裂纹前端低孔隙压力区对岩石断裂性质的影响

Effect of low pore pressure zone around crack front on rock fracture properties

设计加工了可单独控制孔压和围压的三点弯试验装置,对多组均质性较好的岩石试样在不同孔隙压力条件下进行了三点弯试验,同时建立考虑2D流固耦合的有限元数值模型,采用LVDT测量的裂纹张开位移和DIC(数字成像技术)监测裂尖处应变分布,对建立的有限元模型进行验证和标定,得到裂纹扩展过程中尖端附近的孔压分布及裂纹扩展参数。研究结果表明,岩石破裂过程中裂纹尖端前部存在低孔隙压力区,该低孔隙压力区在裂纹尖端附近形成非均匀孔隙压力分布,使岩石I型断裂韧性大于均匀孔隙压力条件下的I型断裂韧性;I型断裂韧性增加幅度与低孔隙压力区孔隙压力梯度正相关。

Pore pressure is a very important parameter in rock mechanics. In previous publications, influence of uniform pore pressure on fracture toughness was considered; however effects of nonuniform pore pressure on fracture properties are not found. A three-point bending test device that controls pore pressure and confining pressure respectively is designed. With this device three-point bending tests are conducted on specimens of different pore pressures. Specimen and fixture are placed in the confining pressure chamber, and the specimen is enclosed in rubber membrane. A 2 D fluid-solid coupling model of finite element is built by XFEM; and the numerical model is verified and calibrated against the crack opening displacement measured by LVDT; and the deformation of crack tip monitored by digital imaging technology. The pore pressure distribution and crack propagation parameters near crack tip is in the rock fracture process are obtained. The results show that there is a low pore pressure zone in the front of the crack tip during rock fracture process. The low pore pressure zone forms a nonuniform pore pressure distribution near the crack tip, which leads to greater mode I fracture toughness than that under uniform pore pressure. The fracture toughness and the pore pressure gradient of low pore pressure zone are positively correlated.