Quantifying the evolution of static elastic properties as crystalline rock approaches failure

The pervasive damage of rock by dilational microcracks strongly influences rock strength and macroscopic elastic properties. In this study, two types of experiment were performed to investigate and quantify the contribution of microcracking to the static elastic response of Westerly granite as it approaches failure. They are (1) increasing-amplitude cyclic loading experiments and (2) constant-amplitude cyclic loading experiments, some of which incorporated a ‘load-hold’ component to explore the elastic response of time-dependent effects such as stress corrosion. We report values for the tangent moduli in the region of the stress–strain curve where the values are the same for an unloading cycle as they are for the subsequent loading cycle. Hence, although there is hysteresis between these two curves, we assume perfect elasticity in this region. This approach allows the evolution of static elastic properties as rock approaches failure to be documented in great detail, in contrast to previous work that has generally been limited to the linear elastic region of the stress–strain curve. Increasing-amplitude stress cycling causes a gradual reduction in sample stiffness, equating to a decrease in Young's modulus (∼11%) and an increase in Poisson's ratio (∼43%) measured at a constant stress level. Elastic properties are also seen to have a strong stress-dependency during loading (∼46% increase in Young's modulus from 20 to 100 MPa). Experiments devised to promote time-dependent microcracking had a negligible contribution to the evolution of static elastic properties over the timeframe and dry conditions under which our experiments were conducted. Such results can be applied to our understanding of the mechanics, stress distribution and fault displacement models within and surrounding fault zones.