The 2-D problem of two collinear radial cracks emanating from a circular pore in a direction parallel to the applied uniaxial compression has been considered. The long crack asymptotic expressions for the stress intensity factor and the area of the crack opening have been obtained. Comparison with published numerical results has shown that the solution can still be used even if the crack length is less than the pore radius.
Two possible mechanisms of dilatancy and fracture in brittle rocks under uniaxial compression are compared on the basis of the above analytical consideration: significant growth of the secondary cracks from i) pre-existing cracks (thin voids, which surfaces can contact due to loading) and ii) initial pores. It has been shown that initial pores are significantly weaker sources of the long secondary cracks than pre-existinH cracks and cannot be considered as the mechanism responsible for dilatancy and fracture of brittle rocks under uniaxial compression.
Non-linear deformation and fracture of brittle rocks under compression is governed, in many cases, by initiation and propagation of secondary cracks, which grow in a stable manner in the direction of the main applied compression load.
The role of sources of the secondary cracks is played by internal pre- existing defects. Two extreme types of the defects are usually considered for modelling of the secondary crack growth: (i) pre-existing shear cracks, inclined with respect to the axis of major compression; and (ii) spherical (cylindrical in 2-D case) pores.
Crack growth from pre-existing cracks was investigated both theoretically and experimentally (e.g., Brace and Bombolakis, 1963; Fairhurst and Cook, 1966; Kachanov, 1982; Moss and Gupta, 1982; Ingraffea, 1983; Zaitsev, 1983; Ashby and Hallam, 1986; Dyskin and Salganik, 1987; Fanella and Krajcinovic, 1988; Nemat-Nasser and Obata, 1988; Ashby and Sammis, 1990; Kemeny and Cook, 1991; Dyskin, Germanovich, and Salganik, 1991).
The crack growth from pores was studied, for example, by Gol'dshtein, Ladygin, and Osipenko (1974), Zaitsev (1983), Galybin (1985), Sammis and Ashby (1986), Germanovich and Cherepanov (1987), Isida and Nemat-Nasser (1987), Murakami (1987), Kemeny and Cook (1991).