Stress levels of precursory strain localization subsequent to the crack damage threshold in brittle rock

Micromechanical cracking processes in rocks directly control macro mechanical responses under compressive stresses. Understanding these micro-scale observations has paramount importance in predicting macro-field problems encountered in rock engineering. Here, our study aims to investigate the development of precursory damage zones resulting from microcracking pertinent to macro-scale rock failure. A series of laboratory tests and three-dimensional (3D) numerical experiments are conducted on andesite samples to reveal the characteristics of damage zones in the form of strain fields. Our results from discrete element methodology (DEM) predict that the crack damage threshold (σ(cd)) values are 61.50% and 67.44% of relevant peak stress under two different confining stresses (σ(3) = 0.1 MPa and σ(3) = 2 MPa), respectively. Our work evaluates the strain fields within the range of the σ(cd) to the peak stress through discrete analysis for both confining stresses. We note that the representative strain field zones of failure are not observed as soon as the σ(cd) is reached. Such localized zones develop approximately 88% of peak stress levels although the confinement value changes the precursory strain localization that appears at similar stress levels. Our results also show that the distinct strain field patterns developed prior to failure control the final size of the macro-damage zone as well as their orientation with respect to the loading direction (e.g 17° and 39°) at the post-failure stage. These findings help to account for many important aspects of precursory strain field analysis in rock mechanics where the damage was rarely quantified subtly.