Numerical studies of the influence of microstructure on rock failure in uniaxial compression - Part I: Effect of heterogeneity

Abstract

A numerical parameter-sensitivity analysis has been conducted to evaluate the effect of heterogeneity on the fracture processes and strength characterization of brittle materials such as rock under uniaxial compression loadings. This was done using the Rock Failure Process Analysis code (RFPA(2D)). Studying the details of macrofracture formation from specimen to specimen due to local variation in a heterogeneous material, a number of features were consistently obtained in the numerical simulations. In relatively homogeneous specimens, the macrofracture nucleated abruptly at a point in the specimen soon after reaching the peak stress. Prior to macrofracture nucleation, a small number of acoustic emission (AE) events or microfractures were distributed randomly throughout the specimen. It is difficult to predict where the macrofracture will initiate for the homogeneous rock type since the failure of the specimen is completely brittle. On the other hand, relatively heterogeneous specimens show a somewhat different response. In this case, more diffused AE events or microfractures appear in the early stage of loading. As opposed to homogeneous specimens, macrofracture nucleation starts well before the peak stress is reached and the fracture propagation, as well as the coalescence, can be traced. These events are precursors for predicting unstable failure of the specimen. For specimens with the same property of heterogeneity, however, the numerical simulations show that the failure modes depend greatly on the fracture initiation location - which is found to be sensitive to local variations within the specimen. Peak strength is dependent on the heterogeneous nature of the specimens. Splitting and faulting failure modes often observed in experiments are also observed in the simulations under uniaxial compression. It is found that tension fractures are the dominant failure mechanism in both splitting and faulting processes. The numerical simulation shows that faulting is mainly a process of tensile fractures, often en echelon fractures, developed in a highly stressed shear band, just is as observed in actual uniaxial compression tests. (C) 2000 Elsevier Science Ltd. All rights reserved.