Study on True triaxial Creep Experiment and Viscoplastic Creep Constitutive Model of Sandstone under Coupled damage Conditions at Submicroscopic Scales

This study investigates the creep behavior of sandstone under various damage conditions through true triaxial tests and submicroscopic mineral degradation analysis. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were employed to characterize submicroscopic mineral degradation, while creep rate, failure modes, long-term strength, and yield strength were determined from true triaxial creep tests. The results show that in its pristine state, sandstone's creep failure is primarily influenced by clay minerals. After reaching the long-term strength threshold, plastic fractures initiate within these minerals and propagate stably. As stress increases, transverse fractures occur, and at elevated temperatures, sandstone experiences microstructural compaction, reducing yield strength. Under the combined effects of high temperature and freeze-thaw cycles, quartz exhibits hydrophobic properties, while other minerals remain hydrophilic, leading to microcrack formation mainly along feldspar. These microcracks coalesce into shear cracks, significantly degrading long-term and yield strengths. The interaction between feldspar, calcite, and acidic hydrogen ions triggers pore invasion, shifting the primary crack propagation mechanism and further weakening the sandstone. A fractional derivative viscoplastic (FDVP) model, incorporating the Mogi yield criterion, was developed and demonstrated a good fit with the experimental data.