Application of a local degradation model to the analysis of brittle fracture of laboratory scale rock specimens under triaxial conditions

Development of brittle fracture and the associated macroscopic behaviour of rock specimens in laboratory tests are simulated using a local degradation model for brittle fracture in heterogeneous rocks. We examine this subject because laboratory uniaxial and triaxial tests are widely used in the rock mechanics community to both characterise rock behaviour and to interpret fracture phenomena observed in natural rock, such as the Earth's crust, and developed around rock engineering structures. In addition, numerous historical efforts related to the detailed study of this subject have made available a great deal of information for use both as model input data and comparison results. A series Of numerical experiments have been performed to investigate the influence of a number of parameters on rock fracture. In particular, rock fracture under various confining stresses has been explored. The results show that the degradation algorithm is capable of reproducing many characteristics associated with brittle fracture in heterogeneous rocks, including: the development of fracture from the elemental scale to the macroscopic scale; fracture pattern as a function of confining pressure; variation of fracture plane angle with respect to confining pressure; the complete stress-strain curve and corresponding strain energy dissipation characteristics; dependence of the stress-strain Curve on confining pressured and loading-unloading hysteresis loops. Independent investigations into the effect on rock fracture of (i) the degradation parameter embodied with the model and (ii) the Weibull shape parameter used to introduce heterogeneity distribution are described. The results indicate that the degradation parameter controls the degree of degradation relative to confining pressure. As this parameter increases, and the elemental degradation decreases, the number of failed sites generated prior to the formation of macroscopic fracture plane increases, and both the peak and ultimate strengths of the model increase. The Weibull parameter influences the formation of the final fracture plane. As this parameter increases, reducing the heterogeneity, the number of diffused failed sites and the angle of the eventual fracture plane to the major principal stress tends to decrease, and the brittleness of the resulting stress-strain curve increases. It is suggested that values in the range 2-4 are appropriate for this parameter in representing elemental strength distribution of rock materials. (C) 2002 Elsevier Science Ltd. All rights reserved.