Understanding the mechanical and failure behavior of rock subjected to dynamic shear load is essential for the scientific design and safe operation of rock engineering. To understand the coupled effect of water and strain rate on the mechanical and failure behavior of sandstone, direct shear tests with constant normal pressure were conducted on sandstone specimens under different water saturations with shear strain rate ranging from 10 s to 10 s. The acoustic emission (AE) and scanning electron microscope (SEM) techniques were applied to analyze shear behavior and reveal failure mechanism under different loading conditions. Results show that at a given strain rate shear strength decreases with increasing water saturation, while it increases with strain rate when the water saturation remains constant. Meanwhile, the water-saturated specimens are more sensitive to strain rate effect than dry specimens, which is mainly the result of the Stefan effect. Both water and strain rate affect AE activity and fracturing of the specimens during shearing process. AE activity was more intense and lasted longer at low water saturation condition. With increasing strain rate, the peak AE counts decreased, yet the peak cumulative energy of AE events increased. The SEM analysis reveals that the development of microcracks is fuller under the conditions of low water saturations and high strain rates. The microcrack network and fracture surfaces are more complicated at lower water saturation, while microcracks are wider and the fracture surfaces are smoother under higher strain rate shearing. The findings of this study contribute to a better understanding of mechanical and failure behavior of sandstone material under the coupled effect of water saturation and dynamic disturbance.

中文翻译：

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恒正压直剪试验中含水饱和度和应变率对砂岩力学及破坏行为的影响

了解岩石在动态剪切载荷作用下的力学和破坏行为对于岩石工程的科学设计和安全运行至关重要。为了了解水和应变率对砂岩力学和破坏行为的耦合影响，对不同含水饱和度下的砂岩试件进行恒正压直剪试验，剪应变率范围为10 s至10 s。应用声发射（AE）和扫描电子显微镜（SEM）技术来分析剪切行为并揭示不同载荷条件下的失效机制。结果表明，在给定应变速率下，剪切强度随着含水饱和度的增加而减小，而当含水饱和度保持恒定时，剪切强度随着应变速率的增加而增加。同时，水饱和试件比干燥试件对应变率效应更敏感，这主要是Stefan效应的结果。水和应变率都会影响剪切过程中样品的声发射活性和断裂。低含水饱和度条件下AE活动更强烈、持续时间更长。随着应变率的增加，AE 峰值计数减少，但 AE 事件的峰值累积能量增加。 SEM分析表明，在低含水饱和度和高应变率条件下，微裂纹发育更加充分。含水饱和度较低时，微裂纹网络和断裂面更加复杂，而应变率剪切较高时，微裂纹更宽，断裂面更加光滑。这项研究的结果有助于更好地理解砂岩材料在水饱和度和动态扰动的耦合作用下的力学和破坏行为。