Study on the Differences in Mechanical Properties and Energy Evolution of Sandstone under Different Loading and Unloading Modes

Abstract

To study the mechanical properties and energy evolution of sandstone under cyclic loading, experiments were conducted on intact prefabricated sandstone specimens using different loading and unloading methods, and the macroscopic failure characteristics of the rock samples were observed and analyzed. The mechanical properties and energy evolution of the rock samples were analyzed based on the stress-strain curves. The results indicate that the hysteresis loop areas differ under the two loading and unloading methods. The method with increasing upper limits mainly shows that the higher the stress peak, the stronger the material's energy dissipation capacity, and the more energy is consumed by plastic deformation and damage. In the method of increasing the upper limit and decreasing the lower limit, the hysteresis loop area gradually increases, indicating that an elevated stress level enhances the material's energy dissipation capacity, and damage induces internal friction or microcrack propagation. In both loading methods, energy evolution and damage failure characteristics are dominated by the stress application pattern: the stress concentration effect in the upper-limit-increasing method accelerates the compaction of the elastic framework and energy dissipation due to damage, resulting in a greater increase in total input energy, elastic energy, and dissipated energy, and the energy conversion efficiency is also higher due to more favorable elastic energy storage. In the upper-limit-increasing and lower-limit-decreasing method, simultaneously adjusting the stress upper and lower limits weakens stress concentration, resulting in smoother energy growth and increased damage energy consumption due to stress fluctuations. The growth rate of dissipated energy density shows a 'burst-decline-leveling-off' trend, corresponding to three damage stages, revealing the non-uniform evolution of damage. The upper-limit-increasing method results in tensile failure dominated by a single vertical main crack, with damage occurring as a directional superposition effect, while the upper-limit-increasing and lower-limit-decreasing method results in dispersed failure driven by a multi-directional crack network, with damage extending at multiple scales.