Uniaxial compression experiments were conducted on Barre granite specimen with a single flaw. The main objective of this study was to identify geophysical signatures of progressive damage processes in prismatic rock specimens with a pre-existing flaw and to develop technique to determine fracture process zone and crack formation in case of uniaxial compression loading. Acoustic Emission (AE) was used to track the spatiotemporal changes in the registered AE events and the two-dimensional digital image correlation (2D-DIC) technique was used to evaluate the accumulated strain and damage around the flaw tips of the uniaxially loaded Barre granite specimens. To observe the microcrack development processes and the transition to macro-cracks, a set of specimens with the same flaw geometry was loaded to the peak compressive stress. The energy of the AE sources was monitored at different stages of cracking and the source mechanism analysis was conducted to characterize the cracks as tensile, shear or mixed mode using the moment tensor inversion. The localized tensile strain profiles were obtained using the 2D-DIC, the size of the damage zone around the crack tips and the nature of the cracks were investigated. The major contribution of the study was to determine the evolution of various types of cracks at different stages of loading and to develop techniques to link the crack mechanism with the 2D-DIC strain measurements.

1. INTRODUCTION

The deformation and cracking behavior of rock masses is of great importance in the field of geomechanics. The rock material contains initial defects in the form of fractures and discontinuities. These naturally occurring features can have a significant influence on the mechanical and hydraulic properties of the rock. Therefore, it is important to study the fracturing process in brittle rocks containing pre-existing flaw (i.e., Diyuan Li et al., 2017). The different stages of cracking defined by Brace (1976) and Beniawiski (1967) in brittle rocks are (1) crack closure, (2) elastic region, (3) crack initiation and stable crack growth, (4) strain localization and unstable crack growth, and (5) failure. The development of macrocracks is associated with a localized region of microcracking. This localized region of microcracks is known as the fracture process zone and the initiation, propagation and coalescence of these microcracks causes the macrocrack formation. Acoustic emission (AE) monitoring has the potential to identify micro fracturing in brittle rocks (Eberhardt et al., 1997). It represents the plastic deformations in a material and the AE signal parameters such as hit, count, duration, energy etc. carry information about the formation of new cracks as well as the propagation and coalescence of existing cracks. AE energy is a key parameter in understanding the damage process in rocks (Lin et al., 2019). The location of the AE sources are determined on the basis of compressional Pwave arrival time. Source location analysis of the AE events help in understanding the progression of crack growth and identifying the area of instability or the damage zone leading to rock failure (Hardy, 1981).

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