The effect of fracture growth rate on fracture process zone development in quasi-brittle rock

Abstract The nonlinear development zone in front of the rock fracture tip, called fracture process zone (FPZ), could affect the propagation of fracture. Understanding the effect of fracture growth rate on FPZ could provide guidance for engineering applications such as hydraulic fracturing. In this paper, a pressure-driven fracture device was adopted to carry out Model-Ⅰ fracture test on sandstone specimens. Based on the cohesive zone model, a criterion was proposed to identify the tip of traction-free fracture and the boundary of FPZ using Digital Image Correlation (DIC) method. The fracture growth rates were calculated during the fracture process, where the FPZ length and shape were characterized. It was found that the fracture growth rate could affect the FPZ length significantly. In the linear fracture growth stage, FPZ is nearly constant in length, semi-elliptical in shape; in the exponential fracture propagation stage, FPZ lengthens with fracture growth rate, and its shape grows from a semi-ellipse to a band. A cohesive zone model considering fracture growth rate was established to describe the softening function, fracture energy, and the characteristic length of FPZ, from which the effect of fracture growth rate on characteristic length was analyzed, indicating that the FPZ length increases at a high fracture propagation rate because of the decrease of brittleness in rock.