复杂应力路径下饱水砂岩宏细观力学特性研究

Research on macro- and meso-mechanical properties of water-saturated sandstone under complex stress path

饱水作用对砂岩变形及强度特征影响显著,考虑岩体实际赋存环境,为了解复杂应力路径下饱水砂岩宏细观力学特性,利用RMT-150C岩石力学多功能试验机及SEM电镜扫描技术,对饱水砂岩进行循环荷载作用后卸荷破坏试验,研究不同影响因素作用下的疲劳损伤特征,重点分析了加载频率、上限应力等因素对卸荷变形、强度及细观损伤特征的影响规律。研究表明,根据试验设计方案获得的饱水砂岩应力-应变曲线大致可以划分为5个典型阶段,其中,同一应力状态下,疲劳损伤阶段轴向不可逆应变在等速变形阶段的应变速率随加载频率的增大迅速增加,这表明上限应力的"门槛值"极有可能是变化的,且与加载频率及循环次数密切相关;同时,卸围压变形破坏阶段的总应变及围压卸荷量均随加载频率(或上限应力)的增大而减小,与该阶段持续时间随加载频率(或上限应力)的变化规律一致。不仅如此,通过分析饱水砂岩破坏面细观损伤特征发现,加载频率与上限应力作用下的细观损伤特征差异显著,其中破坏面微裂隙面积占比随加载频率增大而减小,上限应力则恰好相反。

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-150 C 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.