Interplay between random and correlated fracture processes induced by shock-waves in compressed granites

Abstract

In contrast to damage accumulation in rocks subjected to constant or growing load wherein local failures increase with time, the strongest fracture events at the shock forcing occur at the very beginning of the loading. An inverse trend in the stress variation could be expected to affect some parameters of the rock behavior specific for a quasi-static pressure exposure. In this study, shock waves in uniaxially compressed granites were generated by an electric discharge, and the nucleation of microcracks was detected by the acoustic emission (AE) method. The experiment represented some conditions, under which deep underground rocks response on the impact forcing (explosions, shocks, etc.). The compression varied from zero to a pre-damage level. Fine-grained Sesam Black granite (grain size ∼1 mm) and coarse-grained Rapakivi granite (5–7 mm) were tested. The energy release distributions in all AE time series followed a Gutenberg-Richter type relation (power law) in both compressed and free samples with a departure from the log-linear dependences for the most intensive AE peaks. At the same time in a compressed fine-grained sample, a high-energy reminder of the power law distribution followed a well-defined exponential (Poisson-like) function; in coarse-grained granite, the higher-energy part of the energy distribution could not be described with any explicit expression. The specificities in the mechanical behavior of the tested fine- and coarse-grained granites were considered from the viewpoint of the hierarchical structure of rocks.