Numerical simulation with experimental verification of the fracture behavior in granite under confining pressures based on the tension-softening model

Abstract

The use of the tension-softening model for analyzing fracture processes of rock is examined with special reference to the effect of confining pressure on the fracture extension. Tension-softening curves are measured by means of the J-based technique from unconfined tests performed on compact tension (CT) specimens of granite. On the basis of the determined tension-softening relation, numerical analyses are executed using a boundary element method (BEM) to simulate fracture of the granite under confining pressures. Numerical results are compared to the experimental results of two series of tests for which CT specimens and thick-walled cylindrical specimens were loaded to failure under confining pressures ranging from 0 to 26.5 MPa. It is shown that the BEM analyses can predict the observed fracture behavior. Based on the results, it is demonstrated that the tension-softening relation provides a suitable model to analyze the fracture process in the rock. The source mechanism for the pressure sensitive fracture is discussed by examining the growth of the fracture process zone.

Relationship Between the Orientation of Maximum Permeability and Intermediate Principal Stress in Fractured Rocks

“…The length L of the numerical fracture models is chosen as the shorter edge length in the case of nonsquare experimental fractures. The Young's modulus E equals the reported values, that is, 66 GPa for slate (Bandis, ) and 29.4 GPa for Iidate granite (Hashida et al, ). Poisson's ratio has been chosen as ν = 0.15 for all models.…”

Fracture toughness measurements on igneous rocks using a high-pressure, high-temperature rock fracture mechanics cell

“…The apparatus described here makes use of the short-rod (SR) specimen, one of the standard rock fracture toughness testing geometries (Ouchterlony, 1989) put forward by the ISRM, (1988). Because the studies for which this apparatus is intended require a variety of confining mediums at high temperatures and pressures, existing fracture toughness testing equipment such as those of Meredith and Atkinson (1985), Hashida et al (1993), Duclos and Paquet, (1991) etc. are unsuitable because they cannot accommodate changes in both temperature and pressure, employ non-ISRM standard geometries or are restricted to one type of confining medium.…”

Numerical simulation of high‐pressure rock tensile fracture experiments: Evidence of an increase in fracture energy with pressure?

“…In the present study we use a numerical technique similar to that employed by Hashida et al [1993] to investigate the pressure dependence of rock fracture properties, using previously reported data from high-pressure tensile fracture experiments (hereinafter, by "high pressure" we imply pressures that exceed the peak tensile strength of a material). It should be emphasized that even if the experiments discussed below could be interpreted unambiguously, they are insufficient to reach completely general conclusions regarding rock fracture energy at depth.…”

“…High confining pressure fracture tests of Indiana limestone [Abou-Sayed, 1977] and Iidate granite [Hashida et al, 1993] were simulated using boundary element techniques and a Dugdale-Barenblatt (tension-softening) model of the fracture process zone. Our results suggest a substantial (more than a factor of 2) increase in the fracture energy of Indiana limestone when the confining pressure was increased from zero to only 6-7 MPa.…”