Meso-progressive failure and damage constitutive model of fractured sandstone under uniaxial loading-unloading in freeze-thaw environments

High-altitude cold regions exhibit complex geological and environmental conditions, fostering steep rock slopes with macroscopic joints and mesoscopic freeze-thaw (F-T) damage. Cyclic loading further exacerbates rock instability, yet the fracture mechanisms and load response relationships remain poorly understood. This study prepared intact and fractured sandstone specimens, subjected them to F-T cycles and graded loading-unloading, and monitored their structural evolution via X-ray computed tomography. First, the progressive failure process was investigated from both qualitative morphologic features and quantitative void parameters. The results showed that intact and fractured sandstone instability behaviors are determined by F-T damage and joint arrangement, respectively. However, both indicate that precursory localization of failure can only be detected when heterogeneous damage exists in advance. Furthermore, the void parameters of undamaged intact sandstone exhibit power-law acceleration, while damaged sandstones are characterized by a trend of initial decrease followed by an increase. Subsequently, a damage constitutive model for freeze-thawed fractured sandstone under graded loading-unloading was established. This model is based on the Lemaitre strain equivalence hypothesis and defines the coupled damage variable through multivariable indicators. In this framework, the material damage induced by fractures and F-T is unified and characterized by void parameters; while the load-induced damage is integrated with the energy linear allocation law and defined by damage energy. Thus, the stress-strain theoretical relationship is depicted, and the model is validated as reliable. Finally, a conceptual model of rock damage due to F-T and loading-unloading was proposed by combining the microscopic testing results from X-ray diffraction and scanning electron microscopy. • Failure modes and precursors are employed to qualitatively describe the progressive failure. • Voids evolution trends are used to quantitatively characterize the accelerated damage process. • A damage constitutive model of rocks affected by internal and external forces is established. • A conceptual model of rock damage under freeze-thaw and loading-unloading is proposed.