Evaluating Nonlinear Plastic Dilatation Behaviors of Sandstone Using Stress-Dilatancy Models

Sandstones exhibit a complex, stress-dependent behavior characterized by nonlinearity and inelasticity. This study delves into the mechanical properties of sandstone through two distinct triaxial compression experiments: monotonic and cyclic tests, under confining pressures ranging from 0 to 20 MPa. Based on plastic strain analysis, two stress-dilatancy models were developed to describe nonlinear plastic dilatation behaviors. The introduction of the “plastic dilatancy line” concept, derived from comparing plastic dilatancy stresses with crack damage strengths, marks a significant advancement in understanding sandstone’s inelastic models. It was found that the plastic flow directions are not perpendicular to the yield surfaces marked by the characteristic strengths. This indicated that the non-associated flow rule is suitable to describe the macroscopic plastic deformations of sandstone. Furthermore, it was identified confining pressure as the dominant influence on sandstone failure, with cyclic loading modes playing a secondary role. An increase in confining pressure shifts the macroscopic failure modes from splitting-tension to mixed shear-tension, and ultimately to shear failure. Scanning Electron Microscope (SEM) analyses indicated that loading–unloading (L–U) cycles induce more significant mineral grain fragmentation compared to monotonic testing, thereby markedly decreasing sandstone’s failure strength due to accumulated damage from grain-crushing. Additionally, the dip angles of dominant fractures in samples subjected to cyclic tests are typically smaller than that in monotonic tests. This investigation not only sheds light on the complex mechanical behaviors of sandstones but also provides a vital theoretical and practical framework for future research in this field.