Abstract: Water saturation has an appreciable effect on deformation and strength feature of sandstone. The aim of this paper is to understand micro- and meso-mechanical properties of water-saturated sandstone under complex stress path by considering real occurrence environment of rock mass. A series of unloading tests is conducted on the water-sandstone samples after cyclic loading to examine the fatigue damage feature under different influence factors by using the rock mechanical testing system RMT-150C and scanning electron microscopy(SEM). The effects of loading frequency and upper limit stress on unloading deformation, strength and meso-damage are analyzed emphatically. The study results indicate that the stress-strain curves of water-saturated sandstone can be roughly divided into five stages according to the design scheme of the experiment. Thereinto, under the same stress condition, the axial irreversible strain rate in constant velocity stage rapidly increases with the loading frequency increasing during fatigue damage stage, which implies that “threshold value” of the upper limit stress is likely to be variable, and to be closely related with loading frequency and cycles. Meanwhile, during the unloading confining pressure in deformation failure stage, both the total strain and unloading capacity of confining pressure about water-saturated sandstone decrease with increasing the loading frequency or upper limit stress, which is consistent with the variation law of the duration of this stage with the loading frequency (or the upper limit stress). Moreover, it is found that the meso-damage characteristics under the action of loading frequency and upper limit stress are significant by analyzing meso-characteristics of failure surface, and the proportion of the microcrack area in the failure surface decreases with the increase of loading frequency, and the upper limit stress is just the opposite.

Key words: water-saturated sandstone, loading frequency, upper limit stress, fatigue damage, unloading, meso-damage feature