree of compaction of the sandstone samples rapidly increases in the failure stage. This results in the loss of the failure buffering process of the sandstone sample. Changes in the degree of compaction of the rock samples lead to obvious differences in the irreversible strain of the rock under different initial confining pressures and different limit unloading and loading cyclic confining stresses. The scanning electron microscopy and analysis results demonstrate that the macroscopic mechanical and microscopic physical properties of sandstone show different characteristics under different initial confining stresses.