三轴循环荷载下页岩变形及破坏特征试验研究

前期工作已经研究了含裂隙页岩在单轴循环荷载下的变形及破裂特征,为进一步认清围压与循环荷载耦合作用下的变形及破坏特征,利用MTS815.03岩石刚性压力机展开了对含裂隙页岩的三轴循环加卸载试验研究。试验结果表明:在三轴循环荷载下,含裂隙页岩的破坏形式主要为混合破坏,呈现出拉剪贯通模式;随围压逐渐增大,页岩的峰值强度呈线性增大,完整页岩经11周加卸载循环后,峰值强度增大9.98%~25.03%;而含裂隙页岩经15周加卸载循环后,峰值强度增大1.47%~6.98%;含裂隙页岩在同一循环内的弹性模量大于变形模量,卸载弹性模量大于加载弹性模量,卸载变形模量大于加载变形模量,并且损伤面积系数F越大,卸载弹性模量与加载弹性模量的差值也越大;随循环加卸载次数逐渐增加,加载变形模量与卸载变形模量呈先增大后单调减小的规律,加载弹性模量与卸载弹性模量表现出先增大,后逐渐呈"波浪式"减小的规律,剧烈波动的弹性模量是内部结构不断发生局部调整的有效证据。该研究为揭示在围压与循环荷载耦合作用下含裂隙页岩破坏形成复杂裂缝网的发展机理提供了有益参考。

Experimental research on deformation and fracture characteristics of shale under cyclic loading

The previous researches have studied the deformation and fracture characteristics of shale containing natural cracks subjected to uniaxial compression. In order to further indicate how the deformation and fracture characteristics are influenced under coupling effect of confining pressure and cyclic loading, the equipment of MTS815.03 is employed to conduct loading and unloading cyclic triaxial compression tests. The experimental results show that under triaxial cyclic loading, the main failure mode of the shale samples containing natural cracks is a mixed failure, performing tensile-shear propagating mode, and having at least one tensile crack propagating the natural crack for each sample. With the gradually increasing confining pressure, the peak strength rises linearly. For the entire shale sample already subjected to 11 cycles of loading-unloading, the peak strength increases by 9.98%~25.03%; whereas for the shale sample with crack already subjected to 15 cycles, the peak strength increases by only 1.47%~6.98%. For the sample with crack under the same cycle loading, the elastic modulus is larger than the deformation modulus, the unloading elastic modulus is larger than the loading elastic modulus, and the unloading deforamtion modulus is larger than the loading deformation modulus. Simultaneously, the larger the damage-area coefficient F, the larger the difference between the unloading elastic modulus and the loading elastic modulus. With the increase of the cyclic times, the loading deformation modulus and the unloading deformation modulus firstly increase and then monotonously decrease; whereas, the elastic moduli of both loading and unloading exhibit increasing initially then decreasing gradually with wave-shape. The serious variant of elastic modulus is the direct evidence that the inner structure coordinates in local locations. The research may provide constructive references for the mechanical investigation of crack propagation to form complex crack net.