Abstract: 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 2D 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.

Key words: crack front, confining pressure, low pore pressure zone, fracture toughness