单轴循环荷载下含天然裂隙脆性页岩变形及破裂特征试验研究

Experimental investigation on deformation and fracture characteristics of brittle shale with natural cracks under uniaxial cyclic loading

利用RMT–150C岩石力学测试系统,对重庆彭水含天然裂隙脆性页岩在单轴循环荷载作用下的变形及破裂特征进行了试验研究。研究结果表明：（1）在循环加卸载和裂隙的共同影响下页岩所含天然裂隙使页岩性质局部劣化、加剧裂隙扩展和破坏提前,导致屈服应力、破裂压力和峰值强度等减小,其中峰值强度降低了13.7%~58.3%;（2）轴向应变形成封闭的＂尖叶＂状滞回环,并呈疏—密—疏排列,而横向应变形成上开口＂8＂字形滞回环,并呈密—疏排列,横向应变-循环次数曲线可分为初始变形阶段、小速率等速变形阶段、大速率等速变形阶段和失稳破坏阶段等四阶段演化规律,前期横向应变突变现象可作为天然裂隙和新裂隙扩展、交汇完成,进入大速率等速变形阶段的标志,后期突变可作为整体失稳破坏的前兆;（3）含天然裂隙页岩的破坏模式主要呈拉剪贯通模式和拉贯通模式,两种贯通模式均至少包含一条贯通天然裂隙的拉裂隙;（4）在弹性阶段,有效弹性模量与损伤面积系数呈线性关系,损伤面积系数越大,有效弹性模量越小;（5）在低应力水平内循环,不可逆变形缓慢增加,轴向应变-循环次数曲线始终处于初始变形阶段,试样不发生破坏;在高应力水平内循环,经历3个变形阶段后试样发生破坏;在接近峰值应力的应力水平内循环,曲线直接进入加速变形阶段,几次循环后试样发生破坏。该研究为认清页岩的裂隙扩展形成复杂裂隙网的发展机制提供了有益参考。

To determine the deformation and fracture properties of brittle shale containing natural cracks sampled from Pengshui region of Chongqing City, uniaxial tests under uniaxial cyclic loading have been conducted using the rock mechanics test system RMT-150C. The experimental results show that： （1） Under the combined effect of cyclic load and cracks, the existence of cracks can weaken the local properties, accelerate the cracks propagation and promote the failure of the shale, resulting in the reductions in yield stress, fracture stress and peak strength; namely, the peak strength reduces by 13.7%--58.3%. （2） The axial strain shows a closed ＂sharp-leaf＂ shape hysteresis loop with a ＂sparse-compact-sparse＂ arrangement, whereas the transverse strain displays a 8-shaped hysteresis loop upward opening with a ＂compact-sparse＂ arrangement. The curve of transverse strain versus cyclic number can be divided into four phases： initial deformation phase, constant speed deformation phase with a low speed rate, constant speed deformation phase with a high speed rate, and accelerating deformation phase. The onset of transverse strain within the initial phase can be referred to as the sign of the completion of the pre-existing cracks propagating and coalescing into the new-forming cracks, and then transferring to next phase; a subsequent mutation occurrence can be considered as the omen of whole failure. （3） The main fracture morphologies of shale containing pre-existing cracks are can be categorized into two distinct types： tensile-shear cut-through mode and tensile cut-through mode. Both modes contains at least one tensile crack which cut through the pre-existing crack. （4） During the elastic phase, there is a linear relationship between the effective modulus and the damage area coefficient F, and the larger the damage area coefficient , the lower the effective modulus. （5） Under the cyclic condition of low stress level, irreversible deformation develops slowly, and the curves of axial strain versus times always stay in the initial deformation phase, while the specimens remain stable; under the cyclic condition of high-stress level, after the specimens have experienced the three phases, failures can be observed in the specimens. Under the cyclic condition of a stress level close to the theoretical peak strength, deformation curves directly reaches the acceleration phase and several cycles of loading can result in fracture of the specimens