Abstract: Stress-strain properties and strain-energy conversion behaviors of sandstone are studied by conducting freeze-thaw cycling tests and triaxial tests under unloading conditions to understand the mechanical properties of sandstone surrounding an underground tunnel in a cold region. Results show that with the increasing number of freeze-thaw cycles, peak strength in the unloading yield process is significantly reduced and the strain varies gradually. Time variations in strain energy can be divided into four stages. During the pre-peak unloading, most of the strain energy due to axial pressure is converted into elastic strain energy, and the damage caused by freeze-thaw cycling intensifies this conversion. After the peak, the stored elastic strain energy is rapidly released, and the energy dissipated by plastic deformation and damage is rapidly increased. The strain energy consumed by circumferential deformation also increases rapidly. The failure mode of the specimens shows obvious characteristics of tensile failure. With the increasing freeze-thaw cycles, the number of tensile cracks increases and particle breakage becomes more pronounced. The results reveal the fundamental behaviors of strain-energy conversion and improve our understanding of the damage and failure of frozen rock in unloading.

Key words: freeze-thaw cycles, triaxial confining pressure unloading, sandstone, stress-strain property, strain energy conversion property