Deformation behavior, energy characteristics, and damage evolution of pre-flawed sandstone under triaxial multistage cyclic loading

Many natural flaws with varying inclinations exist in rock masses, which significantly influence the stability of rock engineering. However, the macroscopic responses, microscopic failure mechanisms, and fatigue properties of flawed rocks remain insufficiently investigated. Therefore, this study employs triaxial fatigue cyclic testing, SEM microstructure analysis, and CT imaging techniques to examine the macroscopic properties and microscopic fracture damage mechanisms of sandstone containing pre-existing flaws with different inclination angles ( β = 0°, 30°, 60°, and 90°) under multistage fatigue loading. The results demonstrate that the strength of the specimens increases progressively. The stiffness of pre-flawed sandstone exhibits significant hardening with a staged increase, while the crack strain and strain energy display similar variation patterns. The accumulation of stress-induced crack strain is identified as the dominant factor governing the coupled damage development evolution and eventual specimen failure. The angle β controls the macroscopic failure modes and the characteristics of crack distribution. The non-uniform stress concentration at the pre-flaw tip raises crack propagation. The crack network of the failed specimen ( β = 90°) is the most complex, and the microstructure of the failure surface appears flatter and more uniform. These findings enhance the understanding of fatigue damage mechanisms in rock engineering involving flaws.