Shear fracture energy of Westerly granite from post‐failure behavior

Abstract

We consider in this paper the energetic relationship among four different types of measurements: evaluation of shear fracture energy from seismological data, rock mechanics study of post‐failure behavior in triaxial compression, measurement of specific surface energy of minerals in a tensile mode, and scanning electron microscope (SEM) observation of stress‐induced microstructures. Shear fracture energy can be estimated from post‐failure deformation data following an integration scheme recently suggested by Rice. Values so determined for Westerly granite at pressures up to 250 MPa and temperatures up to 700°C are of the order of 104 J m−2, which is about 2 orders of magnitude higher than the tensile values. Analysis of our data, together with previous room temperature post‐failure data, show that the influence of temperature, pressure, and rock type can change the shear fracture energy by an order of magnitude. Microcracking energy can be estimated by multiplying the stress‐induced crack area (obtained by stereological technique) and the single‐crystal specific surface energy. Energy for the extension of microcracks resolvable under SEM can account for a major portion of the total energy input for pre‐failure deformation. However, for post‐failure deformation the estimate of microcracking energy is smaller than the total by at least an order of magnitude.