The Modified Ring Test is used to study the effect of confining pressure on fracture toughness. After an analysis of the decrease of the stress intensity factor as a function of the confining pressure, experimental results are presented. They show that the increase of fracture toughness with confining pressure depends on rock properties. The nature of the cement and preexisting microcracks appear to be the main parameters in this influence.
Few authors have studied the influence of confining pressure on the fracture toughness value (Schmidt and Hurdle, 1977; Abou-Sayed, 1977; Winter, 1983; M üller, 1986). The difficulties reside not only in performing the experiments, but also in interpreting the results. The general trend is to observe a non-negligible increase of the fracture toughness with an increase of confining pressure. Winter (1983) measured, for instance, an increase in fracture toughness value, at 20 MPa confining pressure, of 100% over the unconfined value for the Ruhr sandstone. Similar increases were reported by M üller (1985) for Iidate Granite. Schmidt and Hurdle only reported an increase of 50% in fracture toughness value at 20.7 MPa confining pressure for Indiana Limestone. Abou-sayed (1977) measured the same amount of increase, but at 6.9 MPa confining pressure. The increase of fracture toughness with confining pressure is generally attributed to a change in the behavior of the decohesion zone (or process zone) which exists ahead of the stress free crack tip. The microcrack model developed by Schmidt (1980) has been used to explain this influence. This model assumes that the process zone ahead of the crack tip results from tensile microcracks which are induced when the singular stress field exceeds the tensile strength of the rock. Using then, Irwin's formula, one can easily determine the size of the process zone: (mathematical equation)(available in full paper)
where r and ¿ are the polar coordinates with respect to the crack tip (figure 1), KIC is the fracture toughness, st is the tensile strength of the rock, and sc is the confining pressure. This formula shows that an increase in confining pressure results in a decrease of the process zone, provided KIC remains constant. As KIC increases with confining pressure, one can assume that the size of the process zone r(and ¿) is a material property, indepeudent of the confining pressure. This assumption allowed Müller to derive an equation which relate KIC at ambient condition to KIC at given confining pressure: (mathematical equation)(available in full paper)
This equation was found in excellent agreement with the data obtained by Winter (1985), on the Ruhr sandstone. However, such model is a simplified model and did not succeed to explain observed behaviors in fracture propagation. Indeed it largely underestimates the size of the process zone ;measured in rock by Swanson et al. (1984) and Labuz et al. (1985). Consequently, there is a need in determining rock fracture toughness as a function of confining pressure, and as a function of rock properties.